TRANSCRIBER’S NOTE
Typographical errors noticed during the preparation of this text have been underlined
like
this. A list has also been placed at the end.
A POPULAR HISTORY OF ASTRONOMY
DURING THE NINETEENTH CENTURY
BY THE SAME AUTHOR
PROBLEMS IN ASTROPHYSICS.
Demy 8vo., cloth. Containing over 100 Illustrations.
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THE SYSTEM OF THE STARS.
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A. AND C. BLACK, SOHO SQUARE, LONDON, W.[Pg i]
A POPULAR
HISTORY OF ASTRONOMY
DURING
THE NINETEENTH CENTURY
BY
AGNES M. CLERKE
LONDON
ADAM AND CHARLES BLACK
1908
First Edition, Post 8vo., published 1885
Second Edition, Post 8vo., published 1887
Third Edition, Demy 8vo., published 1893
Fourth Edition, Demy 8vo., published 1902
Fourth Edition, Post 8vo., reprinted February, 1908
PREFACE TO THE FOURTH EDITION
Since the third edition of the present work issued from the press,
the nineteenth century has run its course and finished its record.
A new era has dawned, not by chronological prescription alone, but
to the vital sense of humanity. Novel thoughts are rife; fresh
impulses stir the nations; the soughing of the wind of progress
strikes every ear. “The old order changeth” more and more
swiftly as mental activity becomes intensified. Already many of
the scientific doctrines implicitly accepted fifteen years ago begin
to wear a superannuated aspect. Dalton’s atoms are in process of
disintegration; Kirchhoff’s theorem visibly needs to be modified;
Clerk Maxwell’s medium no longer figures as an indispensable
factotum; “absolute zero” is known to be situated on an asymptote
to the curve of cold. Ideas, in short, have all at once become
plastic, and none more completely so than those relating to
astronomy. The physics of the heavenly bodies, indeed, finds its
best opportunities in unlooked-for disclosures; for it deals with
transcendental conditions, and what is strange to terrestrial experience
may serve admirably to expound what is normal in the skies.
In celestial science especially, facts that appear subversive are often
the most illuminative, and the prospect of its advance widens and
brightens with each divagation enforced or permitted from the
strait paths of rigid theory.
This readiness for innovation has undoubtedly its dangers and
drawbacks. To the historian, above all, it presents frequent occasions
of embarrassment. The writing of history is a strongly
selective operation, the outcome being valuable just in so far as the
choice what to reject and what to include has been judicious; and
the task is no light one of discriminating between barren speculations
and ideas pregnant with coming truth. To the possession of[Pg vi]
such prescience of the future as would be needed to do this effectually
I can lay no claim; but diligence and sobriety of thought are
ordinarily within reach, and these I shall have exercised to good
purpose if I have succeeded in rendering the fourth edition of A
Popular History of Astronomy during the Nineteenth Century not wholly
unworthy of a place in the scientific literature of the twentieth
century.
My thanks are due to Sir David Gill for the use of his photograph
of the great comet of 1901, which I have added to my list of illustrations,
and to the Council of the Royal Astronomical Society for
the loan of glass positives needed for the reproduction of those
included in the third edition.
London, July, 1902.
PREFACE TO THE FIRST EDITION
The progress of astronomy during the last hundred years has been
rapid and extraordinary. In its distinctive features, moreover, the
nature of that progress has been such as to lend itself with facility
to untechnical treatment. To this circumstance the present volume
owes its origin. It embodies an attempt to enable the ordinary
reader to follow, with intelligent interest, the course of modern
astronomical inquiries, and to realize (so far as it can at present be
realized) the full effect of the comprehensive change in the whole
aspect, purposes, and methods of celestial science introduced by the
momentous discovery of spectrum analysis.
Since Professor Grant’s invaluable work on the History of Physical
Astronomy was published, a third of a century has elapsed. During
the interval a so-called “new astronomy” has grown up by the side
of the old. One effect of its advent has been to render the science
of the heavenly bodies more popular, both in its needs and in its
nature, than formerly. More popular in its needs, since its progress
now primarily depends upon the interest in, and consequent efforts
towards its advancement of the general public; more popular in
its nature, because the kind of knowledge it now chiefly tends to
accumulate is more easily intelligible—less remote from ordinary
experience—than that evolved by the aid of the calculus from
materials collected by the use of the transit-instrument and chronograph.
It has thus become practicable to describe in simple language the
most essential parts of recent astronomical discoveries, and, being
practicable, it could not be otherwise than desirable to do so. The
service to astronomy itself would be not inconsiderable of enlisting
wider sympathies on its behalf, while to help one single mind
towards a fuller understanding of the manifold works which have[Pg viii]
in all ages irresistibly spoken to man of the glory of God might
well be an object of no ignoble ambition.
The present volume does not profess to be a complete or exhaustive
history of astronomy during the period covered by it. Its design
is to present a view of the progress of celestial science, on its most
characteristic side, since the time of Herschel. Abstruse mathematical
theories, unless in some of their more striking results, are
excluded from consideration. These, during the eighteenth century,
constituted the sum and substance of astronomy, and their fundamental
importance can never be diminished, and should never be
ignored. But as the outcome of the enormous development given
to the powers of the telescope in recent times, together with the
swift advance of physical science, and the inclusion, by means of the
spectroscope, of the heavenly bodies within the domain of its
inquiries, much knowledge has been acquired regarding the nature
and condition of those bodies, forming, it might be said, a science
apart, and disembarrassed from immediate dependence upon intricate,
and, except to the initiated, unintelligible formulæ. This
kind of knowledge forms the main subject of the book now offered
to the public.
There are many reasons for preferring a history to a formal
treatise on astronomy. In a treatise, what we know is set forth. A
history tells us, in addition, how we came to know it. It thus
places facts before us in the natural order of their ascertainment,
and narrates instead of enumerating. The story to be told leaves
the marvels of imagination far behind, and requires no embellishment
from literary art or high-flown phrases. Its best ornament is
unvarnished truthfulness, and this, at least, may confidently be
claimed to be bestowed upon it in the ensuing pages.
In them unity of treatment is sought to be combined with a due
regard to chronological sequence by grouping in separate chapters
the various events relating to the several departments of descriptive
astronomy. The whole is divided into two parts, the line between
which is roughly drawn at the middle of the present century.
Herschel’s inquiries into the construction of the heavens strike the
keynote of the first part; the discoveries of sun-spot and magnetic
periodicity and of spectrum analysis determine the character of the
second. Where the nature of the subject required it, however, this
arrangement has been disregarded. Clearness and consistency
should obviously take precedence of method. Thus, in treating of[Pg ix]
the telescopic scrutiny of the various planets, the whole of the
related facts have been collected into an uninterrupted narrative.
A division elsewhere natural and helpful would here have been
purely artificial, and therefore confusing.
The interests of students have been consulted by a full and
authentic system of references to the sources of information relied
upon. Materials have been derived, as a rule with very few exceptions,
from the original authorities. The system adopted has been
to take as little as possible at second-hand. Much pains have been
taken to trace the origin of ideas, often obscurely enunciated long
before they came to resound through the scientific world, and to
give to each individual discoverer, strictly and impartially, his due.
Prominence has also been assigned to the biographical element, as
underlying and determining the whole course of human endeavour.
The advance of knowledge may be called a vital process. The
lives of men are absorbed into and assimilated by it. Inquiries
into the kind and mode of the surrender in each separate case must
always possess a strong interest, whether for study or for example.
The acknowledgments of the writer are due to Professor
Edward S. Holden, director of the Washburn Observatory, Wisconsin,
and to Dr. Copeland, chief astronomer of Lord Crawford’s
Observatory at Dunecht, for many valuable communications.
London, September, 1885.
[Pg x]
[Pg xi]
CONTENTS
Three Kinds of Astronomy—Progress of the Science during the Eighteenth
Century—Popularity and Rapid Advance during the Nineteenth
Century
PART I
PROGRESS OF ASTRONOMY DURING THE FIRST HALF OF THE
NINETEENTH CENTURY
FOUNDATION OF SIDEREAL ASTRONOMY
State of Knowledge regarding the Stars in the Eighteenth Century—Career
of Sir William Herschel—Constitution of the Stellar System—Double
Stars—Herschel’s Discovery of their Revolutions—His Method of Star-gauging—Discoveries
of Nebulæ—Theory of their Condensation into
Stars—Summary of Results
PROGRESS OF SIDEREAL ASTRONOMY
Exact Astronomy in Germany—Career of Bessel—His Fundamenta Astronomiæ—Career
of Fraunhofer—Parallaxes of Fixed Stars—Translation
of the Solar System—Astronomy of the Invisible—Struve’s Researches
in Double Stars—Sir John Herschel’s Exploration of the Heavens—Fifty
Years’ Progress
PROGRESS OF KNOWLEDGE REGARDING THE SUN
Early Views as to the Nature of Sun-spots—Wilson’s Observations and
Reasonings—Sir William Herschel’s Theory of the Solar Constitution—Sir
John Herschel’s Trade-Wind Hypothesis—Baily’s Beads—Total
Solar Eclipse of 1842—Corona and Prominences—Eclipse of 1851
PLANETARY DISCOVERIES
Bode’s Law—Search for a Missing Planet—Its Discovery by Piazzi—Further
Discoveries of Minor Planets—Unexplained Disturbance of
Uranus—Discovery of Neptune—Its Satellite—An Eighth Saturnian
Moon—Saturn’s Dusky Ring—The Uranian System
COMETS
Predicted Return of Halley’s Comet—Career of Olbers—Acceleration of
Encke’s Comet—Biela’s Comet—Its Duplication—Faye’s Comet—Comet
of 1811—Electrical Theory of Cometary Emanations—The
Earth in a Comet’s Tail—Second Return of Halley’s Comet—Great
Comet of 1843—Results to Knowledge
INSTRUMENTAL ADVANCES
Two Principles of Telescopic Construction—Early Reflectors—Three Varieties—Herschel’s
Specula—High Magnifying Powers—Invention of the
Achromatic Lens—Guinand’s Optical Glass—The Great Rosse Reflector—Its
Disclosures—Mounting of Telescopes—Astronomical Circles—Personal
Equation
PART II
RECENT PROGRESS OF ASTRONOMY
FOUNDATION OF ASTRONOMICAL PHYSICS
Schwabe’s Discovery of a Decennial Sun-spot Period—Coincidence with
Period of Magnetic Disturbance—Sun-spots and Weather—Spectrum
Analysis—Preliminary Inquiries—Fraunhofer Lines—Kirchhoff’s Principle—Anticipations—Elementary
Principles of Spectrum Analysis—Unity
of Nature
SOLAR OBSERVATIONS AND THEORIES
Black Openings in Spots—Carrington’s Observations—Rotation of the
Sun—Kirchhoff’s Theory of the Solar Constitution—Faye’s Views—Solar
Photography—Kew Observations—Spectroscopic Method—Cyclonic
Theory of Sun-spots—Volcanic Hypothesis—A Solar Outburst—Sun-spot
Periodicity—Planetary Influence—Structure of the Photosphere
RECENT SOLAR ECLIPSES
Expeditions to Spain—Great Indian Eclipse—New Method of Viewing
Prominences—Total Eclipse Visible in North America—Spectrum of
the Corona—Eclipse of 1870—Young’s Reversing Layer—Eclipse of
1871—Corona of 1878—Varying Coronal Types—Egyptian Eclipse—Daylight
Coronal Photography—Observations at Caroline Island—Photographs
of Corona in 1886 and 1889—Eclipses of 1896, 1898, 1900,
and 1901—Mechanical Theory of Corona—Electro-Magnetic Theories—Nature
of Corona
SOLAR SPECTROSCOPY
Chemistry of Prominences—Study of their Forms—Two Classes—Photographs
and Spectrographs of Prominences—Their Distribution—Structure
of the Chromosphere—Spectroscopic Measurement of Radial
Movements—Spectroscopic Determination of Solar Rotation—Velocities
of Transport in the Sun—Lockyer’s Theory of Dissociation—Solar
Constituents—Oxygen Absorption in Solar Spectrum
TEMPERATURE OF THE SUN
Thermal Power of the Sun—Radiation and Temperature—Estimates of
Solar Temperature—Rosetti’s and Wilson’s Results—Zöllner’s Method—Langley’s
Experiment at Pittsburg—The Sun’s Atmosphere—Langley’s
Bolometric Researches—Selective Absorption by our Air—The Solar
Constant
THE SUN’S DISTANCE
Difficulty of the Problem—Oppositions of Mars—Transits of Venus—Lunar
Disturbance—Velocity of Light—Transit of 1874—Inconclusive
Result—Opposition of Mars in 1877—Measurements of Minor Planets—Transit
of 1882—Newcomb’s Determination of the Velocity of Light—Combined
Result
PLANETS AND SATELLITES
Schröter’s Life and Work—Luminous Appearances during Transits of
Mercury—Mountains of Mercury—Intra-Mercurian Planets—Schiaparelli’s
Results for the Rotation of Mercury and Venus—Illusory
Satellite—Mountains and Atmosphere of Venus—Ashen Light—Solidity
of the Earth—Variation of Latitude—Secular Changes of
Climate—Figure of the Globe—Study of the Moon’s Surface—Lunar
Atmosphere—New Craters—Thermal Energy of Moonlight—Tidal
Friction
PLANETS AND SATELLITES—(continued)
Analogy between Mars and the Earth—Martian Snowcaps, Seas, and Continents—Climate
and Atmosphere—Schiaparelli’s Canals—Discovery
of Two Martian Satellites—Photographic Detection of Minor Planets—Orbit
of Eros—Distribution of the Minor Planets—Their Collective
Mass and Estimated Diameters—Condition of Jupiter—His Spectrum—Transits
of his Satellites—Discovery of a Fifth Satellite—The Great
Red Spot—Constitution of Saturn’s Rings—Period of Rotation of the
Planet—Variability of Japetus—Equatorial Markings on Uranus—His
Spectrum—Rotation of Neptune—Trans-Neptunian Planets
THEORIES OF PLANETARY EVOLUTION
Origin of the World according to Kant—Laplace’s Nebular Hypothesis—Maintenance
of the Sun’s Heat—Meteoric Hypothesis—Radiation as
an Effect of Contraction—Regenerative Theory—Faye’s Scheme of
Planetary Development—Origin of the Moon—Effects of Tidal
RECENT COMETS
Donati’s Comet—The Earth again Involved in a Comet’s Tail—Comets of
the August and November Meteors—Star Showers—Comets and
Meteors—Biela’s Comet and the Andromedes—Holmes’s Comet—Deflection
of the Leonids—Orbits of Meteorites—Meteors with Stationary
Radiants—Spectroscopic Analysis of Cometary Light—Comet of 1901—Coggia’s
Comet
RECENT COMETS—(continued)
Forms of Comets’ Tails—Electrical Repulsion—Brédikhine’s Three Types—Great
Southern Comet—Supposed Previous Appearances—Tebbutt’s
Comet and the Comet of 1807—Successful Photographs—Schaeberle’s
Comet—Comet Wells—Sodium Blaze in Spectrum—Great Comet of
1882—Transit across the Sun—Relation to Comets of 1843 and 1880—Cometary
Systems—Spectral Changes in Comet of 1882—Brooks’s
Comet of 1889—Swift’s Comet of 1892—Origin of Comets
STARS AND NEBULÆ
Stellar Chemistry—Four Orders of Stars—Their Relative Ages—Gaseous
Stars—Spectroscopic Star-Catalogues—Stellar Chemistry—Hydrogen
Spectrum in Stars—The Draper Catalogue—Velocities of Stars in Line
of Sight—Spectroscopic Binaries—Eclipses of Algol—Catalogues of
Variables—New Stars—Outbursts in Nebulæ—Nova Aurigæ—Nova
Persei—Gaseous Nebulæ—Variable Nebulæ—Movements of Nebulæ—Stellar
[Pg xv]
Persei—Gaseous Nebulæ—Variable Nebulæ—Movements of Nebulæ—Stellar
and Nebular Photography—Nebulæ in the Pleiades—Photographic
Star-charting—Stellar Parallax—Double Stars—Stellar Photometry—Status
of Nebulæ—Photographs and Drawings of the Milky
Way—Star Drift
METHODS OF RESEARCH
Development of Telescopic Power—Silvered Glass Reflectors—Giant Refractors—Comparison
with Reflectors—The Yerkes Telescope—Atmospheric
Disturbance—The Lick Observatory—Mechanical Difficulties—The
Equatoreal Coudé—The Photographic Camera—Retrospect and
Conclusion
Chronology, 1774-1893—Chemical Elements in the Sun (Rowland, 1891)—Epochs
of Sun-spot Maximum and Minimum from 1610 to 1901—Movements
of Sun and Stars—List of Great Telescopes—List of Observatories
employed in the Construction of the Photographic Chart
and Catalogue of the Heavens
[Pg xvii]
LIST OF ILLUSTRATIONS
Photograph of the Great Nebula in Orion, 1883 Frontispiece
Photographs of Jupiter, 1879, and of Saturn, 1885 Vignette
Plate I. Photographs of the Solar Chromosphere and Prominences To face p. 198
Plate III. The Great Comet of September, (Photographed at the Cape of Good Hope)
Plate IV. Photographs of Swift’s Comet, 1892
Plate V. Photographic and Visual Spectrum of Nova Aurigæ
Plate VI. Photograph of the Milky Way in Sagittarius
[Pg xviii]
HISTORY OF ASTRONOMY
DURING THE NINETEENTH CENTURY
We can distinguish three kinds of astronomy, each with a different
origin and history, but all mutually dependent, and composing, in
their fundamental unity, one science. First in order of time came
the art of observing the returns, and measuring the places, of the
heavenly bodies. This was the sole astronomy of the Chinese and
Chaldeans; but to it the vigorous Greek mind added a highly
complex geometrical plan of their movements, for which Copernicus
substituted a more harmonious system, without as yet any idea of
a compelling cause. The planets revolved in circles because it was
their nature to do so, just as laudanum sets to sleep because it
possesses a virtus dormitiva. This first and oldest branch is known
as “observational,” or “practical astronomy.” Its business is to
note facts as accurately as possible; and it is essentially unconcerned
with schemes for connecting those facts in a manner satisfactory to
the reason.
The second kind of astronomy was founded by Newton. Its nature
is best indicated by the term “gravitational”; but it is also called
“theoretical astronomy.”[1] It is based on the idea of cause; and
the whole of its elaborate structure is reared according to the
dictates of a single law, simple in itself, but the tangled web of
whose consequences can be unravelled only by the subtle agency of
an elaborate calculus.
The third and last division of celestial science may properly be
termed “physical and descriptive astronomy.” It seeks to know
what the heavenly bodies are in themselves, leaving the How? and[Pg 2]
the Wherefore? of their movements to be otherwise answered.
Now, such inquiries became possible only through the invention
of the telescope, so that Galileo was, in point of fact, their
originator. But Herschel first gave them a prominence which the
whole progress of science during the nineteenth century served to
confirm and render more exclusive. Inquisitions begun with the
telescope have been extended and made effective in unhoped-for
directions by the aid of the spectroscope and photographic camera;
and a large part of our attention in the present volume will be
occupied with the brilliant results thus achieved.
The unexpected development of this new physical-celestial science
is the leading fact in recent astronomical history. It was out of the
regular course of events. In the degree in which it has actually
occurred it could certainly not have been foreseen. It was a seizing
of the prize by a competitor who had hardly been thought qualified
to enter the lists. Orthodox astronomers of the old school looked
with a certain contempt upon observers who spent their nights in
scrutinising the faces of the moon and planets rather than in timing
their transits, or devoted daylight energies, not to reductions and
computations, but to counting and measuring spots on the sun.
They were regarded as irregular practitioners, to be tolerated
perhaps, but certainly not encouraged.
The advance of astronomy in the eighteenth century ran in general
an even and logical course. The age succeeding Newton’s had for
its special task to demonstrate the universal validity, and trace the
complex results, of the law of gravitation. The accomplishment
of that task occupied just one hundred years. It was virtually
brought to a close when Laplace explained to the French Academy,
November 19, 1787, the cause of the moon’s accelerated motion.
As a mere machine, the solar system, so far as it was then known,
was found to be complete and intelligible in all its parts; and in
the Mécanique Céleste its mechanical perfections were displayed under
a form of majestic unity which fitly commemorated the successive
triumphs of analytical genius over problems amongst the most
arduous ever dealt with by the mind of man.
Theory, however, demands a practical test. All its data are
derived from observation; and their insecurity becomes less tolerable
as it advances nearer to perfection. Observation, on the other hand,
is the pitiless critic of theory; it detects weak points, and provokes
reforms which may be the beginnings of discovery. Thus, theory
and observation mutually act and react, each alternately taking the
lead in the endless race of improvement.
Now, while in France Lagrange and Laplace were bringing the
gravitational theory of the solar system to completion, work of[Pg 3]
a very different kind, yet not less indispensable to the future welfare
of astronomy, was being done in England. The Royal Observatory
at Greenwich is one of the few useful institutions which date their
origin from the reign of Charles II. The leading position which it
still occupies in the science of celestial observation was, for near a
century and a half after its foundation, an exclusive one. Delambre
remarked that, had all other materials of the kind been destroyed,
the Greenwich records alone would suffice for the restoration of
astronomy. The establishment was indeed absolutely without a
rival.[2] Systematic observations of sun, moon, stars, and planets
were during the whole of the eighteenth century made only at
Greenwich. Here materials were accumulated for the secure correction
of theory, and here refinements were introduced by which
the exquisite accuracy of modern practice in astronomy was eventually
attained.
The chief promoter of these improvements was James Bradley.
Few men have possessed in an equal degree with him the power
of seeing accurately, and reasoning on what they see. He let
nothing pass. The slightest inconsistency between what appeared
and what was to be expected roused his keenest attention; and he
never relaxed his mental grip of a subject until it had yielded to
his persistent inquisition. It was to these qualities that he owed
his discoveries of the aberration of light and the nutation of the
earth’s axis. The first was announced in 1729. What is meant by
it is that, owing to the circumstance of light not being instantaneously
transmitted, the heavenly bodies appear shifted from their true
places by an amount depending upon the ratio which the velocity of
light bears to the speed of the earth in its orbit. Because light
travels with enormous rapidity, the shifting is very slight; and each
star returns to its original position at the end of a year.
Bradley’s second great discovery was finally ascertained in 1748.
Nutation is a real “nodding” of the terrestrial axis produced by the
dragging of the moon at the terrestrial equatorial protuberance.
From it results an apparent displacement of the stars, each of them
describing a little ellipse about its true or “mean” position, in a
period of nearly nineteen years.
Now, an acquaintance with the fact and the laws of each of
these minute irregularities is vital to the progress of observational
astronomy; for without it the places of the heavenly bodies could
never be accurately known or compared. So that Bradley, by their
detection, at once raised the science to a higher grade of precision.
Nor was this the whole of his work. Appointed Astronomer-Royal
in 1742, he executed during the years 1750-62 a series of observations[Pg 4]
which formed the real beginning of exact astronomy. Part of their
superiority must, indeed, be attributed to the co-operation of John
Bird, who provided Bradley in 1750 with a measuring instrument
of till then unequalled excellence. For not only was the art of
observing in the eighteenth century a peculiarly English art, but
the means of observing were furnished almost exclusively by British
artists. John Dollond, the son of a Spitalfields weaver, invented
the achromatic lens in 1758, removing thereby the chief obstacle to
the development of the powers of refracting telescopes; James Short,
of Edinburgh, was without a rival in the construction of reflectors;
the sectors, quadrants, and circles of Graham, Bird, Ramsden, and
Cary were inimitable by Continental workmanship.
Thus practical and theoretical astronomy advanced on parallel
lines in England and France respectively, the improvement of
their several tools—the telescope and the quadrant on the one side,
and the calculus on the other—keeping pace. The whole future
of the science seemed to be theirs. The cessation of interest
through a too speedy attainment of the perfection towards which
each spurred the other, appeared to be the only danger it held in
store for them. When all at once, a rival stood by their side—not,
indeed, menacing their progress, but threatening to absorb their
popularity.
The rise of Herschel was the one conspicuous anomaly in the
astronomical history of the eighteenth century. It proved decisive
of the course of events in the nineteenth. It was unexplained
by anything that had gone before; yet all that came after hinged
upon it. It gave a new direction to effort; it lent a fresh impulse
to thought. It opened a channel for the widespread public
interest which was gathering towards astronomical subjects to
flow in.
Much of this interest was due to the occurrence of events
calculated to arrest the attention and excite the wonder of the
uninitiated. The predicted return of Halley’s comet in 1759
verified, after an unprecedented fashion, the computations of
astronomers. It deprived such bodies for ever of their portentous
character; it ranked them as denizens of the solar system. Again,
the transits of Venus in 1761 and 1769 were the first occurrences
of the kind since the awakening of science to their consequence.
Imposing preparations, journeys to remote and hardly accessible
regions, official expeditions, international communications, all for
the purpose of observing them to the best advantage, brought
their high significance vividly to the public consciousness; a result
aided by the facile pen of Lalande, in rendering intelligible the
means by which these elaborate arrangements were to issue in an[Pg 5]
accurate knowledge of the sun’s distance. Lastly, Herschel’s
discovery of Uranus, March 13, 1781, had the surprising effect of
utter novelty. Since the human race had become acquainted with
the company of the planets, no addition had been made to their
number. The event thus broke with immemorial traditions, and
seemed to show astronomy as still young and full of unlooked-for
possibilities.
Further popularity accrued to the science from the sequel of a
career so strikingly opened. Herschel’s huge telescopes, his
detection by their means of two Saturnian and as many Uranian
moons, his piercing scrutiny of the sun, picturesque theory of its
constitution, and sagacious indication of the route pursued by it
through space; his discovery of stellar revolving systems, his bold
soundings of the universe, his grandiose ideas, and the elevated
yet simple language in which they were conveyed—formed a
combination powerfully effective to those least susceptible of new
impressions. Nor was the evoked enthusiasm limited to the
British Isles. In Germany, Schröter followed—longo intervallo—in
Herschel’s track. Von Zach set on foot from Gotha that
general communication of ideas which gives life to a forward
movement. Bode wrote much and well for unlearned readers.
Lalande, by his popular lectures and treatises, helped to form an
audience which Laplace himself did not disdain to address in the
Exposition du Système du Monde.
This great accession of public interest gave the impulse to
the extraordinarily rapid progress of astronomy in the nineteenth
century. Official patronage combined with individual zeal sufficed
for the elder branches of the science. A few well-endowed institutions
could accumulate the materials needed by a few isolated
thinkers for the construction of theories of wonderful beauty and
elaboration, yet precluded, by their abstract nature, from winning
general applause. But the new physical astronomy depends for
its prosperity upon the favour of the multitude whom its striking
results are well fitted to attract. It is, in a special manner, the
science of amateurs. It welcomes the most unpretending co-operation.
There is no one “with a true eye and a faithful
hand” but can do good work in watching the heavens. And not
unfrequently, prizes of discovery which the most perfect appliances
failed to grasp, have fallen to the share of ignorant or ill-provided
assiduity.
Observers, accordingly, have multiplied; observatories have been
founded in all parts of the world; associations have been constituted
for mutual help and counsel. A formal astronomical congress met
in 1789 at Gotha—then, under Duke Ernest II. and Von Zach, the[Pg 6]
focus of German astronomy—and instituted a combined search for
the planet suspected to revolve undiscovered between the orbits of
Mars and Jupiter. The Astronomical Society of London was
established in 1820, and the similar German institution in 1863.
Both have been highly influential in promoting the interests, local
and general, of the science they are devoted to forward; while
functions corresponding to theirs have been discharged elsewhere
by older or less specially constituted bodies, and new ones of a more
popular character are springing up on all sides.
Modern facilities of communication have helped to impress more
deeply upon modern astronomy its associative character. The
electric telegraph gives a certain ubiquity which is invaluable to an
observer of the skies. With the help of a wire, a battery, and a
code of signals, he sees whatever is visible from any portion of our
globe, depending, however, upon other eyes than his own, and so
entering as a unit into a widespread organisation of intelligence.
The press, again, has been a potent agent of co-operation. It has
mainly contributed to unite astronomers all over the world into a
body animated by the single aim of collecting “particulars” in their
special branch for what Bacon termed a History of Nature, eventually
to be interpreted according to the sagacious insight of some one
among them gifted above his fellows. The first really effective
astronomical periodical was the Monatliche Correspondenz, started by
Von Zach in the year 1800. It was followed in 1822 by the
Astronomische Nachrichten, later by the Memoirs and Monthly Notices
of the Astronomical Society, and by the host of varied publications
which now, in every civilised country, communicate the discoveries
made in astronomy to divers classes of readers, and so incalculably
quicken the current of its onward flow.
Public favour brings in its train material resources. It is represented
by individual enterprise, and finds expression in an ample
liberality. The first regular observatory in the Southern Hemisphere
was founded at Paramatta by Sir Thomas Makdougall
Brisbane in 1821. The Royal Observatory at the Cape of Good
Hope was completed in 1829. Similar establishments were set
to work by the East India Company at Madras, Bombay, and
St. Helena, during the first third of the nineteenth century. The
organisation of astronomy in the United States of America was due
to a strong wave of popular enthusiasm. In 1825 John Quincy
Adams vainly urged upon Congress the foundation of a National
Observatory; but in 1843 the lectures on celestial phenomena
of Ormsby MacKnight Mitchel stirred an impressionable audience
to the pitch of providing him with the means of erecting at Cincinnati
the first astronomical establishment worthy the name in that[Pg 7]
great country. On the 1st of January, 1882, no less than one
hundred and forty-four were active within its boundaries.
The apparition of the great comet of 1843 gave an additional
fillip to the movement. To the excitement caused by it the Harvard
College Observatory—called the “American Pulkowa”—directly
owed its origin; and the example was not ineffective elsewhere.
The United States Naval Observatory was built in 1844, Lieutenant
Maury being its first Director. Corporations, universities, municipalities,
vied with each other in the creation of such institutions;
private subscriptions poured in; emissaries were sent to Europe to
purchase instruments and to procure instruction in their use. In
a few years the young Republic was, in point of astronomical
efficiency, at least on a level with countries where the science had
been fostered since the dawn of civilisation.
A vast widening of the scope of astronomy has accompanied, and
in part occasioned, the great extension of its area of cultivation
which our age has witnessed. In the last century its purview was
a comparatively narrow one. Problems lying beyond the range
of the solar system were almost unheeded, because they seemed
inscrutable. Herschel first showed the sidereal universe as accessible
to investigation, and thereby offered to science new worlds—majestic,
manifold, “infinitely infinite” to our apprehension in
number, variety, and extent—for future conquest. Their gradual
appropriation has absorbed, and will long continue to absorb, the
powers which it has served to develop.
But this is not the only direction in which astronomy has enlarged,
or rather has levelled, its boundaries. The unification of the physical
sciences is perhaps the greatest intellectual feat of recent times.
The process has included astronomy; so that, like Bacon, she may
now be said to have “taken all knowledge” (of that kind) “for
her province.” In return, she proffers potent aid for its increase.
Every comet that approaches the sun is the scene of experiments in
the electrical illumination of rarefied matter, performed on a huge
scale for our benefit. The sun, stars, and nebulæ form so many
celestial laboratories, where the nature and mutual relations of the
chemical “elements” may be tried by more stringent tests than
sublunary conditions afford. The laws of terrestrial magnetism can
be completely investigated only with the aid of a concurrent study
of the face of the sun. The solar spectrum will perhaps one day,
by its recurrent modifications, tell us something of impending
droughts, famines, and cyclones.
Astronomy generalises the results of the other sciences. She
exhibits the laws of Nature working over a wider area, and under
more varied conditions, than ordinary experience presents. Ordinary[Pg 8]
experience, on the other hand, has become indispensable to her
progress. She takes in at one view the indefinitely great and the
indefinitely little. The mutual revolutions of the stellar multitude
during tracts of time which seem to lengthen out to eternity as the
mind attempts to traverse them, she does not admit to be beyond
her ken; nor is she indifferent to the constitution of the minutest
atom of matter that thrills the ether into light. How she entered
upon this vastly expanded inheritance, and how, so far, she has
dealt with it, is attempted to be set forth in the ensuing chapters.[Pg 9]
FOOTNOTES:
[1] The denomination “physical astronomy,” first used by Kepler, and long
appropriated to this branch of the science, has of late been otherwise applied.
[2] Histoire de l’Astronomie au xviiie Siècle, p. 267.
PART I
PROGRESS OF ASTRONOMY DURING THE FIRST HALF OF
THE NINETEENTH CENTURY
CHAPTER I
FOUNDATION OF SIDEREAL ASTRONOMY
Until nearly a hundred years ago the stars were regarded by
practical astronomers mainly as a number of convenient fixed points
by which the motions of the various members of the solar system
could be determined and compared. Their recognised function, in
fact, was that of milestones on the great celestial highway traversed
by the planets, as well as on the byways of space occasionally
pursued by comets. Not that curiosity as to their nature, and even
conjecture as to their origin, were at any period absent. Both were
from time to time powerfully stimulated by the appearance of
startling novelties in a region described by philosophers as “incorruptible,”
or exempt from change. The catalogue of Hipparchus
probably, and certainly that of Tycho Brahe, some seventeen centuries
later, owed each its origin to the temporary blaze of a new
star. The general aspect of the skies was thus (however imperfectly)
recorded from age to age, and with improved appliances the
enumeration was rendered more and more accurate and complete;
but the secrets of the stellar sphere remained inviolate.
In a qualified though very real sense, Sir William Herschel may
be called the Founder of Sidereal Astronomy. Before his time
some curious facts had been noted, and some ingenious speculations
hazarded, regarding the condition of the stars, but not even the
rudiments of systematic knowledge had been acquired. The facts
ascertained can be summed up in a very few sentences.
Giordano Bruno was the first to set the suns of space in motion;
but in imagination only. His daring surmise was, however, confirmed
in 1718, when Halley announced[3] that Sirius, Aldebaran,[Pg 10]
Betelgeux, and Arcturus had unmistakably shifted their quarters in
the sky since Ptolemy assigned their places in his catalogue. A
similar conclusion was reached by J. Cassini in 1738, from a
comparison of his own observations with those made at Cayenne
by Richer in 1672; and Tobias Mayer drew up in 1756 a list
showing the direction and amount of about fifty-seven proper
motions,[4] founded on star-places determined by Olaus Römer fifty
years previously. Thus the stars were no longer regarded as
“fixed,” but the question remained whether the movements
perceived were real or only apparent; and this it was not yet
found possible to answer. Already, in the previous century, the
ingenious Robert Hooke had suggested an “alteration of the very
system of the sun,”[5] to account for certain suspected changes in
stellar positions; Bradley in 1748, and Lambert in 1761, pointed
out that such apparent displacements (by that time well ascertained)
were in all probability a combined effect of motions both of sun
and stars; and Mayer actually attempted the analysis, but without
result.
On the 13th of August, 1596, David Fabricius, an unprofessional
astronomer in East Friesland, saw in the neck of the Whale a star
of the third magnitude, which by October had disappeared. It
was, nevertheless, visible in 1603, when Bayer marked it in his
catalogue with the Greek letter ο, and was watched, in 1638-39,
through its phases of brightening and apparent extinction by a
Dutch professor named Holwarda.[6] From Hevelius this first-known
periodical star received the name of “Mira,” or the Wonderful,
and Boulliaud in 1667 fixed the length of its cycle of change at
334 days. It was not a solitary instance. A star in the Swan
was perceived by Janson in 1600 to show fluctuations of light,
and Montanari found in 1669 that Algol in Perseus shared the
same peculiarity to a marked degree. Altogether the class
embraced in 1782 half-a-dozen members. When it is added that
a few star-couples had been noted in singularly, but it was
supposed accidentally, close juxtaposition, and that the failure
of repeated attempts to measure stellar parallaxes pointed to
distances at least 400,000 times that of the earth from the sun,[7] the[Pg 11]
picture of sidereal science, when the last quarter of the eighteenth
century began, is practically complete. It included three items of
information: that the stars have motions, real or apparent; that
they are immeasurably remote; and that a few shine with a
periodically variable light. Nor were these scantily collected
facts ordered into any promise of further development. They
lay at once isolated and confused before the inquirer. They
needed to be both multiplied and marshalled, and it seemed as if
centuries of patient toil must elapse before any reliable conclusions
could be derived from them. The sidereal world was thus
the recognised domain of far-reaching speculations, which remained
wholly uncramped by systematic research until Herschel
entered upon his career as an observer of the heavens.
The greatest of modern astronomers was born at Hanover,
November 15, 1738. He was the fourth child of Isaac Herschel,
a hautboy-player in the band of the Hanoverian Guard, and was
early trained to follow his father’s profession. On the termination,
however, of the disastrous campaign of 1757, his parents removed
him from the regiment, there is reason to believe, in a somewhat
unceremonious manner. Technically, indeed, he incurred the
penalties of desertion, remitted—according to the Duke of Sussex’s
statement to Sir George Airy—by a formal pardon handed to him
personally by George III. on his presentation in 1782.[8] At the
age of nineteen, then, his military service having lasted four years,
he came to England to seek his fortune. Of the life of struggle
and privation which ensued little is known beyond the circumstances
that in 1760 he was engaged in training the regimental
band of the Durham Militia, and that in 1765 he was appointed
organist at Halifax. In the following year he removed to Bath as
oboist in Linley’s orchestra, and in October 1767 was promoted
to the post of organist in the Octagon Chapel. The tide of
prosperity now began to flow for him. The most brilliant and
modish society in England was at that time to be met at Bath,
and the young Hanoverian quickly found himself a favourite and
the fashion in it. Engagements multiplied upon him. He became
director of the public concerts; he conducted oratorios, engaged
singers, organised rehearsals, composed anthems, chants, choral
services, besides undertaking private tuitions, at times amounting
to thirty-five or even thirty-eight lessons a week. He in fact
personified the musical activity of a place then eminently and
energetically musical.
But these multifarious avocations did not take up the whole of
his thoughts. His education, notwithstanding the poverty of his[Pg 12]
family, had not been neglected, and he had always greedily assimilated
every kind of knowledge that came in his way. Now that he
was a busy and a prosperous man, it might have been expected that
he would run on in the deep professional groove laid down for him.
On the contrary, his passion for learning seemed to increase with the
diminution of the time available for its gratification. He studied
Italian, Greek, mathematics; Maclaurin’s Fluxions served to “unbend
his mind”; Smith’s Harmonics and Optics and Ferguson’s
Astronomy were the nightly companions of his pillow. What he
read stimulated without satisfying his intellect. He desired not
only to know, but to discover. In 1772 he hired a small telescope,
and through it caught a preliminary glimpse of the rich and varied
fields in which for so many years he was to expatiate. Henceforward
the purpose of his life was fixed: it was to obtain “a
knowledge of the construction of the heavens”;[9] and this sublime
ambition he cherished to the end.
A more powerful instrument was the first desideratum; and here
his mechanical genius came to his aid. Having purchased the
apparatus of a Quaker optician, he set about the manufacture of
specula with a zeal which seemed to anticipate the wonders they
were to disclose to him. It was not until fifteen years later that his
grinding and polishing machines were invented, so the work had at
that time to be entirely done by hand. During this tedious and
laborious process (which could not be interrupted without injury,
and lasted on one occasion sixteen hours), his strength was supported
by morsels of food put into his mouth by his sister,[10] and his
mind amused by her reading aloud to him the Arabian Nights, Don
Quixote, or other light works. At length, after repeated failures, he
found himself provided with a reflecting telescope—a 5-1/2-foot Gregorian—of
his own construction. A copy of his first observation
with it, on the great Nebula in Orion—an object of continual amazement
and assiduous inquiry to him—is preserved by the Royal
Society. It bears the date March 4, 1774.[11]
In the following year he executed his first “review of the
heavens,” memorable chiefly as an evidence of the grand and novel
conceptions which already inspired him, and of the enthusiasm with
which he delivered himself up to their guidance. Overwhelmed
with professional engagements, he still contrived to snatch some[Pg 13]
moments for the stars; and between the acts at the theatre was
often seen running from the harpsichord to his telescope, no doubt
with that “uncommon precipitancy which accompanied all his
actions.”[12] He now rapidly increased the power and perfection of
his telescopes. Mirrors of seven, ten, even twenty feet focal length,
were successively completed, and unprecedented magnifying powers
employed. His energy was unceasing, his perseverance indomitable.
In the course of twenty-one years no less than 430 parabolic specula
left his hands. He had entered upon his forty-second year when he
sent his first paper to the Philosophical Transactions; yet during the
ensuing thirty-nine years his contributions—many of them elaborate
treatises—numbered sixty-nine, forming a series of extraordinary
importance to the history of astronomy. As a mere explorer of the
heavens his labours were prodigious. He discovered 2,500 nebulæ,
806 double stars, passed the whole firmament in review four several
times, counted the stars in 3,400 “gauge-fields,” and executed a
photometric classification of the principal stars, founded on an
elaborate (and the first systematically conducted) investigation of
their relative brightness. He was as careful and patient as he was
rapid; spared no time and omitted no precaution to secure accuracy
in his observations; yet in one night he would examine, singly and
attentively, up to 400 separate objects.
The discovery of Uranus was a mere incident of the scheme he
had marked out for himself—a fruit, gathered as it were by the
way. It formed, nevertheless, the turning-point in his career. From
a star-gazing musician he was at once transformed into an eminent
astronomer; he was relieved from the drudgery of a toilsome profession,
and installed as Royal Astronomer, with a modest salary of
£200 a year; funds were provided for the construction of the forty-foot
reflector, from the great space-penetrating power of which he
expected unheard-of revelations; in fine, his future work was not
only rendered possible, but it was stamped as authoritative.[13] On
Whit-Sunday 1782, William and Caroline Herschel played and sang
in public for the last time in St. Margaret’s Chapel, Bath; in August
of the same year the household was moved to Datchet, near Windsor,
and on April 3, 1786, to Slough. Here happiness and honours
crowded on the fortunate discoverer. In 1788 he married Mary,
only child of James Baldwin, a merchant of the city of London,
and widow of Mr. John Pitt—a lady whose domestic virtues were
enhanced by the possession of a large jointure. The fruit of their
union was one son, of whose work—the worthy sequel of his father’s—we
shall have to speak further on. Herschel was created a Knight[Pg 14]
of the Hanoverian Guelphic Order in 1816, and in 1821 he became
the first President of the Royal Astronomical Society, his son being
its first Foreign Secretary. But his health had now for some years
been failing, and on August 25, 1822, he died at Slough, in the
eighty-fourth year of his age, and was buried in Upton churchyard.
His epitaph claims for him the lofty praise of having “burst the
barriers of heaven.” Let us see in what sense this is true.
The first to form any definite idea as to the constitution of the
stellar system was Thomas Wright, the son of a carpenter living at
Byer’s Green, near Durham. With him originated what has been
called the “Grindstone Theory” of the universe, which regarded the
Milky Way as the projection on the sphere of a stratum or disc of
stars (our sun occupying a position near the centre), similar in magnitude
and distribution to the lucid orbs of the constellations.[14] He
was followed by Kant,[15] who transcended the views of his predecessor
by assigning to nebulæ the position they long continued to
occupy, rather on imaginative than scientific grounds, of “island
universes,” external to, and co-equal with, the Galaxy. Johann
Heinrich Lambert,[16] a tailor’s apprentice from Mühlhausen, followed,
but independently. The conceptions of this remarkable man were
grandiose, his intuitions bold, his views on some points a singular
anticipation of subsequent discoveries. The sidereal world presented
itself to him as a hierarchy of systems, starting from the planetary
scheme, rising to throngs of suns within the circuit of the Milky
Way—the “ecliptic of the stars,” as he phrased it—expanding to
include groups of many Milky Ways; these again combining to
form the unit of a higher order of assemblage, and so onwards and
upwards until the mind reels and sinks before the immensity of the
contemplated creations.
“Thus everything revolves—the earth round the sun; the sun
round the centre of his system; this system round a centre common
to it with other systems; this group, this assemblage of systems,
round a centre which is common to it with other groups of the same
kind; and where shall we have done?”[17]
The stupendous problem thus speculatively attempted, Herschel[Pg 15]
undertook to grapple with experimentally. The upshot of this
memorable inquiry was the inclusion, for the first time, within the
sphere of human knowledge, of a connected body of facts, and
inferences from facts, regarding the sidereal universe; in other
words, the foundation of what may properly be called a science of
the stars.
Tobias Mayer had illustrated the perspective effects which must
ensue in the stellar sphere from a translation of the solar system,
by comparing them to the separating in front and closing up behind
of trees in a forest to the eye of an advancing spectator;[18] but the
appearances which he thus correctly described he was unable to
detect. By a more searching analysis of a smaller collection of
proper motions, Herschel succeeded in rendering apparent the very
consequences foreseen by Mayer. He showed, for example, that
Arcturus and Vega did, in fact, appear to recede from, and Sirius
and Aldebaran to approach, each other by very minute amounts;
and, with a striking effort of divinatory genius, placed the “apex,”
or point of direction of the sun’s motion, close to the star λ in the
constellation Hercules,[19] within a few degrees of the spot indicated
by later and indefinitely more refined methods of research. He
resumed the subject in 1805,[20] but though employing a more
rigorous method, was scarcely so happy in his result. In 1806,[21] he
made a preliminary attempt to ascertain the speed of the sun’s
journey, fixing it, by doubtless much too low an estimate, at about
three miles a second. Yet the validity of his general conclusion as
to the line of solar travel, though long doubted, has been triumphantly
confirmed. The question as to the “secular parallax” of the fixed
stars was in effect answered.
With their annual parallax, however, the case was very different.
The search for it had already led Bradley to the important discoveries
of the aberration of light and the nutation of the earth’s
axis; it was now about to lead Herschel to a discovery of a different,
but even more elevated character. Yet in neither case was the object
primarily sought attained.
From the very first promulgation of the Copernician theory the
seeming immobility of the stars had been urged as an argument
against its truth; for if the earth really travelled in a vast orbit[Pg 16]
round the sun, objects in surrounding space should appear to change
their positions, unless their distances were on a scale which, to the
narrow ideas of the universe then prevailing, seemed altogether
extravagant.[22] The existence of such apparent or “parallactic” displacements
was accordingly regarded as the touchstone of the new
views, and their detection became an object of earnest desire to
those interested in maintaining them. Copernicus himself made the
attempt; but with his “Triquetrum,” a jointed wooden rule with
the divisions marked in ink, constructed by himself,[23] he was hardly
able to measure angles of ten minutes, far less fractions of a second.
Galileo, a more impassioned defender of the system, strained his
ears, as it were, from Arcetri, in his blind and sorrowful old age, for
news of a discovery which two more centuries had still to wait for.
Hooke believed he had found a parallax for the bright star in the
Head of the Dragon; but was deceived. Bradley convinced himself
that such effects were too minute for his instruments to measure.
Herschel made a fresh attempt by a practically untried method.
It is a matter of daily experience that two objects situated at
different distances seem to a beholder in motion to move relatively
to each other. This principle Galileo, in the third of his Dialogues
on the Systems of the World,[24] proposed to employ for the determination
of stellar parallax; for two stars, lying apparently close
together, but in reality separated by a great gulf of space, must shift
their mutual positions when observed from opposite points of the
earth’s orbit; or rather, the remoter forms a virtually fixed point, to
which the movements of the other can be conveniently referred. By
this means complications were abolished more numerous and perplexing
than Galileo himself was aware of, and the problem was
reduced to one of simple micrometrical measurement. The “double-star
method” was also suggested by James Gregory in 1675, and
again by Wallis in 1693;[25] Huygens first, and afterwards Dr. Long
of Cambridge (about 1750), made futile experiments with it; and it
eventually led, in the hands of Bessel, to the successful determination
of the parallax of 61 Cygni.
Its advantages were not lost upon Herschel. His attempt to
assign definite distances to the nearest stars was no isolated effort,
but part of the settled plan upon which his observations were conducted.
He proposed to sound the heavens, and the first requisite
was a knowledge of the length of his sounding-line. Thus it came
about that his special attention was early directed to double stars.
“I resolved,” he writes,[26] “to examine every star in the heavens[Pg 17]
with the utmost attention and a very high power, that I might
collect such materials for this research as would enable me to fix
my observations upon those that would best answer my end. The
subject has already proved so extensive, and still promises so rich a
harvest to those who are inclined to be diligent in the pursuit, that
I cannot help inviting every lover of astronomy to join with me in
observations that must inevitably lead to new discoveries.”
The first result of these inquiries was a classed catalogue of
269 double stars presented to the Royal Society in 1782, followed,
after three years, by an additional list of 434. In both these
collections the distances separating the individuals of each pair were
carefully measured, and (with a few exceptions) the angles made
with the hour-circle by the lines joining their centres (technically
called “angles of position”) were determined with the aid of a
“revolving-wire micrometer,” specially devised for the purpose.
Moreover, an important novelty was introduced by the observation
of the various colours visible in the star-couples, the singular and
vivid contrasts of which were now for the first time described.
Double stars were at that time supposed to be a purely optical
phenomenon. Their components, it was thought, while in reality
indefinitely remote from each other, were brought into fortuitous
contiguity by the chance of lying nearly in the same line of sight
from the earth. Yet Bradley had noticed a change of 30°, between
1718 and 1759, in the position-angle of the two stars forming
Castor, and was thus within a hair’s breadth of the discovery of
their physical connection.[27] While the Rev. John Michell, arguing
by the doctrine of probabilities, wrote as follows in 1767:—”It is
highly probable in particular, and next to a certainty in general,
that such double stars as appear to consist of two or more stars
placed very near together, do really consist of stars placed near
together, and under the influence of some general law.”[28] And in
1784:[29] “It is not improbable that a few years may inform us that
some of the great number of double, triple stars, etc., which have
been observed by Mr. Herschel, are systems of bodies revolving
about each other.”
This remarkable speculative anticipation had a practical counterpart
in Germany. Father Christian Mayer, a Jesuit astronomer at
Mannheim, set himself, in January 1776, to collect examples of
stellar pairs, and shortly after published the supposed discovery of
“satellites” to many of the principal stars.[30] But his observations[Pg 18]
were neither exact nor prolonged enough to lead to useful results in
such an inquiry. His disclosures were derided; his planet-stars
treated as results of hallucination. On n’a point cru à des choses aussi
extraordinaires, wrote Lalande[31] within one year of a better-grounded
announcement to the same effect.
Herschel at first shared the general opinion as to the merely
optical connection of double stars. Of this the purpose for which
he made his collection is in itself sufficient evidence, since what
may be called the differential method of parallaxes depends, as we
have seen, for its efficacy upon disparity of distance. It was
“much too soon,” he declared in 1782,[32] “to form any theories of
small stars revolving round large ones;” while in the year following,[33]
he remarked that the identical proper motions of the two
stars forming, to the naked eye, the single bright orb of Castor
could only be explained as both equally due to the “systematic
parallax” caused by the sun’s movement in space. Plainly showing
that the notion of a physical tie, compelling the two bodies to
travel together, had not as yet entered into his speculations. But
he was eminently open to conviction, and had, moreover, by
observations unparalleled in amount as well as in kind, prepared
ample materials for convincing himself and others. In 1802 he
was able to announce the fact of his discovery, and in the two
ensuing years, to lay in detail before the Royal Society proofs,
gathered from the labours of a quarter of a century, of orbital
revolution in the case of as many as fifty double stars, henceforth,
he declared, to be held as real binary combinations, “intimately
held together by the bond of mutual attraction.”[34] The fortunate
preservation in Dr. Maskelyne’s note-book of a remark made by
Bradley about 1759, to the effect that the line joining the components
of Castor was an exact prolongation of that joining Castor
with Pollux, added eighteen years to the time during which the
pair were under scrutiny, and confirmed the evidence of change
afforded by more recent observations. Approximate periods were
fixed for many of the revolving suns—for Castor 342 years; for γ
Leonis, 1200, δ Serpentis, 375, ε Bootis, 1681 years; ε Lyræ was
noted as a “double-double-star,” a change of relative situation
having been detected in each of the two pairs composing the
group; and the occultation was described of one star by another in
the course of their mutual revolutions, as exemplified in 1795 by the
rapidly circulating system of ζ Herculis.
Thus, by the sagacity and perseverance of a single observer, a
firm basis was at last provided upon which to raise the edifice of[Pg 19]
sidereal science. The analogy long presumed to exist between
the mighty star of our system and the bright points of light
spangling the firmament was shown to be no fiction of the
imagination, but a physical reality; the fundamental quality of
attractive power was proved to be common to matter so far as
the telescope was capable of exploring, and law, subordination,
and regularity to give testimony of supreme and intelligent
design no less in those limitless regions of space than in our
narrow terrestrial home. The discovery was emphatically (in
Arago’s phrase) “one with a future,” since it introduced the
element of precise knowledge where more or less probable conjecture
had previously held almost undivided sway; and precise
knowledge tends to propagate itself and advance from point to point.
We have now to speak of Herschel’s pioneering work in the
skies. To explore with line and plummet the shining zone of the
Milky Way, to delineate its form, measure its dimensions, and
search out the intricacies of its construction, was the primary
task of his life, which he never lost sight of, and to which all
his other investigations were subordinate. He was absolutely
alone in this bold endeavour. Unaided, he had to devise methods,
accumulate materials, and sift out results. Yet it may safely be
asserted that all the knowledge we possess on this sublime subject
was prepared, and the greater part of it anticipated, by him.
The ingenious method of “star-gauging,” and its issue in the
delineation of the sidereal system as an irregular stratum of
evenly-scattered suns, is the best-known part of his work. But
it was, in truth, only a first rude approximation, the principle of
which maintained its credit in the literature of astronomy a full
half-century after its abandonment by its author. This principle
was the general equality of star distribution. If equal portions
of space really held equal numbers of stars, it is obvious that the
number of stars visible in any particular direction would be strictly
proportional to the range of the system in that direction, apparent
accumulation being produced by real extent. The process of
“gauging the heavens,” accordingly, consisted in counting the
stars in successive telescopic fields, and calculating thence the
depths of space necessary to contain them. The result of 3,400
such operations was the plan of the Galaxy familiar to every
reader of an astronomical text-book. Widely-varying evidence
was, as might have been expected, derived from an examination
of different portions of the sky. Some fields of view were almost
blank, while others (in or near the Milky Way) blazed with the
radiance of many hundred stars compressed into an area about
one-fourth that of the full-moon. In the most crowded parts[Pg 20]
116,000 were stated to have been passed in review within a
quarter of an hour. Here the “length of his sounding-line” was
estimated by Herschel at about 497 times the distance of Sirius—in
other words, the bounding orb, or farthest sun of the system in
that direction, so far as could be seen with the 20-foot reflector,
was thus inconceivably remote. But since the distance of Sirius,
no less than of every other fixed star, was as yet an unknown
quantity, the dimensions inferred for the Galaxy were of course
purely relative; a knowledge of its form and structure might
(admitting the truth of the fundamental hypothesis) be obtained,
but its real or absolute size remained altogether undetermined.
Even as early as 1785, however, Herschel perceived traces of a
tendency which completely invalidated the supposition of any
approach to an average uniformity of distribution. This was the
action of what he called a “clustering power” in the Milky Way.
“Many gathering clusters”[35] were already discernible to him even
while he endeavoured to obtain a “true mean result” on the
assumption that each star in space was separated from its
neighbours as widely as the sun from Sirius. “It appears,” he
wrote in 1789, “that the heavens consist of regions where suns
are gathered into separate systems”; and in certain assemblages
he was able to trace “a course or tide of stars setting towards a
centre,” denoting, not doubtfully, the presence of attractive forces.[36]
Thirteen years later, he described our sun and his constellated
companions as surrounded by “a magnificent collection of
innumerable stars, called the Milky Way, which must occasion
a very powerful balance of opposite attractions to hold the
intermediate stars at rest. For though our sun, and all the stars
we see, may truly be said to be in the plane of the Milky Way,
yet I am now convinced, by a long inspection and continued
examination of it, that the Milky Way itself consists of stars very
differently scattered from those which are immediately about us.”
“This immense aggregation,” he added, “is by no means uniform.
Its component stars show evident signs of clustering together into
many separate allotments.”[37]
The following sentences, written in 1811, contain a definite
retractation of the view frequently attributed to him:—
“I must freely confess,” he says, “that by continuing my
sweeps of the heavens my opinion of the arrangement of the stars
and their magnitudes, and of some other particulars, has undergone
a gradual change; and indeed, when the novelty of the subject is
considered, we cannot be surprised that many things formerly taken[Pg 21]
for granted should on examination prove to be different from what
they were generally but incautiously supposed to be. For instance,
an equal scattering of the stars may be admitted in certain calculations;
but when we examine the Milky Way, or the closely compressed
clusters of stars of which my catalogues have recorded so
many instances, this supposed equality of scattering must be given
up.”[38]
Another assumption, the fallacy of which he had not the means
of detecting since become available, was retained by him to the
end of his life. It was that the brightness of a star afforded an
approximate measure of its distance. Upon this principle he
founded in 1817 his method of “limiting apertures,”[39] by which
two stars, brought into view in two precisely similar telescopes,
were “equalised” by covering a certain portion of the object-glass
collecting the more brilliant rays. The distances of the orbs
compared were then taken to be in the ratio of the reduced to the
original apertures of the instruments with which they were
examined. If indeed the absolute lustre of each were the same,
the result might be accepted with confidence; but since we have
no warrant for assuming a “standard star” to facilitate our
computations, but much reason to suppose an indefinite range,
not only of size but of intrinsic brilliancy, in the suns of our
firmament, conclusions drawn from such a comparison are entirely
worthless.
In another branch of sidereal science besides that of stellar
aggregation, Herschel may justly be styled a pioneer. He was
the first to bestow serious study on the enigmatical objects known
as “nebulæ.” The history of the acquaintance of our race with
them is comparatively short. The only one recognised before the
invention of the telescope was that in the girdle of Andromeda,
certainly familiar in the middle of the tenth century to the Persian
astronomer Abdurrahman Al-Sûfi; and marked with dots on
Spanish and Dutch constellation-charts of the fourteenth and
fifteenth centuries.[40] Yet so little was it noticed that it might
practically be said—as far as Europe is concerned—to have been
discovered in 1612 by Simon Marius (Mayer of Genzenhausen),
who aptly described its appearance as that of a “candle shining
through horn.” The first mention of the great Orion nebula is
by a Swiss Jesuit named Cysatus, who succeeded Father Scheiner[Pg 22]
in the chair of mathematics at Ingolstadt. He used it, apparently
without any suspicion of its novelty, as a term of comparison for
the comet of December 1618.[41] A novelty, nevertheless, to
astronomers it still remained in 1656, when Huygens discerned,
“as it were, an hiatus in the sky, affording a glimpse of a more
luminous region beyond.”[42] Halley in 1716 knew of six nebulæ,
which he believed to be composed of a “lucid medium” diffused
through the ether of space.[43] He appears, however, to have been
unacquainted with some previously noticed by Hevelius. Lacaille
brought back with him from the Cape a list of forty-two—the
first-fruits of observation in Southern skies—arranged in three
numerically equal classes;[44] and Messier (nicknamed by Louis XV.
the “ferret of comets”), finding such objects a source of extreme
perplexity in the pursuit of his chosen game, attempted to eliminate
by methodising them, and drew up a catalogue comprising, in 1781,
103 entries.[45]
These preliminary attempts shrank into insignificance when
Herschel began to “sweep the heavens” with his giant telescopes.
In 1786 he presented to the Royal Society a descriptive catalogue
of 1,000 nebulæ and clusters, followed, three years later, by a
second of as many more; to which he added in 1802 a further
gleaning of 500. On the subject of their nature his views underwent
a remarkable change. Finding that his potent instruments
resolved into stars many nebulous patches in which no signs of
such a structure had previously been discernible, he naturally
concluded that “resolvability” was merely a question of distance
and telescopic power. He was (as he said himself) led on by
almost imperceptible degrees from evident clusters, such as the
Pleiades, to spots without a trace of stellar formation, the gradations
being so well connected as to leave no doubt that all these
phenomena were equally stellar. The singular variety of their
appearance was thus described by him:—
“I have seen,” he says, “double and treble nebulæ variously
arranged; large ones with small, seeming attendants; narrow,
but much extended lucid nebulæ or bright dashes; some of the
shape of a fan, resembling an electric brush, issuing from a lucid
point; others of the cometic shape, with a seeming nucleus in the
centre, or like cloudy stars surrounded with a nebulous atmosphere;
a different sort, again, contain a nebulosity of the milky kind, like
that wonderful, inexplicable phenomenon about θ Orionis; while[Pg 23]
others shine with a fainter, mottled kind of light, which denotes
their being resolvable into stars.”[46]
“These curious objects” he considered to be “no less than whole
sidereal systems,”[47] some of which might “well outvie our Milky
Way in grandeur.” He admitted, however, a wide diversity in
condition as well as compass. The system to which our sun belongs
he described as “a very extensive branching congeries of many
millions of stars, which probably owes its origin to many remarkably
large as well as pretty closely scattered small stars, that may have
drawn together the rest.”[48] But the continued action of this same
“clustering power” would, he supposed, eventually lead to the
breaking-up of the original majestic Galaxy into two or three
hundred separate groups, already visibly gathering. Such minor
nebulæ, due to the “decay” of other “branching nebulæ” similar
to our own, he recognised by the score, lying, as it were, stratified
in certain quarters of the sky. “One of these nebulous beds,” he
informs us, “is so rich that in passing through a section of it, in
the time of only thirty-six minutes, I detected no less than thirty-one
nebulæ, all distinctly visible upon a fine blue sky.” The
stratum of Coma Berenices he judged to be the nearest to our
system of such layers; nor did the marked aggregation of nebulæ
towards both poles of the circle of the Milky Way escape his
notice.
By a continuation of the same process of reasoning, he was
enabled (as he thought) to trace the life-history of nebulæ from a
primitive loose and extended formation, through clusters of gradually
increasing compression, down to the kind named by him “Planetary”
because of the defined and uniform discs which they present. These
he regarded as “very aged, and drawing on towards a period of
change or dissolution.”[49]
“This method of viewing the heavens,” he concluded, “seems to
throw them into a new kind of light. They now are seen to
resemble a luxuriant garden which contains the greatest variety
of productions in different flourishing beds; and one advantage we
may at least reap from it is, that we can, as it were, extend the
range of our experience to an immense duration. For, to continue
the simile which I have borrowed from the vegetable kingdom, is
it not almost the same thing whether we live successively to
witness the germination, blooming, foliage, fecundity, fading,
withering, and corruption of a plant, or whether a vast number
of specimens, selected from every stage through which the plant[Pg 24]
passes in the course of its existence, be brought at once to our
view?”[50]
But already this supposed continuity was broken. After mature
deliberation on the phenomena presented by nebulous stars, Herschel
was induced, in 1791, to modify essentially his original opinion.
“When I pursued these researches,” he says, “I was in the
situation of a natural philosopher who follows the various species
of animals and insects from the height of their perfection down to
the lowest ebb of life; when, arriving at the vegetable kingdom, he
can scarcely point out to us the precise boundary where the animal
ceases and the plant begins; and may even go so far as to suspect
them not to be essentially different. But, recollecting himself, he
compares, for instance, one of the human species to a tree, and all
doubt upon the subject vanishes before him. In the same manner
we pass through gentle steps from a coarse cluster of stars, such as
the Pleiades … till we find ourselves brought to an object such
as the nebula in Orion, where we are still inclined to remain in the
once adopted idea of stars exceedingly remote and inconceivably
crowded, as being the occasion of that remarkable appearance. It
seems, therefore, to require a more dissimilar object to set us right
again. A glance like that of the naturalist, who casts his eye from
the perfect animal to the perfect vegetable, is wanting to remove
the veil from the mind of the astronomer. The object I have
mentioned above is the phenomenon that was wanting for this
purpose. View, for instance, the 19th cluster of my 6th class, and
afterwards cast your eye on this cloudy star, and the result will be
no less decisive than that of the naturalist we have alluded to. Our
judgment, I may venture to say, will be, that the nebulosity about the
star is not of a starry nature.”[51]
The conviction thus arrived at of the existence in space of a
widely diffused “shining fluid” (a conviction long afterwards fully
justified by the spectroscope) led him into a field of endless speculation.
What was its nature? Should it “be compared to the
coruscation of the electric fluid in the aurora borealis? or to the
more magnificent cone of the zodiacal light?” Above all, what was
its function in the cosmos? And on this point he already gave a
hint of the direction in which his mind was moving by the remark
that this self-luminous matter seemed “more fit to produce
a star by its condensation, than to depend on the star for its
existence.”[52]
This was not a novel idea. Tycho Brahe had tried to explain
the blaze of the star of 1572 as due to a sudden concentration of[Pg 25]
nebulous material in the Milky Way, even pointing out the space
left dark and void by the withdrawal of the luminous stuff; and
Kepler, theorising on a similar stellar apparition in 1604, followed
nearly in the same track. But under Herschel’s treatment the
nebular origin of stars first acquired the consistency of a formal
theory. He meditated upon it long and earnestly, and in two
elaborate treatises, published respectively in 1811 and 1814, he at
length set forth the arguments in its favour. These rested entirely
upon the “principle of continuity.” Between the successive classes
of his assortment of developing objects there was, as he said,
“perhaps not so much difference as would be in an annual description
of the human figure, were it given from the birth of a child
till he comes to be a man in his prime.”[53] From diffused nebulosity,
barely visible in the most powerful light-gathering instruments, but
which he estimated to cover nearly 152 square degrees of the
heavens,[54] to planetary nebulæ, supposed to be already centrally
solid, instances were alleged of every stage and phase of condensation.
The validity of his reasoning, however, was evidently impaired
by his confessed inability to distinguish between the dim rays of
remote clusters and the milky light of true gaseous nebulæ.
It may be said that such speculations are futile in themselves,
and necessarily barren of results. But they gratify an inherent
tendency of the human mind, and, if pursued in a becoming spirit,
should be neither reproved nor disdained. Herschel’s theory still
holds the field, the testimony of recent discoveries with regard to
it having proved strongly confirmatory of its principle, although not
of its details. Strangely enough, it seems to have been propounded
in complete independence of Laplace’s nebular hypothesis as to the
origin of the solar system. Indeed, it dated, as we have seen, in its
first inception, from 1791, while the French geometrician’s view was
not advanced until 1796.
We may now briefly sum up the chief results of Herschel’s long
years of “watching the heavens.” The apparent motions of the
stars had been disentangled; one portion being clearly shown to be
due to a translation towards a point in the constellation Hercules of
the sun and his attendant planets; while a large balance of displacement
was left to be accounted for by real movements, various in
extent and direction, of the stars themselves. By the action of a
central force similar to, if not identical with, gravity, suns of every
degree of size and splendour, and sometimes brilliantly contrasted
in colour, were seen to be held together in systems, consisting of
two, three, four, even six members, whose revolutions exhibited a
wide range of variety both in period and in orbital form. A new[Pg 26]
department of physical astronomy was thus created,[55] and rigid
calculation for the first time made possible within the astral region.
The vast problem of the arrangement and relations of the millions
of stars forming the Milky Way was shown to be capable of
experimental treatment, and of at least partial solution, notwithstanding
the variety and complexity seen to prevail, to an extent
previously undreamt of, in the arrangement of that majestic system.
The existence of a luminous fluid, diffused through enormous tracts
of space, and intimately associated with stellar bodies, was virtually
demonstrated, and its place and use in creation attempted to be
divined by a bold but plausible conjecture. Change on a stupendous
scale was inferred or observed to be everywhere in progress.
Periodical stars shone out and again decayed; progressive ebbings
or flowings of light were indicated as probable in many stars under
no formal suspicion of variability; forces were everywhere perceived
to be at work, by which the very structure of the heavens themselves
must be slowly but fundamentally modified. In all directions
groups were seen to be formed or forming; tides and streams of
suns to be setting towards powerful centres of attraction; new
systems to be in process of formation, while effete ones hastened to
decay or regeneration when the course appointed for them by
Infinite Wisdom was run. And thus, to quote the words of the
observer who “had looked farther into space than ever human being
did before him,”[56] the state into which the incessant action of the
clustering power has brought the Milky Way at present, is a kind
of chronometer that may be used to measure the time of its past and
future existence; and although we do not know the rate of going of
this mysterious chronometer, it is nevertheless certain that, since
the breaking-up of the parts of the Milky Way affords a proof that
it cannot last for ever, it equally bears witness that its past duration
cannot be admitted to be infinite.[57]
FOOTNOTES:
[3] Phil. Trans., vol. xxx., p. 737.
[4] Out of eighty stars compared, fifty-seven were found to have changed their
places by more than 10″. Lesser discrepancies were at that time regarded as
falling within the limits of observational error. Tobiæ Mayeri Op. Inedita,
t. i., pp. 80, 81, and Herschel in Phil. Trans., vol. lxxiii., pp. 275-278.
[5] Posthumous Works, p. 701.
[6] Arago in Annuaire du Bureau des Longitudes, 1842, p. 313.
[7] Bradley to Halley, Phil. Trans., vol. xxxv. (1728), p. 660. His observations
were directly applicable to only two stars, γ Draconis and η Ursæ Majoris,
but some lesser ones were included in the same result.
[8] Holden, Sir William Herschel, his Life and Works, p. 17.
[9] Phil. Trans., vol. ci., p. 269.
[10] Caroline Lucretia Herschel, born at Hanover, March 16, 1750, died in the
same place, January 9, 1848. She came to England in 1772, and was her
brother’s devoted assistant, first in his musical undertakings, and afterwards,
down to the end of his life, in his astronomical labours.
[11] Holden, op. cit., p. 39.
[12] Memoir of Caroline Herschel, p. 37.
[13] See Holden’s Sir William Herschel, p. 54.
[14] An Original Theory or New Hypothesis of the Universe, London, 1750. See
also De Morgan’s summary of his views in Philosophical Magazine, April, 1848.
[15] Allgemeine Naturgeschichte und Theorie des Himmels, 1755.
[16] Cosmologische Briefe, Augsburg, 1761.
[17] The System of the World, p. 125, London, 1800 (a translation of Cosmologische
Briefe). Lambert regarded nebulæ as composed of stars crowded together, but
not as external universes. In the case of the Orion nebula, indeed, he throws
out such a conjecture, but afterwards suggests that it may form a centre for that
one of the subordinate systems composing the Milky Way to which our sun
belongs.
[18] Opera Inedita, t. i., p. 79.
[19] Phil. Trans., vol. lxxiii. (1783), p. 273. Pierre Prévost’s similar investigation,
communicated to the Berlin Academy of Sciences four months later,
July 3, 1783, was inserted in the Memoirs of that body for 1781, and thus seems
to claim a priority not its due. Georg Simon Klügel at Halle gave about the
same time an analytical demonstration of Herschel’s result. Wolf, Gesch. der
Astronomie, p. 733.
[20] Phil. Trans., vol. xcv., p. 233.
[21] Ibid., vol. xcvi., p. 205.
[22] “Ingens bolus devorandus est,” Kepler admitted to Herwart in May, 1603.
[23] Described in “Præfatio Editoris” to De Revolutionibus, p. xix. (ed. 1854).
[24] Opere, t. i., p. 415.
[25] Phil. Trans., vol. xvii., p. 848.
[26] Ibid., vol. lxxii., p. 97.
[27] Doberck, Observatory, vol. ii., p. 110.
[28] Phil. Trans., vol. lvii., p. 249.
[29] Ibid., vol. lxxiv., p. 56.
[30] Beobachtungen von Fixsterntrabanten, 1778; and De Novis in Cœlo Sidereo
Phænomenis, 1779.
[31] Bibliographie, p. 569.
[32] Phil. Trans., vol. lxxii., p. 162.
[33] Ibid., vol. lxxiii., p. 272.
[34] Ibid., vol. xciii., p. 340.
[35] Phil. Trans., vol. lxxv., p. 255.
[36] Ibid., vol. lxxix., pp. 214, 222.
[37] Ibid., vol. xcii., pp. 479, 495.
[38] Phil. Trans., vol. ci., p. 269.
[39] Ibid., vol. cvii., p. 311.
[40] Bullialdus, De Nebulosâ Stellâ in Cingulo Andromedæ (1667); see also
G. P. Bond, Mém. Am. Ac., vol. iii., p. 75, Holden’s Monograph on the Orion
Nebula, Washington Observations, vol. xxv., 1878 (pub. 1882), and Lady
Huggins’s drawing, Atlas of Spectra, p. 119.
[41] Mathemata Astronomica, p. 75.
[42] Systema Saturnium, p. 9.
[43] Phil. Trans., vol. xxix., p. 390.
[44] Mém. Ac. des Sciences, 1755.
[45] Conn. des Temps, 1784 (pub. 1781), p. 227. A previous list of forty-five had
appeared in Mém. Ac. des Sciences, 1771.
[46] Phil. Trans., vol. lxxiv., p. 442.
[47] Ibid., vol. lxxix., p. 213.
[48] Ibid., vol. lxxv., p. 254.
[49] Ibid., vol. lxxix., p. 225.
[50] Phil. Trans., vol. lxxix., p. 226.
[51] Ibid., vol. lxxxi., p. 72.
[52] Ibid., p. 85.
[53] Phil. Trans., vol. ci., p. 271.
[54] Ibid., p. 277.
[55] J. Herschel, Phil. Trans., vol. cxvi., part iii., p. 1.
[56] His own words to the poet Campbell cited by Holden, Life and Works,
p. 109.
[57] Phil. Trans., vol. civ., p. 283.
CHAPTER II
PROGRESS OF SIDEREAL ASTRONOMY
We have now to consider labours of a totally different character
from those of Sir William Herschel. Exploration and discovery do
not constitute the whole business of astronomy; the less adventurous,
though not less arduous, task of gaining a more and more complete
mastery over the problems immemorially presented to her, may, on
the contrary, be said to form her primary duty. A knowledge of
the movements of the heavenly bodies has, from the earliest times,
been demanded by the urgent needs of mankind; and science finds
its advantage, as in many cases it has taken its origin, in condescension
to practical claims. Indeed, to bring such knowledge as near
as possible to absolute precision has been defined by no mean
authority[58] as the true end of astronomy.
Several causes concurred about the beginning of the last century
to give a fresh and powerful impulse to investigations having
this end in view. The rapid progress of theory almost compelled
a corresponding advance in observation; instrumental
improvements rendered such an advance possible; Herschel’s
discoveries quickened public interest in celestial inquiries; royal,
imperial, and grand-ducal patronage widened the scope of individual
effort. The heart of the new movement was in Germany. Hitherto
the observatory of Flamsteed and Bradley had been the acknowledged
centre of practical astronomy; Greenwich observations were the
standard of reference all over Europe; and the art of observing
prospered in direct proportion to the fidelity with which Greenwich
methods were imitated. Dr. Maskelyne, who held the post of
Astronomer Royal during forty-six years (from 1765 to 1811), was
no unworthy successor to the eminent men who had gone before
him. His foundation of the Nautical Almanac (in 1767) alone
constitutes a valid title to fame; he introduced at the Observatory
the important innovation of the systematic publication of results;
and the careful and prolonged series of observations executed by[Pg 28]
him formed the basis of the improved theories, and corrected tables
of the celestial movements, which were rapidly being brought to
completion abroad. His catalogue of thirty-six “fundamental”
stars was besides excellent in its way, and most serviceable. Yet
he was devoid of Bradley’s instinct for divining the needs of the
future. He was fitted rather to continue a tradition than to found
a school. The old ways were dear to him; and, indefatigable as
he was, a definite purpose was wanting to compel him, by its
exigencies, along the path of progress. Thus, for almost fifty years
after Bradley’s death, the acquisition of a small achromatic[59] was the
only notable change made in the instrumental equipment of the
Observatory. The transit, the zenith sector, and the mural
quadrant, with which Bradley had done his incomparable work,
retained their places long after they had become deteriorated by
time and obsolete by the progress of invention; and it was not
until the very close of his career that Maskelyne, compelled by
Pond’s detection of serious errors, ordered a Troughton’s circle,
which he did not live to employ.
Meanwhile, the heavy national disasters with which Germany was
overwhelmed in the early part of the nineteenth century seemed
to stimulate rather than impede the intellectual revival already for
some years in progress there. Astronomy was amongst the first of
the sciences to feel the new impulse. By the efforts of Bode, Olbers,
Schröter, and Von Zach, just and elevated ideas on the subject
were propagated, intelligence was diffused, and a firm ground
prepared for common action in mutual sympathy and disinterested
zeal. They received powerful aid through the foundation, in 1804,
by a young artillery officer named Von Reichenbach, of an Optical
and Mechanical Institute at Munich. Here the work of English
instrumental artists was for the first time rivalled, and that of
English opticians—when Fraunhofer entered the new establishment—far
surpassed. The development given to the refracting telescope
by this extraordinary man was indispensable to the progress of that
fundamental part of astronomy which consists in the exact determination
of the places of the heavenly bodies. Reflectors are brilliant
engines of discovery, but they lend themselves with difficulty to the
prosaic work of measuring right ascensions and polar distances. A
signal improvement in the art of making and working flint-glass
thus most opportunely coincided with the rise of a German school of
scientific mechanicians, to furnish the instrumental means needed
for the reform which was at hand. Of the leader of that reform it
is now time to speak.
Friedrich Wilhelm Bessel was born at Minden, in Westphalia,[Pg 29]
July 22, 1784. A certain taste for figures, coupled with a still
stronger distaste for the Latin accidence, directed his inclination
and his father’s choice towards a mercantile career. In his fifteenth
year, accordingly, he entered the house of Kuhlenkamp and Sons,
in Bremen, as an apprenticed clerk. He was now thrown completely
upon his own resources. From his father, a struggling Government
official, heavily weighted with a large family, he was well aware
that he had nothing to expect; his dormant faculties were roused
by the necessity for self-dependence, and he set himself to push
manfully forward along the path that lay before him. The post of
supercargo on one of the trading expeditions sent out from the
Hanseatic towns to China and the East Indies was the aim of his
boyish ambition, for the attainment of which he sought to qualify
himself by the industrious acquisition of suitable and useful knowledge.
He learned English in two or three months; picked up
Spanish with the casual aid of a gunsmith’s apprentice; studied
the geography of the distant lands which he hoped to visit; collected
information as to their climates, inhabitants, products, and the
courses of trade. He desired to add some acquaintance with the
art (then much neglected) of taking observations at sea; and thus,
led on from navigation to astronomy, and from astronomy to
mathematics, he groped his way into a new world.
It was characteristic of him that the practical problems of
science should have attracted him before his mind was as yet
sufficiently matured to feel the charm of its abstract beauties.
His first attempt at observation was made with a sextant,
rudely constructed under his own directions, and a common
clock. Its object was the determination of the longitude of
Bremen, and its success, he tells us himself,[60] filled him with a
rapture of delight, which, by confirming his tastes, decided his
destiny. He now eagerly studied Bode’s Jahrbuch and Von Zach’s
Monatliche Correspondenz, overcoming each difficulty as it arose with
the aid of Lalande’s Traité d’Astronomie, and supplying, with amazing
rapidity, his early deficiency in mathematical training. In two
years he was able to attack a problem which would have tasked
the patience, if not the skill, of the most experienced astronomer.
Amongst the Earl of Egremont’s papers Von Zach had discovered
Harriot’s observations on Halley’s comet at its appearance in 1607,
and published them as a supplement to Bode’s Annual. With an
elaborate care inspired by his youthful ardour, though hardly
merited by their loose nature, Bessel deduced from them an orbit
for that celebrated body, and presented the work to Olbers, whose
reputation in cometary researches gave a special fitness to the[Pg 30]
proffered homage. The benevolent physician-astronomer of Bremen
welcomed with surprised delight such a performance emanating from
such a source. Fifteen years previously, the French Academy had
crowned a similar work; now its equal was produced by a youth
of twenty, busily engaged in commercial pursuits, self-taught, and
obliged to snatch from sleep the hours devoted to study. The paper
was immediately sent to Von Zach for publication, with a note from
Olbers explaining the circumstances of its author; and the name
of Bessel became the common property of learned Europe.
He had, however, as yet no intention of adopting astronomy
as his profession. For two years he continued to work in the
counting-house by day, and to pore over the Mécanique Céleste and
the Differential Calculus by night. But the post of assistant
in Schröter’s observatory at Lilienthal having become vacant by
the removal of Harding to Göttingen in 1805, Olbers procured
for him the offer of it. It was not without a struggle that
he resolved to exchange the desk for the telescope. His reputation
with his employers was of the highest; he had thoroughly
mastered the details of the business, which his keen practical intelligence
followed with lively interest; his years of apprenticeship were
on the point of expiring, and an immediate, and not unwelcome
prospect of comparative affluence lay before him. The love of
science, however, prevailed; he chose poverty and the stars, and
went to Lilienthal with a salary of a hundred thalers yearly. Looking
back over his life’s work, Olbers long afterwards declared that
the greatest service which he had rendered to astronomy was that of
having discerned, directed, and promoted the genius of Bessel.[61]
For four years he continued in Schröter’s employment. At the
end of that time the Prussian Government chose him to superintend
the erection of a new observatory at Königsberg, which after many
vexatious delays, caused by the prostrate condition of the country,
was finished towards the end of 1813. Königsberg was the first
really efficient German observatory. It became, moreover, a centre
of improvement, not for Germany alone, but for the whole astronomical
world. During two-and-thirty years it was the scene of
Bessel’s labours, and Bessel’s labours had for their aim the reconstruction,
on an amended and uniform plan, of the entire science of
observation.
A knowledge of the places of the stars is the foundation of
astronomy.[62] Their configuration lends to the skies their distinctive
features, and marks out the shifting tracks of more mobile objects
with relatively fixed, and generally unvarying points of light. A
more detailed and accurate acquaintance with the stellar multitude,[Pg 31]
regarded from a purely uranographical point of view, has accordingly
formed at all times a primary object of celestial science, and was,
during the last century, cultivated with a zeal and success by which
all previous efforts were dwarfed into insignificance. In Lalande’s
Histoire Céleste, published in 1801, the places of no less than 47,390
stars were given, but in the rough, as it were, and consequently
needing laborious processes of calculation to render them available
for exact purposes. Piazzi set an example of improved methods of
observation, resulting in the publication, in 1803 and 1814, of two
catalogues of about 7,600 stars—the second being a revision and
enlargement of the first—which for their time were models of what
such works should be.[63] Stephen Groombridge at Blackheath was
similarly and most beneficially active. But something more was
needed than the diligence of individual observers. A systematic
reform was called for; and it was this which Bessel undertook and
carried through.
Direct observation furnishes only what has been called the “raw
material” of the positions of the heavenly bodies.[64] A number of
highly complex corrections have to be applied before their mean can
be disengaged from their apparent places on the sphere. Of these,
the most considerable and familiar is atmospheric refraction, by
which objects seem to stand higher in the sky than they in reality
do, the effect being evanescent at the zenith, and attaining, by
gradations varying with conditions of pressure and temperature, a
maximum at the horizon. Moreover, the points to which measurements
are referred are themselves in motion, either continually in
one direction, or periodically to and fro. The precession of the
equinoxes is slowly progressive, or rather retrogressive; the nutation
of the pole oscillatory in a period of about eighteen years. Added
to which, the non-instantaneous transmission of light, combined
with the movement of the earth in its orbit, causes a small annual
displacement known as aberration.
Now it is easy to see that any uncertainty in the application of
these corrections saps the very foundations of exact astronomy.
Extremely minute quantities, it is true, are concerned; but the life
and progress of modern celestial science depends upon the sure recognition
of extremely minute quantities. In the early years of the
nineteenth century, however, no uniform system of “reduction” (so
the complete correction of observational results is termed) had been
established. Much was left to the individual caprice of observers,
who selected for the several “elements” of reduction such values as[Pg 32]
seemed best to themselves. Hence arose much hurtful confusion,
tending to hinder united action and mar the usefulness of laborious
researches. For this state of things, Bessel, by the exercise of consummate
diligence, sagacity, and patience, provided an entirely
satisfactory remedy.
His first step was an elaborate investigation of the precious
series of observations made by Bradley at Greenwich from 1750
until his death in 1762. The catalogue of 3,222 stars which he extracted
from them gave the earliest example of the systematic
reduction on a uniform plan of such a body of work. It is difficult,
without entering into details out of place in a volume like the
present, to convey an idea of the arduous nature of this task. It
involved the formation of a theory of the errors of each of Bradley’s
instruments, and a difficult and delicate inquiry into the true value
of each correction to be applied, before the entries in the Greenwich
journals could be developed into a finished and authentic catalogue.
Although completed in 1813, it was not until five years later that
the results appeared with the proud, but not inappropriate title of
Fundamenta Astronomiæ. The eminent value of the work consisted
in this, that by providing a mass of entirely reliable information as
to the state of the heavens at the epoch 1755, it threw back the
beginning of exact astronomy almost half a century. By comparison
with Piazzi’s catalogues the amount of precession was more
accurately determined, the proper motions of a considerable
number of stars became known with certainty, and definite prediction—the
certificate of initiation into the secrets of Nature—at
last became possible as regards the places of the stars. Bessel’s
final improvements in the methods of reduction were published in
1830 in his Tabulæ Regiomontanæ. They not only constituted an
advance in accuracy, but afforded a vast increase of facility in
application, and were at once and everywhere adopted. Thus
astronomy became a truly universal science; uncertainties and
disparities were banished, and observations made at all times and
places rendered mutually comparable.[65]
More, however, yet remained to be done. In order to verify with
greater strictness the results drawn from the Bradley and Piazzi
catalogues, a third term of comparison was wanted, and this Bessel
undertook to supply. By a course of 75,011 observations, executed
during the years 1821-33, with the utmost nicety of care, the
number of accurately known stars was brought up to above 50,000,
and an ample store of trustworthy facts laid up for the use of future
astronomers. In this department Argelander, whom he attracted
from finance to astronomy, and trained in his own methods, was his[Pg 33]
assistant and successor. The great “Bonn Durchmusterung,”[66] in
which 324,198 stars visible in the northern hemisphere are
enumerated, and the corresponding “Atlas” published in 1857-63,
constituting a picture of our sidereal surroundings of heretofore
unapproached completeness, may be justly said to owe their origin
to Bessel’s initiative, and to form a sequel to what he commenced.
But his activity was not solely occupied with the promotion of a
comprehensive reform in astronomy; it embraced special problems
as well. The long-baffled search for a parallax of the fixed stars
was resumed with fresh zeal as each mechanical or optical improvement
held out fresh hopes of a successful issue. Illusory results
abounded. Piazza in 1805 perceived, as he supposed, considerable
annual displacements in Vega, Aldebaran, Sirius, and Procyon; the
truth being that his instruments were worn out with constant use,
and could no longer be depended upon.[67] His countryman, Calandrelli,
was similarly deluded. The celebrated controversy between
the Astronomer Royal and Dr. Brinkley, Director of the Dublin
College Observatory, turned on the same subject. Brinkley, who
was in possession of a first-rate meridian-circle, believed himself to
have discovered relatively large parallaxes for four of the brightest
stars; Pond, relying on the testimony of the Greenwich instruments,
asserted their nullity. The dispute, protracted for fourteen years,
from 1810 until 1824, was brought to no definite conclusion; but
the strong presumption on the negative side was abundantly justified
in the event.
There was good reason for incredulity in the matter of parallaxes.
Announcements of their detection had become so frequent as to be
discredited before they were disproved; and Struve, who investigated
the subject at Dorpat in 1818-21, had clearly shown that
the quantities concerned were too small to come within the reliable
measuring powers of any instrument then in use. Already, however,
the means were being prepared of giving to those powers a
large increase.
On the 21st July, 1801, two old houses in an alley of Munich
tumbled down, burying in their ruins the occupants, of whom one alone
was extricated alive, though seriously injured. This was an orphan
lad of fourteen named Joseph Fraunhofer. The Elector Maximilian
Joseph was witness of the scene, became interested in the survivor,
and consoled his misfortune with a present of eighteen ducats.
Seldom was money better bestowed. Part of it went to buy books
and a glass-polishing machine, with the help of which young Fraunhofer
studied mathematics and optics, and secretly exercised himself
in the shaping and finishing of lenses; the remainder purchased his[Pg 34]
release from the tyranny of one Weichselberger, a looking-glass
maker by trade, to whom he had been bound apprentice on the
death of his parents. A period of struggle and privation followed,
during which, however, he rapidly extended his acquirements; and
was thus eminently fitted for the task awaiting him, when, in 1806,
he entered the optical department of the establishment founded two
years previously by Von Reichenbach and Utzschneider. He now
zealously devoted himself to the improvement of the achromatic
telescope; and, after a prolonged study of the theory of lenses, and
many toilsome experiments in the manufacture of flint-glass, he
succeeded in perfecting, December 12, 1817, an object-glass of exquisite
quality and finish, 9-1/2 inches in diameter, and of 14 feet
focal length.
This (as it was then considered) gigantic lens was secured by
Struve for the Russian Government, and the “great Dorpat refractor”—the
first of the large achromatics which have played such
an important part in modern astronomy—was, late in 1824, set up
in the place which it still occupies. By ingenious improvements in
mounting and fitting, it was adapted to the finest micrometrical
work, and thus offered unprecedented facilities both for the examination
of double stars (in which Struve chiefly employed it),
and for such subtle measurements as might serve to reveal or disprove
the existence of a sensible stellar parallax. Fraunhofer,
moreover, constructed for the observatory at Königsberg the first
really available heliometer. The principle of this instrument (termed
with more propriety a “divided object-glass micrometer”) is the
separation, by a strictly measurable amount, of two distinct images
of the same object. If a double star, for instance, be under examination,
the two half-lenses into which the object-glass is divided are
shifted until the upper star (say) in one image is brought into
coincidence with the lower star in the other, when their distance
apart becomes known by the amount of motion employed.[68]
This virtually new engine of research was delivered and mounted
in 1829, three years after the termination of the life of its deviser.
The Dorpat lens had brought to Fraunhofer a title of nobility and
the sole management of the Munich Optical Institute (completely
separated since 1814 from the mechanical department). What he
had achieved, however, was but a small part of what he meant to
achieve. He saw before him the possibility of nearly quadrupling
the light-gathering capacity of the great achromatic acquired by[Pg 35]
Struve; he meditated improvements in reflectors as important as
those he had already effected in refractors; and was besides eagerly
occupied with investigations into the nature of light, the momentous
character of which we shall by-and-by have an opportunity of
estimating. But his health was impaired, it is said, from the
weakening effects of his early accident, combined with excessive and
unwholesome toil, and, still hoping for its restoration from a projected
journey to Italy, he died of consumption, June 7, 1826, aged
thirty-nine years. His tomb in Munich bears the concise eulogy,
Approximavit sidera.
Bessel had no sooner made himself acquainted with the exquisite
defining powers of the Königsberg heliometer, than he resolved to
employ them in an attack upon the now secular problem of star-distances.
But it was not until 1837 that he found leisure to pursue
the inquiry. In choosing his test-star he adopted a new principle.
It had hitherto been assumed that our nearest neighbours in space
must be found among the brightest ornaments of our skies. The
knowledge of stellar proper motions afforded by the critical comparison
of recent with earlier star-places, suggested a different
criterion of distance. It is impossible to escape from the conclusion
that the apparently swiftest-moving stars are, on the whole, also the
nearest to us, however numerous the individual exceptions to the
rule. Now, as early as 1792,[69] Piazzi had noted as an indication of
relative vicinity to the earth, the unusually large proper motion (5·2′
annually) of a double star of the fifth magnitude in the constellation
of the Swan. Still more emphatically in 1812[70] Bessel drew the
attention of astronomers to the fact, and 61 Cygni became known
as the “flying star.” The seeming rate of its flight, indeed, is of so
leisurely a kind, that in a thousand years it will have shifted its
place by less than 3-1/2 lunar diameters, and that a quarter of a
million would be required to carry it round the entire circuit of
the visible heavens. Nevertheless, it has few rivals in rapidity of
movement, the apparent displacement of the vast majority of stars
being, by comparison, almost insensible.
This interesting, though inconspicuous object, then, was chosen
by Bessel to be put to the question with his heliometer, while
Struve made a similar and somewhat earlier trial with the bright
gem of the Lyre, whose Arabic title of the “Falling Eagle” survives
as a time-worn remnant in “Vega.” Both astronomers agreed
to use the “differential” method, for which their instruments and
the vicinity to their selected stars of minute, physically detached
companions offered special facilities. In the last month of 1838[Pg 36]
Bessel made known the result of one year’s observations, showing
for 61 Cygni a parallax of about a third of a second (0·3136′).[71] He
then had his heliometer taken down and repaired, after which he
resumed the inquiry, and finally terminated a series of 402 measures
in March 1840.[72] The resulting parallax of 0·3483′ (corresponding
to a distance about 600,000 times that of the earth from the sun),
seemed to be ascertained beyond the possibility of cavil, and is
memorable as the first published instance of the fathom-line, so
industriously thrown into celestial space, having really and indubitably
touched bottom. It was confirmed in 1842-43 with curious
exactness by C. A. F. Peters at Pulkowa; but later researches
showed that it required increase to nearly half a second.[73]
Struve’s measurements inspired less confidence. They extended
over three years (1835-38), but were comparatively few, and were
frequently interrupted. The parallax, accordingly, of about a
quarter of a second (0·2613′) which he derived from them for α
Lyræ, and announced in 1840,[74] has proved considerably too large.[75]
Meanwhile a result of the same kind, but of a more striking
character than either Bessel’s or Struve’s, had been obtained, one
might almost say casually, by a different method and in a distant
region. Thomas Henderson, originally an attorney’s clerk in his
native town of Dundee, had become known for his astronomical
attainments, and was appointed in 1831 to direct the recently
completed observatory at the Cape of Good Hope. He began
observing in April, 1832, and, the serious shortcomings of his
instrument notwithstanding, executed during the thirteen months
of his tenure of office a surprising amount of first-rate work.
With a view to correcting the declination of the lustrous double
star α Centauri (which ranks after Sirius and Canopus as the third
brightest orb in the heavens), he effected a number of successive
determinations of its position, and on being informed of its very
considerable proper motion (3·6′ annually), he resolved to examine
the observations already made for possible traces of parallactic
displacement. This was done on his return to Scotland, where he
filled the office of Astronomer Royal from 1834 until his premature
death in 1844. The result justified his expectations. From the[Pg 37]
declination measurements made at the Cape and duly reduced,
a parallax of about one second of arc clearly emerged (diminished
by Gill’s and Elkin’s observations, 1882-1883, to O·75′); but, by
perhaps an excess of caution, was withheld from publication until
fuller certainty was afforded by the concurrent testimony of
Lieutenant Meadows’s determinations of the same star’s right
ascension.[76] When at last, January 9, 1839, Henderson communicated
his discovery to the Astronomical Society, he could no
longer claim the priority which was his due. Bessel had anticipated
him with the parallax of 61 Cygni by just two months.
Thus from three different quarters, three successful and almost
simultaneous assaults were delivered upon a long-beleaguered citadel
of celestial secrets. The same work has since been steadily pursued,
with the general result of showing that, as regards their overwhelming
majority, the stars are far too remote to show even the slightest
trace of optical shifting from the revolution of the earth in its orbit.
In nearly a hundred cases, however, small parallaxes have been
determined, some certainly (that is, within moderate limits of error),
others more or less precariously. The list is an instructive one,
in its omissions no less than in its contents. It includes stars of
many degrees of brightness, from Sirius down to a nameless
telescopic star in the Great Bear;[77] yet the vicinity to the earth of
this minute object is so much greater than that of the brilliant
Vega, that the latter transported to its place would increase in
lustre thirty-eight times. Moreover, many of the brightest stars
are found to have no sensible parallax, while the majority of those
ascertained to be nearest to the earth are of fifth, sixth, even ninth
magnitudes. The obvious conclusions follow that the range of
variety in the sidereal system is enormously greater than had been
supposed, and that estimates of distance based upon apparent
magnitude must be wholly futile. Thus, the splendid Canopus,
Betelgeux, and Rigel can be inferred, from their indefinite remoteness,
to exceed our sun thousands of times in size and lustre; while
many inconspicuous objects, which prove to be in our relative
vicinity, must be notably his inferiors. The limits of real stellar
magnitude are then set very widely apart. At the same time,
the so-called “optical” and “geometrical” methods of relatively
estimating star-distances are both seen to have a foundation of fact,
although so disguised by complicated relations as to be of very
doubtful individual application. On the whole, the chances are in[Pg 38]
favour of the superior vicinity of a bright star over a faint one;
and, on the whole, the stars in swiftest apparent motion are amongst
those whose actual remoteness is least. Indeed, there is no escape
from either conclusion, unless on the supposition of special arrangements
in themselves highly improbable, and, we may confidently
say, non-existent.
The distances even of the few stars found to have measurable
parallaxes are on a scale entirely beyond the powers of the human
mind to conceive. In the attempt both to realize them distinctly,
and to express them conveniently, a new unit of length, itself of
bewildering magnitude, has originated. This is what we may call
the light-journey of one year. The subtle vibrations of the ether,
propagated on all sides from the surface of luminous bodies, travel
at the rate of 186,300 miles a second, or (in round numbers) six
billions of miles a year. Four and a third such measures are needed
to span the abyss that separates us from the nearest fixed star. In
other words, light takes four years and four months to reach the
earth from α Centauri; yet α Centauri lies some ten billions of miles
nearer to us (so far as is yet known) than any other member of the
sidereal system!
The determination of parallax leads, in the case of stars revolving
in known orbits, to the determination of mass; for the distance
from the earth of the two bodies forming a binary system being
ascertained, the seconds of arc apparently separating them from
each other can be translated into millions of miles; and we only
need to add a knowledge of their period to enable us, by an easy
sum in proportion, to find their combined mass in terms of that of
the sun. Thus, since—according to Dr. Doberck’s elements—the
components of α Centauri revolve round their common centre of
gravity at a mean distance nearly 25 times the radius of the earth’s
orbit, in a period of 88 years, the attractive force of the two together
must be just twice the solar. We may gather some idea of their
relations by placing in imagination a second luminary like our sun
in circulation between the orbits of Neptune and Uranus. But
systems of still more majestic proportions are reduced by extreme
remoteness to apparent insignificance. A double star of the fourth
magnitude in Cassiopeia (Eta), to which a small parallax is ascribed
on the authority of O. Struve, appears to be above eight times
as massive as the central orb of our world; while a much less
conspicuous pair—85 Pegasi—exerts, if the available data can be
depended upon, no less than thirteen times the solar gravitating
power.
Further, the actual rate of proper motions, so far as regards that
part of them which is projected upon the sphere, can be ascertained[Pg 39]
for stars at known distance. The annual journey, for instance,
of 61 Cygni across the line of sight amounts to 1,000, and that of α
Centauri to 446 millions of miles. A small star, numbered 1,830
in Groombridge’s Circumpolar Catalogue, “devours the way” at the
rate of at least 150 miles a second—a speed, in Newcomb’s opinion,
beyond the gravitating power of the entire sidereal system to
control; and μ Cassiopeiæ possesses above two-thirds of that
surprising velocity; while for both objects, radial movements of
just sixty miles a second were disclosed by Professor Campbell’s
spectroscopic measurements.
Herschel’s conclusion as to the advance of the sun among the
stars was not admitted as valid by the most eminent of his successors.
Bessel maintained that there was absolutely no preponderating
evidence in favour of its supposed direction towards
a point in the constellation Hercules.[78] Biot, Burckhardt, even
Herschel’s own son, shared his incredulity. But the appearance of
Argelander’s prize-essay in 1837[79] changed the aspect of the question.
Herschel’s first memorable solution in 1783 was based upon
the motions of thirteen stars, imperfectly known; his second, in
1805, upon those of no more than six. Argelander now obtained
an entirely concordant result from the large number of 390, determined
with the scrupulous accuracy characteristic of Bessel’s
work and his own. The reality of the fact thus persistently disclosed
could no longer be doubted; it was confirmed five years
later by the younger Struve, and still more strikingly in
1847[80] by Galloway’s investigations, founded exclusively on the
apparent displacements of southern stars. In 1859 and 1863, Sir
George Airy and Mr. Dunkin (1821-1898),[81] employing all the
resources of modern science, and commanding the wealth of material
furnished by 1,167 proper motions carefully determined by Mr. Main,
reached conclusions closely similar to that indicated nearly eighty
years previously by the first great sidereal astronomer; which
Mr. Plummer’s reinvestigation of the subject in 1883[82] served but
slightly to modify. Yet astronomers were not satisfied. Dr. Auwers
of Berlin completed in 1866 a splendid piece of work, for which he
received in 1888 the Gold Medal of the Royal Astronomical Society.
It consisted in reducing afresh, with the aid of the most refined
modern data, Bradley’s original stars, and comparing their places
thus obtained for the year 1755 with those assigned to them from
observations made at Greenwich after the lapse of ninety years. In
the interval, as was to be anticipated, most of them were found to[Pg 40]
have travelled over some small span of the heavens, and there
resulted a stock of nearly three thousand highly authentic proper
motions. These ample materials were turned to account by M.
Ludwig Struve[83] for a discussion of the sun’s motion, of which the
upshot was to shift its point of aim to the bordering region of the
constellations Hercules and Lyra. And the more easterly position
of the solar apex was fully confirmed by the experiments, with
variously assorted lists of stars, of Lewis Boss of Albany,[84] and Oscar
Stumpe of Bonn.[85] Fresh precautions of refinement were introduced
into the treatment of the subject by Ristenpart of Karlsruhe,[86] by
Kapteyn of Groningen,[87] by Newcomb[88] and Porter[89] in America, who
ably availed themselves of the copious materials accumulated before
the close of the century. Their results, although not more closely
accordant than those of their predecessors, combined to show that
the journey of our system is directed towards a point within a circle
about ten degrees in radius, having the brilliant Vega for its centre.
To determine its rate was a still more arduous problem. It involved
the assumption, very much at discretion, of an average parallax for
the stars investigated; and Otto Struve’s estimate of 154 million
miles as the span yearly traversed was hence wholly unreliable.
Fortunately, however, as will be seen further on, a method of
determining the sun’s velocity independently of any knowledge of
star-distances, has now become available.
As might have been expected, speculation has not been idle
regarding the purpose and goal of the strange voyage of discovery
through space upon which our system is embarked; but altogether
fruitlessly. The variety of the conjectures hazarded in the matter
is in itself a measure of their futility. Long ago, before the construction
of the heavens had as yet been made the subject of
methodical inquiry, Kant was disposed to regard Sirius as the
“central sun” of the Milky Way; while Lambert surmised that
the vast Orion nebula might serve as the regulating power of a
subordinate group including our sun. Herschel threw out the
hint that the great cluster in Hercules might prove to be the
supreme seat of attractive force;[90] Argelander placed his central
body in the constellation Perseus;[91] Fomalhaut, the brilliant of
the Southern Fish, was set in the post of honour by Boguslawski[Pg 41]
of Breslau. Mädler (who succeeded Struve at Dorpat in 1839)
concluded from a more formal inquiry that the ruling power
in the sidereal system resided, not in any single prepondering
mass, but in the centre of gravity of the self-controlled revolving
multitude.[92] In the former case (as we know from the example
of the planetary scheme), the stellar motions would be most
rapid near the centre; in the latter, they would become accelerated
with remoteness from it.[93] Mädler showed that no part of the
heavens could be indicated as a region of exceptionally swift
movements, such as would result from the presence of a gigantic
(though possibly obscure) ruling body; but that a community
of extremely sluggish movements undoubtedly existed in and near
the group of the Pleiades, where, accordingly, he placed the centre
of gravity of the Milky Way.[94] The bright star Alcyone thus
became the “central sun,” but in a purely passive sense, its headship
being determined by its situation at the point of neutralisation
of opposing tendencies, and of consequent rest. By an avowedly
conjectural method, the solar period of revolution round this point
was fixed at 18,200,000 years.
The scheme of sidereal government framed by the Dorpat
astronomer was, it may be observed, of the most approved constitutional
type; deprivation, rather than increase of influence
accompanying the office of chief dignitary. But while we are still
ignorant, and shall perhaps ever remain so, of the fundamental plan
upon which the Galaxy is organised, recent investigations tend more
and more to exhibit it, not as monarchical (so to speak), but as
federative. The community of proper motions detected by Mädler
in the vicinity of the Pleiades may accordingly possess a significance
altogether different from what he imagined.
Bessel’s so-called “foundation of an Astronomy of the Invisible”
now claims attention.[95] His prediction regarding the planet Neptune
does not belong to the present division of our subject; a strictly
analogous discovery in the sidereal system was, however, also very
clearly foreshadowed by him. His earliest suspicions of non-uniformity
in the proper motion of Sirius dated from 1834; they
extended to Procyon in 1840; and after a series of refined measurements
with the new Repsold circle, he announced in 1844 his
conclusion that these irregularities were due to the presence of[Pg 42]
obscure bodies round which the two bright Dog-stars revolved as
they pursued their way across the sphere.[96] He even assigned to
each an approximate period of half a century. “I adhere to the
conviction,” he wrote later to Humboldt, “that Procyon and Sirius
form real binary systems, consisting of a visible and an invisible
star. There is no reason to suppose luminosity an essential quality
of cosmical bodies. The visibility of countless stars is no argument
against the invisibility of countless others.”[97]
An inference so contradictory to received ideas obtained little
credit, until Peters found, in 1851,[98] that the apparent anomalies in
the movements of Sirius could be completely explained by an orbital
revolution in a period of fifty years. Bessel’s prevision was destined
to be still more triumphantly vindicated. On the 31st of January,
1862, while in the act of trying a new 18-inch refractor, Mr. Alvan
G. Clark (one of the celebrated firm of American opticians) actually
discovered the hypothetical Sirian companion in the precise position
required by theory. It has now been watched through nearly an
entire revolution (period 49·4 years), and proves to be very slightly
luminous in proportion to its mass. Its attractive power, in fact,
is nearly half that of its primary, while it emits only 1/10000th of its
light. Sirius itself, on the other hand, possesses a far higher radiative
intensity than our sun. It gravitates—admitting Sir David Gill’s
parallax of 0·38′ to be exact—like two suns, but shines like twenty.
Possibly it is much distended by heat, and undoubtedly its atmosphere
intercepts a very much smaller proportion of its light than in
stars of the solar class. As regards Procyon, visual verification was
awaited until November 13, 1896, when Professor Schaeberle, with
the great Lick refractor, detected the long-sought object in the
guise of a thirteenth-magnitude star. Dr. See’s calculations[99]
showed it to possess one-fifth the mass of its primary, or rather
more than half that of our sun.[100] Yet it gives barely 1/20000th of the
sun’s light, so that it is still nearer to total obscurity than the dusky
satellite of Sirius. The period of forty years assigned to the system
by Auwers in 1862[101] appears to be singularly exact.
But Bessel was not destined to witness the recognition of
“the invisible” as a legitimate and profitable field for astronomical
research. He died March 17, 1846, just six months before the discovery
of Neptune, of an obscure disease, eventually found to be
occasioned by an extensive fungus-growth in the stomach. The[Pg 43]
place which he left vacant was not one easy to fill. His life’s work
might be truly described as “epoch-making.” Rarely indeed shall
we find one who reconciled with the same success the claims of
theoretical and practical astronomy, or surveyed the science which
he had made his own with a glance equally comprehensive, practical,
and profound.
The career of Friedrich Georg Wilhelm Struve illustrates the
maxim that science differentiates as it develops. He was, while
much besides, a specialist in double stars. His earliest recorded
use of the telescope was to verify Herschel’s conclusion as to the
revolving movement of Castor, and he never varied from the
predilection which this first observation at once indicated and
determined. He was born at Altona, of a respectable yeoman
family, April 15, 1793, and in 1811 took a degree in philology at
the new Russian University of Dorpat. He then turned to science,
was appointed in 1813 to a professorship of astronomy and mathematics,
and began regular work in the Dorpat Observatory just
erected by Parrot for Alexander I. It was not, however, until 1819
that the acquisition of a 5-foot refractor by Troughton enabled him to
take the position-angles of double stars with regularity and tolerable
precision. The resulting catalogue of 795 stellar systems gave the
signal for a general resumption of the Herschelian labours in this
branch. His success, so far, and the extraordinary facilities for
observation afforded by the Fraunhofer achromatic encouraged him
to undertake, February 11, 1825, a review of the entire heavens
down to 15° south of the celestial equator, which occupied more than
two years, and yielded, from an examination of above 120,000 stars,
a harvest of about 2,200 previously unnoticed composite objects. The
ensuing ten years were devoted to delicate and patient measurements,
the results of which were embodied in Mensuræ Micrometricæ,
published at St. Petersburg in 1837. This monumental work gives
the places, angles of position, distances, colours, and relative brightness
of 3,112 double and multiple stars, all determined with the
utmost skill and care. The record is one which gains in value with
the process of time, and will for ages serve as a standard of reference
by which to detect change or confirm discovery.
It appears from Struve’s researches that about one in forty of
all stars down to the ninth magnitude is composite, but that the
proportion is doubled in the brighter orders.[102] This he attributed
to the difficulty of detecting the faint companions of very remote orbs.
It was also noticed, both by him and Bessel, that double stars are
in general remarkable for large proper motions. Struve’s catalogue
included no star of which the components were more than 32′ apart,[Pg 44]
because beyond that distance the chances of merely optical juxtaposition
become considerable; but the immense preponderance of
extremely close over (as it were) loosely yoked bodies is such as to
demonstrate their physical connection, even if no other proof were
forthcoming. Many stars previously believed to be single divided
under the scrutiny of the Dorpat refractor; while in some cases, one
member of a supposed binary system revealed itself as double,
thus placing the surprised observer in the unexpected presence of
a triple group of suns. Five instances were noted of two pairs
lying so close together as to induce a conviction of their mutual
dependence;[103] besides which, 124 examples occurred of triple, quadruple,
and multiple combinations, the reality of which was open to
no reasonable doubt.[104]
It was first pointed out by Bessel that the fact of stars exhibiting
a common proper motion might serve as an unfailing test
of their real association into systems. This was, accordingly, one
of the chief criteria employed by Struve to distinguish true
binaries from merely optical couples. On this ground alone, 61
Cygni was admitted to be a genuine double star; and it was
shown that, although its components appeared to follow almost
strictly rectilinear paths, yet the probability of their forming a
connected pair is actually greater than that of the sun rising
to-morrow morning.[105] Moreover, this tie of an identical movement
was discovered to unite bodies[106] far beyond the range of distance
ordinarily separating the members of binary systems, and to
prevail so extensively as to lead to the conclusion that single do not
outnumber conjoined stars more than twice or thrice.[107]
In 1835 Struve was summoned by the Emperor Nicholas to
superintend the erection of a new observatory at Pulkowa, near
St. Petersburg, destined for the special cultivation of sidereal
astronomy. Boundless resources were placed at his disposal, and
the institution created by him was acknowledged to surpass all
others of its kind in splendour, efficiency, and completeness. Its
chief instrumental glory was a refractor of fifteen inches aperture by
Merz and Mahler (Fraunhofer’s successors), which left the famous
Dorpat telescope far behind, and remained long without a rival. On
the completion of this model establishment, August 19, 1839, Struve
was installed as its director, and continued to fulfil the important
duties of the post with his accustomed vigour until 1858, when[Pg 45]
illness compelled his virtual resignation in favour of his son Otto
Struve, born at Dorpat in 1819. He died November 23, 1864.
An inquiry into the laws of stellar distribution, undertaken
during the early years of his residence at Pulkowa, led Struve to
confirm in the main the inferences arrived at by Herschel as to the
construction of the heavens. According to his view, the appearance
known as the Milky Way is produced by a collection of irregularly
condensed star-clusters, within which the sun is somewhat eccentrically
placed. The nebulous ring which thus integrates the light
of countless worlds was supposed by him to be made up of stars
scattered over a bent or “broken plane,” or to lie in two planes
slightly inclined to each other, our system occupying a position near
their intersection.[108] He further attempted to show that the limits
of this vast assemblage must remain for ever shrouded from human
discernment, owing to the gradual extinction of light in its passage
through space,[109] and sought to confer upon this celebrated hypothesis
a definiteness and certainty far beyond the aspirations of its earlier
advocates, Chéseaux and Olbers; but arbitrary assumptions vitiated
his reasonings on this, as well as on some other points.[110]
In his special line as a celestial explorer of the most comprehensive
type, Sir William Herschel had but one legitimate successor,
and that successor was his son. John Frederick William Herschel
was born at Slough, March 17, 1792, graduated with the highest
honours from St. John’s College, Cambridge, in 1813, and entered
upon legal studies with a view to being called to the Bar. But his
share in an early compact with Peacock and Babbage, “to do their
best to leave the world wiser than they found it,” was not thus
to be fulfilled. The acquaintance of Dr. Wollaston decided his
scientific vocation. Already, in 1816, we find him reviewing some
of his father’s double stars; and he completed in 1820 the 18-inch
speculum which was to be the chief instrument of his investigations.
Soon afterwards, he undertook, in conjunction with Mr. (later
Sir James) South, a series of observations, issuing in the presentation
to the Royal Society of a paper[111] containing micrometrical
measurements of 380 binary stars, by which the elder Herschel’s
inferences of orbital motion were, in many cases, strikingly confirmed.
A star in the Northern Crown, for instance (η Coronæ), had completed
more than one entire circuit since its first discovery; another,
τ Ophiuchi, had closed up into apparent singleness; while the motion
of a third, ξ Ursæ Majoris, in an obviously eccentric orbit, was so[Pg 46]
rapid as to admit of being traced and measured from month to
month.
It was from the first confidently believed that the force retaining
double stars in curvilinear paths was identical with that governing
the planetary revolutions. But that identity was not ascertained
until Savary of Paris showed, in 1827,[112] that the movements of the
above-named binary in the Great Bear could be represented with all
attainable accuracy by an ellipse calculated on orthodox gravitational
principles with a period of 58-1/4 years. Encke followed at Berlin
with a still more elegant method; and Sir John Herschel, pointing
out the uselessness of analytical refinements where the data were
necessarily so imperfect, described in 1832 a graphical process by
which “the aid of the eye and hand” was brought in “to guide the
judgment in a case where judgment only, and not calculation, could
be of any avail.”[113] Improved methods of the same kind were
published by Dr. See in 1893,[114] and by Mr. Burnham in 1894;[115] and
our acquaintance with stellar orbits is steadily gaining precision,
certainty, and extent.
In 1825 Herschel undertook, and executed with great assiduity
during the ensuing eight years, a general survey of the northern
heavens, directed chiefly towards the verification of his father’s
nebular discoveries. The outcome was a catalogue of 2,306 nebulæ
and clusters, of which 525 were observed for the first time, besides
3,347 double stars discovered almost incidentally.[116] “Strongly
invited,” as he tells us himself, “by the peculiar interest of the
subject, and the wonderful nature of the objects which presented
themselves,” he resolved to attempt the completion of the survey in
the southern hemisphere. With this noble object in view, he
embarked his family and instruments on board the Mount Stewart
Elphinstone, and, after a prosperous voyage, landed at Cape Town
on the 16th of January, 1834. Choosing as the scene of his observations
a rural spot under the shelter of Table Mountain, he began
regular “sweeping” on the 5th of March. The site of his great
reflector is now marked with an obelisk, and the name of Feldhausen
has become memorable in the history of science; for the four years’
work done there may truly be said to open the chapter of our knowledge
as regards the southern skies.
The full results of Herschel’s journey to the Cape were not made
public until 1847, when a splendid volume[117] embodying them was[Pg 47]
brought out at the expense of the Duke of Northumberland. They
form a sequel to his father’s labours such as the investigations of
one man have rarely received from those of another. What the
elder observer did for the northern heavens, the younger did for
the southern, and with generally concordant results. Reviving
the paternal method of “star-gauging,” he showed, from a count of
2,299 fields, that the Milky Way surrounds the solar system as a
complete annulus of minute stars; not, however, quite symmetrically,
since the sun was thought to lie somewhat nearer to those portions
visible in the southern hemisphere, which display a brighter lustre
and a more complicated structure than the northern branches.
The singular cosmical agglomerations known as the “Magellanic
Clouds” were now, for the first time, submitted to a detailed, though
admittedly incomplete, examination, the almost inconceivable richness
and variety of their contents being such that a lifetime might
with great profit be devoted to their study. In the Greater
Nubecula, within a compass of forty-two square degrees, Herschel
reckoned 278 distinct nebulæ and clusters, besides fifty or sixty
outliers, and a large number of stars intermixed with diffused
nebulosity—in all, 919 catalogued objects, and, for the Lesser
Cloud, 244. Yet this was only the most conspicuous part of what
his twenty-foot revealed. Such an extraordinary concentration of
bodies so various led him to the inevitable conclusion that “the
Nubeculæ are to be regarded as systems sui generis, and which have
no analogues in our hemisphere.”[118] He noted also the blankness of
surrounding space, especially in the case of Nubecula Minor, “the
access to which on all sides,” he remarked, “is through a desert;”
as if the cosmical material in the neighbourhood had been swept up
and garnered in these mighty groups.[119]
Of southern double stars, he discovered and gave careful measurements
of 2,102, and described 1,708 nebulæ, of which at least 300
were new. The list was illustrated with a number of drawings,
some of them extremely beautiful and elaborate.
Sir John Herschel’s views as to the nature of nebulæ were considerably
modified by Lord Rosse’s success in “resolving” with
his great reflectors a crowd of these objects into stars. His former
somewhat hesitating belief in the existence of phosphorescent matter,
“disseminated through extensive regions of space in the manner of
a cloud or fog,”[120] was changed into a conviction that no valid distinction
could be established between the faintest wisp of cosmical
vapour just discernible in a powerful telescope, and the most
brilliant and obvious cluster. He admitted, however, an immense[Pg 48]
range of possible variety in the size and mode of aggregation of the
stellar constituents of various nebulæ. Some might appear nebulous
from the closeness of their parts; some from their smallness. Others,
he suggested, might be formed of “discrete luminous bodies floating
in a non-luminous medium;”[121] while the annular kind probably
consisted of “hollow shells of stars.”[122] That a physical, and not
merely an optical, connection unites nebulæ with the embroidery (so
to speak) of small stars with which they are in many instances
profusely decorated, was evident to him, as it must be to all who
look as closely and see as clearly as he did. His description of
No. 2,093 in his northern catalogue as “a network or tracery of
nebula following the lines of a similar network of stars,”[123] would
alone suffice to dispel the idea of accidental scattering; and many
other examples of a like import might be quoted. The remarkably
frequent occurrence of one or more minute stars in the close vicinity
of “planetary” nebulæ led him to infer their dependent condition;
and he advised the maintenance of a strict watch for evidences of
circulatory movements, not only over these supposed stellar satellites,
but also over the numerous “double nebulæ,” in which, as he pointed
out, “all the varieties of double stars as to distance, position, and
relative brightness, have their counterparts.” He, moreover,
investigated the subject of nebular distribution by the simple and
effectual method of graphic delineation or “charting,” and succeeded
in showing that while a much greater uniformity of scattering
prevails in the southern than in the northern heavens, a condensation
is nevertheless perceptible about the constellations Pisces and
Cetus, roughly corresponding to the “nebular region” in Virgo by
its vicinity (within 20° or 30°) to the opposite pole of the Milky
Way. He concluded “that the nebulous system is distinct from the
sidereal, though involving, and perhaps to a certain extent intermixed
with, the latter.”[124]
Towards the close of his residence at Feldhausen, Herschel was
fortunate enough to witness one of those singular changes in the
aspect of the firmament which occasionally challenge the attention
even of the incurious, and excite the deepest wonder of the
philosophical observer. Immersed apparently in the Argo nebula
is a star denominated η Carinæ. When Halley visited St. Helena
in 1677, it seemed of the fourth magnitude; but Lacaille in
the middle of the following century, and others after him, classed
it as of the second. In 1827 the traveller Burchell, being then at
St. Paul, near Rio Janeiro, remarked that it had unexpectedly
assumed the first rank—a circumstance the more surprising to him[Pg 49]
because he had frequently, when in Africa during the years 1811 to
1815, noted it as of only fourth magnitude. This observation,
however, did not become generally known until later. Herschel,
on his arrival at Feldhausen, registered the star as a bright second,
and had no suspicion of its unusual character until December 16,
1837, when he suddenly perceived its light to be almost tripled. It
then far outshone Rigel in Orion, and on the 2nd of January
following it very nearly matched α Centauri. From that date it
declined; but a second and even brighter maximum occurred in
April, 1843, when Maclear, then director of the Cape Observatory,
saw it blaze out with a splendour approaching that of Sirius. Its
waxings and wanings were marked by curious “trepidations” of
brightness extremely perplexing to theory. In 1863 it had sunk
below the fifth magnitude, and in 1869 was barely visible to the
naked eye; yet it was not until eighteen years later that it touched
a minimum of 7·6 magnitude. Soon afterwards a recovery of
brightness set in, but was not carried very far; and the star now
shines steadily as of the seventh magnitude, its reddish light contrasting
effectively with the silvery rays of the surrounding nebula.
An attempt to include its fluctuations within a cycle of seventy
years[125] has signally failed; the extent and character of the vicissitudes
to which it is subject stamping it rather as a species of connecting
link between periodic and temporary stars.[126]
Among the numerous topics which engaged Herschel’s attention
at the Cape was that of relative stellar brightness. Having contrived
an “astrometer” in which an “artificial star,” formed by the
total reflection of moonlight from the base of a prism, served as a
standard of comparison, he was able to estimate the lustre of the
natural stars examined by the distances at which the artificial object
appeared equal respectively to each. He thus constructed a table
of 191 of the principal stars,[127] both in the northern and southern
hemispheres, setting forth the numerical values of their apparent
brightness relatively to that of α Centauri, which he selected as a
unit of measurement. Further, the light of the full moon being
found by him to exceed that of his standard star 27,408 times, and
Dr. Wollaston having shown that the light of the full moon is to
that of the sun as 1:801,072[128] (Zöllner made the ratio 1:618,000),
it became possible to compare stellar with solar radiance. Hence
was derived, in the case of the few stars at ascertained distances,
a knowledge of real lustre. Alpha Centauri, for example, emits less[Pg 50]
than twice, Capella one hundred times as much light as our sun;
while Arcturus, at its enormous distance, must display the splendour
of 1,300 such luminaries.
Herschel returned to England in the spring of 1838, bringing
with him a wealth of observation and discovery such as had
perhaps never before been amassed in so short a time. Deserved
honours awaited him. He was created a baronet on the occasion
of the Queen’s coronation (he had been knighted in 1831);
universities and learned societies vied with each other in showering
distinctions upon him; and the success of an enterprise in which
scientific zeal was tinctured with an attractive flavour of adventurous
romance, was justly regarded as a matter of national pride. His
career as an observing astronomer was now virtually closed, and
he devoted his leisure to the collection and arrangement of the
abundant trophies of his father’s and his own activity. The resulting
great catalogue of 5,079 nebulæ (including all then certainly
known), published in the Philosophical Transactions for 1864, is, and
will probably long remain, the fundamental source of information
on the subject;[129] but he unfortunately did not live to finish the companion
work on double stars, for which he had accumulated a vast
store of materials.[130] He died at Collingwood in Kent, May 11, 1871,
in the eightieth year of his age, and was buried in Westminster
Abbey, close beside the grave of Sir Isaac Newton.
The consideration of Sir John Herschel’s Cape observations
brings us to the close of the period we are just now engaged in
studying. They were given to the world, as already stated, three
years before the middle of the century, and accurately represent the
condition of sidereal science at that date. Looking back over the
fifty years traversed, we can see at a glance how great was the stride
made in the interval. Not alone was acquaintance with individual
members of the cosmos vastly extended, but their mutual relations,
the laws governing their movements, their distances from the
earth, masses, and intrinsic lustre, had begun to be successfully
investigated. Begun to be; for only regarding a scarcely perceptible
minority had even approximate conclusions been arrived at. Nevertheless
the whole progress of the future lay in that beginning; it
was the thin end of the wedge of exact knowledge. The principle[Pg 51]
of measurement had been substituted for that of probability; a basis
had been found large and strong enough to enable calculation to
ascend from it to the sidereal heavens; and refinements had been
introduced, fruitful in performance, but still more in promise.
Thus, rather the kind than the amount of information collected was
significant for the time to come—rather the methods employed than
the results actually secured rendered the first half of the nineteenth
century of epochal importance in the history of our knowledge of
the stars.[Pg 52]
FOOTNOTES:
[58] Bessel, Populäre Vorlesungen, pp. 6, 408.
[59] Fitted to the old transit instrument, July 11, 1772.
[60] Briefwechsel mit Olbers, p. xvi.
[61] R. Wolf, Gesch. der Astron., p. 518.
[62] Bessel, Pop. Vorl., p. 22.
[63] A new reduction of the observations upon which they were founded was
undertaken in 1896 by Herman S. Davis, of the U.S. Coast Survey.
[64] Bessel, Pop. Vorl., p. 440.
[65] Durège, Bessel’s Leben und Wirken, p. 28.
[66] Bonner Beobachtungen, Bd. iii.-v., 1859-62.
[67] Bessel, Pop. Vorl., p. 238.
[68] The heads of the screws applied to move the halves of the object-glass in the
Königsberg heliometer are of so considerable a size that a thousandth part of a
revolution, equivalent to 1/20 of a second of arc, can be measured with the utmost
accuracy. Main, R. A. S. Mem., vol. xii., p. 53.
[69] Specola Astronomica di Palermo, lib. vi., p. 10, note.
[70] Monatliche Correspondenz, vol. xxvi., p. 162.
[71] Astronomische Nachrichten, Nos. 365-366. It should be explained that what
is called the “annual parallax” of a star is only half its apparent displacement.
In other words, it is the angle subtended at the distance of that particular star
by the radius of the earth’s orbit.
[72] Astr. Nach., Nos. 401-402.
[73] Sir R. Ball’s measurements at Dunsink gave to 61 Cygni a parallax of 0·47′;
Professor Pritchard obtained, by photographic determinations, one of 0·43′.
[74] Additamentum in Mensuras Micrometricas, p. 28.
[75] Elkin’s corrected result (in 1897) for the parallax of Vega is 0·082′.
[76] Mem. Roy. Astr. Soc., vol. xi., p. 61.
[77] That numbered 21,185 in Lalande’s Hist. Cél., found by Argelander to have
a proper motion of 4·734′, and by Winnecke a parallax of O·511′. Month. Not.,
vol. xviii., p. 289.
[78] Fund. Astr., p. 309.
[79] Mém. Prés. à l’Ac. de St. Pétersb., t. iii.
[80] Phil. Trans., vol. cxxxvii., p. 79.
[81] Mem. Roy. Astr. Soc., vols. xxviii. and xxxii.
[82] Ibid., vol. xlvii., p. 327.
[83] Mémoires de St. Pétersbourg, t. xxxv., No. 3, 1887; revised in Astr. Nach.,
Nos. 3,729-30, 1901.
[84] Astronomical Journal, Nos. 213, 501.
[85] Astr. Nach., Nos. 2,999, 3,000.
[86] Veröffentlichungen der Grossh. Sternwarte zu Karlsruhe, Bd. iv., 1892.
[87] Proceedings Amsterdam Acad. of Sciences, Jan. 27, 1900.
[88] Astr. Jour., No. 457.
[89] Ibid., Nos. 276, 497.
[90] Phil. Trans., vol. xcvi., p. 230.
[91] Mém. Prés. à l’Ac. de St. Pétersbourg, t. iii., p. 603 (read Feb. 5, 1837).
[92] Die Centralsonne, Astr. Nach., Nos. 566-567, 1846.
[93] Sir J. Herschel, note to Treatise on Astronomy, and Phil. Trans., vol. cxxiii.,
part ii., p. 502.
[94] The position is (as Sir J. Herschel pointed out, Outlines of Astronomy, p. 631,
10th ed.) placed beyond the range of reasonable probability by its remoteness
(fully 26°) from the galactic plane.
[95] Mädler in Westermann’s Jahrbuch, 1867, p. 615.
[96] Letter from Bessel to Sir J. Herschel, Month. Not., vol. vi., p. 139.
[97] Wolf, Gesch. d. Astr., p. 743, note.
[98] Astr. Nach., Nos. 745-748.
[99] Astr. Jour., No. 440.
[100] Adopting Elkin’s revised parallax for Procyon of 0·325′.
[101] Astr. Nach., Nos. 1371-1373.
[102] Ueber die Doppelsterne, Bericht, 1827, p. 22.
[103] Ueber die Doppelsterne, Bericht, 1827, p. 25.
[104] Mensuræ Micr., p. xcix.
[105] Stellarum Fixarum imprimis Duplicium et Multiplicum Positiones Mediæ,
pp. cxc., cciii.
[106] For instance, the southern stars, 36A Ophiuchi (itself double) and 30
Scorpii, which are 12′ 10″ apart. Ibid., p. cciii.
[107] Stellarum Fixarum, etc., p. ccliii.
[108] Études d’Astronomie Stellaire, 1847, p. 82.
[109] Ibid., p. 86.
[110] See Encke’s criticism in Astr. Nach., No. 622.
[111] Phil. Trans., vol. cxiv., part iii., 1824.
[112] Conn. d. Temps, 1830.
[113] R. A. S. Mem., vol. v., p. 178, 1833.
[114] Astr. and Astrophysics, vol. xii., p. 581.
[115] Popular Astr., vol. i., p. 243.
[116] Phil. Trans., vol. cxxiii., and Results, etc., Introd.
[117] Results of Astronomical Observations made during the years 1834-8 at the
Cape of Good Hope.
[118] Results, etc., p. 147.
[119] See Proctor’s Universe of Stars, p. 92.
[120] A Treatise on Astronomy, 1833, p. 406.
[121] Results, etc., p. 139.
[122] Ibid., pp. 24, 142.
[123] Phil. Trans., vol. cxxiii., p. 503.
[124] Results, etc., p. 136.
[125] Loomis, Month. Not., vol. xxix., p. 298.
[126] See the Author’s System of the Stars, pp. 116-120.
[127] Outlines of Astr., App. I.
[128] Phil. Trans., vol. cxix., p. 27.
[129] Dr. Dreyer’s New General Catalogue, published in 1888 as vol. xlix. of the
Royal Astronomical Society’s Memoirs, is an enlargement of Herschel’s work. It
includes 7,840 entries, and was supplemented, in 1895, by an “Index Catalogue”
of 1,529 nebulæ discovered 1888 to 1894. Mem. R. A. S., vol. li.
[130] A list of 10,320 composite stars was drawn out by him in order of right
ascension, and has been published in vol. xl. of Mem. R. A. S.; but the data
requisite for their formation into a catalogue were not forthcoming. See Main’s
and Pritchard’s Preface to above, and Dunkin’s Obituary Notices, p. 73.
CHAPTER III
PROGRESS OF KNOWLEDGE REGARDING THE SUN
The discovery of sun-spots in 1610 by Fabricius and Galileo
first opened a way for inquiry into the solar constitution; but it
was long before that way was followed with system or profit.
The seeming irregularity of the phenomena discouraged continuous
attention; casual observations were made the basis of arbitrary conjectures,
and real knowledge received little or no increase. In 1620
we find Jean Tarde, Canon of Sarlat, arguing that because the sun
is “the eye of the world,” and the eye of the world cannot suffer from
ophthalmia, therefore the appearances in question must be due, not to
actual specks or stains on the bright solar disc, but to the transits
of a number of small planets across it! To this new group of
heavenly bodies he gave the name of “Borbonia Sidera,” and they
were claimed in 1633 for the House of Hapsburg, under the title of
“Austriaca Sidera” by Father Malapertius, a Belgian Jesuit.[131] A
similar view was temporarily maintained against Galileo by the
justly celebrated Father Scheiner of Ingolstadt, and later by
William Gascoigne, the inventor of the micrometer; but most of
those who were capable of thinking at all on such subjects (and they
were but few) adhered either to the cloud theory or to the slag theory
of sun-spots. The first was championed by Galileo, the second
by Simon Marius, “astronomer and physician” to the brother
Margraves of Brandenburg. The latter opinion received a further
notable development from the fact that in 1618, a year remarkable
for the appearance of three bright comets, the sun was almost free
from spots; whence it was inferred that the cindery refuse from the
great solar conflagration, which usually appeared as dark blotches
on its surface, was occasionally thrown off in the form of comets,
leaving the sun, like a snuffed taper, to blaze with renewed
brilliancy.[132]
In the following century, Derham gathered from observations
carried on during the years 1703-11, “That the spots on the sun are
caused by the eruption of some new volcano therein, which at first
pouring out a prodigious quantity of smoke and other opacous
matter, causeth the spots; and as that fuliginous matter decayeth
and spendeth itself, and the volcano at last becomes more torrid and
flaming, so the spots decay, and grow to umbræ, and at last to
faculæ.”[133]
The view, confidently upheld by Lalande,[134] that spots were rocky
elevations uncovered by the casual ebbing of a luminous ocean, the
surrounding penumbræ representing shoals or sandbanks, had even
less to recommend it than Derham’s volcanic theory. Both were,
however, significant of a growing tendency to bring solar phenomena
within the compass of terrestrial analogies.
For 164 years, then, after Galileo first levelled his telescope at
the setting sun, next to nothing was learned as to its nature; and
the facts immediately ascertained, of its rotation on an axis nearly
erect to the plane of the ecliptic, in a period of between twenty-five
and twenty-six days, and of the virtual limitation of the spots to a
so-called “royal” zone extending some thirty degrees north and
south of the solar equator, gained little either in precision or
development from five generations of astronomers.
But in November, 1769, a spot of extraordinary size engaged the
attention of Alexander Wilson, professor of astronomy in the
University of Glasgow. He watched it day by day, and to good
purpose. As the great globe slowly revolved, carrying the spot
towards its western edge, he was struck with the gradual contraction
and final disappearance of the penumbra on the side next the
centre of the disc; and when on the 6th of December the same spot re-emerged
on the eastern limb, he perceived, as he had anticipated, that
the shady zone was now deficient on the opposite side, and resumed its
original completeness as it returned to a central position. In other
spots subsequently examined by him, similar perspective effects were
visible, and he proved in 1774,[135] by strict geometrical reasoning, that
they could only arise in vast photospheric excavations. It was not,[Pg 54]
indeed, the first time that such a view had been suggested. Father
Scheiner’s later observations plainly foreshadowed it;[136] a conjecture
to the same effect was emitted by Leonard Rost of Nuremburg early
in the eighteenth century;[137] both by Lahire in 1703 and by
J. Cassini in 1719 spots had been seen as notches on the solar limb;
while in 1770 Pastor Schülen of Essingen, from the careful study of
phenomena similar to those noted by Wilson, concluded their depressed
nature.[138] Modern observations, nevertheless, prove those
phenomena to be by no means universally present.
Wilson’s general theory of the sun was avowedly tentative. It
took the modest form of an interrogatory. “Is it not reasonable to
think,” he asks, “that the great and stupendous body of the sun is
made up of two kinds of matter, very different in their qualities;
that by far the greater part is solid and dark, and that this immense
and dark globe is encompassed with a thin covering of that resplendent
substance from which the sun would seem to derive the
whole of his vivifying heat and energy?”[139] He further suggests that
the excavations or spots may be occasioned “by the working of some
sort of elastic vapour which is generated within the dark globe,” and
that the luminous matter, being in some degree fluid, and being
acted upon by gravity, tends to flow down and cover the nucleus.
From these hints, supplemented by his own diligent observations
and sagacious reasonings, Herschel elaborated a scheme of solar constitution
which held its ground until the physics of the sun were
revolutionised by the spectroscope.
A cool, dark, solid globe, its surface diversified with mountains
and valleys, clothed in luxuriant vegetation, and “richly stored with
inhabitants,” protected by a heavy cloud-canopy from the intolerable
glare of the upper luminous region, where the dazzling coruscations
of a solar aurora some thousands of miles in depth evolved the
stores of light and heat which vivify our world—such was the
central luminary which Herschel constructed with his wonted
ingenuity, and described with his wonted eloquence.
“This way of considering the sun and its atmosphere,” he says,[140]
“removes the great dissimilarity we have hitherto been used to find
between its condition and that of the rest of the great bodies of
the solar system. The sun, viewed in this light, appears to be
nothing else than a very eminent, large, and lucid planet, evidently
the first, or, in strictness of speaking, the only primary one of our
system; all others being truly secondary to it. Its similarity to
the other globes of the solar system with regard to its solidity, its[Pg 55]
atmosphere, and its diversified surface, the rotation upon its axis,
and the fall of heavy bodies, leads us on to suppose that it is most
probably also inhabited, like the rest of the planets, by beings
whose organs are adapted to the peculiar circumstances of that
vast globe.”
We smile at conclusions which our present knowledge condemns
as extravagant and impossible, but such incidental flights of fancy
in no way derogate from the high value of Herschel’s contributions
to solar science. The cloud-like character which he attributed to
the radiant shell of the sun (first named by Schröter the “photosphere”)
is borne out by all recent investigations; he observed its
mottled or corrugated aspect, resembling, as he described it, the
roughness on the rind of an orange; showed that “faculæ” are
elevations or heaped-up ridges of the disturbed photospheric matter;
and threw out the idea that spots may ensue from an excess of the
ordinary luminous emissions. A certain “empyreal” gas was, he
supposed (very much as Wilson had done), generated in the body
of the sun, and rising everywhere by reason of its lightness, made
for itself, when in moderate quantities, small openings or “pores,”[141]
abundantly visible as dark points on the solar disc. But should an
uncommon quantity be formed, “it will,” he maintained, “burst
through the planetary[142] regions of clouds, and thus will produce
great openings; then, spreading itself above them, it will occasion
large shallows (penumbræ), and mixing afterwards gradually with
other superior gases, it will promote the increase, and assist in the
maintenance, of the general luminous phenomena.”[143]
This partial anticipation of the modern view that the solar radiations
are maintained by some process of circulation within the solar
mass, was reached by Herschel through prolonged study of the
phenomena in question. The novel and important idea contained in
it, however, it was at that time premature to attempt to develop.
But though many of the subtler suggestions of Herschel’s genius
passed unnoticed by his contemporaries, the main result of his solar
researches was an unmistakable one. It was nothing less than the
definitive introduction into astronomy of the paradoxical conception
of the central fire and hearth of our system as a cold, dark,
terrestrial mass, wrapt in a mantle of innocuous radiance—an earth,
so to speak, within—a sun without.
Let us pause for a moment to consider the value of this remarkable
innovation. It certainly was not a step in the direction of[Pg 56]
truth. On the contrary, the crude notions of Anaxagoras and Xeno
approached more nearly to what we now know of the sun, than the
complicated structure devised for the happiness of a nobler race
of beings than our own by the benevolence of eighteenth-century
astronomers. And yet it undoubtedly constituted a very important
advance in science. It was the first earnest attempt to bring solar
phenomena within the compass of a rational system; to put together
into a consistent whole the facts ascertained; to fabricate, in short,
a solar machine that would in some fashion work. It is true that
the materials were inadequate and the design faulty. The resulting
construction has not proved strong enough to stand the wear and
tear of time and discovery, but has had to be taken to pieces and
remodelled on a totally different plan. But the work was not therefore
done in vain. None of Bacon’s aphorisms show a clearer
insight into the relations between the human mind and the external
world than that which declares “Truth to emerge sooner from error
than from confusion.”[144] A definite theory (even if a false one) gives
holding-ground to thought. Facts acquire a meaning with reference
to it. It affords a motive for accumulating them and a means of
co-ordinating them; it provides a framework for their arrangement,
and a receptacle for their preservation, until they become too strong
and numerous to be any longer included within arbitrary limits, and
shatter the vessel originally framed to contain them.
Such was the purpose subserved by Herschel’s theory of the sun.
It helped to clarify ideas on the subject. The turbid sense of
groping and viewless ignorance gave place to the lucidity of a
possible scheme. The persuasion of knowledge is a keen incentive
to its increase. Few men care to investigate what they are obliged
to admit themselves entirely ignorant of; but once started on the
road of knowledge, real or supposed, they are eager to pursue it.
By the promulgation of a confident and consistent view regarding
the nature of the sun, accordingly, research was encouraged, because
it was rendered hopeful, and inquirers were shown a path leading
indefinitely onwards where an impassable thicket had before seemed
to bar the way.
We have called the “terrestrial” theory of the sun’s nature an
innovation, and so, as far as its general acceptance is concerned, it
may justly be termed; but, like all successful innovations, it was a
long time brewing. It is extremely curious to find that Herschel
had a predecessor in its advocacy who never looked through a
telescope (nor, indeed, imagined the possibility of such an instrument),
who knew nothing of sun-spots, was still (mistaken assertions[Pg 57]
to the contrary notwithstanding) in the bondage of the geocentric
system, and regarded nature from the lofty standpoint of an idealist
philosophy. This was the learned and enlightened Cardinal Cusa, a
fisherman’s son from the banks of the Moselle, whose distinguished
career in the Church and in literature extended over a considerable
part of the fifteenth century (1401-64). In his singular treatise De
Doctâ Ignorantiâ, one of the most notable literary monuments of the
early Renaissance, the following passage occurs:—”To a spectator
on the surface of the sun, the splendour which appears to us would
be invisible, since it contains, as it were, an earth for its central mass,
with a circumferential envelope of light and heat, and between the
two an atmosphere of water and clouds and translucent air.” The
luminary of Herschel’s fancy could scarcely be more clearly portrayed;
some added words, however, betray the origin of the Cardinal’s idea.
“The earth also,” he says, “would appear as a shining star to any
one outside the fiery element.” It was, in fact, an extension to the
sun of the ancient elemental doctrine; but an extension remarkable
at that period, as premonitory of the tendency, so powerfully
developed by subsequent discoveries, to assimilate the orbs of heaven
to the model of our insignificant planet, and to extend the brotherhood
of our system and our species to the farthest limit of the
visible or imaginable universe.
In later times we find Flamsteed communicating to Newton,
March 7, 1681, his opinion “that the substance of the sun is
terrestrial matter, his light but the liquid menstruum encompassing
him.”[145] Bode in 1776 arrived independently at the conclusion that
“the sun is neither burning nor glowing, but in its essence a dark
planetary body, composed like our earth of land and water, varied
by mountains and valleys, and enveloped in a vaporous atmosphere”;[146]
and the learned in general applauded and acquiesced. The view,
however, was in 1787 still so far from popular, that the holding of
it was alleged as a proof of insanity in Dr. Elliot when accused of a
murderous assault on Miss Boydell. His friend Dr. Simmons stated
on his behalf that he had received from him in the preceding January
a letter giving evidence of a deranged mind, wherein he asserted
“that the sun is not a body of fire, as hath been hitherto supposed,
but that its light proceeds from a dense and universal aurora, which
may afford ample light to the inhabitants of the surface beneath,
and yet be at such a distance aloft as not to annoy them. No
objection, he saith, ariseth to that great luminary’s being inhabited;
vegetation may obtain there as well as with us. There may be
water and dry land, hills and dales, rain and fair weather; and as[Pg 58]
the light, so the season must be eternal, consequently it may easily
be conceived to be by far the most blissful habitation of the whole
system!” The Recorder, nevertheless, objected that if an extravagant
hypothesis were to be adduced as proof of insanity, the same
might hold good with regard to some other speculators, and desired
Dr. Simmons to tell the court what he thought of the theories of
Burnet and Buffon.[147]
Eight years later, this same “extravagant hypothesis,” backed by
the powerful recommendation of Sir William Herschel, obtained
admittance to the venerable halls of science, there to abide undisturbed
for nearly seven decades. Individual objectors, it is true,
made themselves heard, but their arguments had little effect on the
general body of opinion. Ruder blows were required to shatter an
hypothesis flattering to human pride of invention in its completeness,
in the plausible detail of observations by which it seemed to be
supported, and in its condescension to the natural pleasure in discovering
resemblance under all but total dissimilarity.
Sir John Herschel included among the results of his multifarious
labours at the Cape of Good Hope a careful study of the sun-spots
conspicuously visible towards the end of the year 1836 and in the
early part of 1837. They were remarkable, he tells us, for their
forms and arrangement, as well as for their number and size; one
group, measured on the 29th of March in the latter year, covering
(apart from what may be called its outlying dependencies) the vast
area of five square minutes or 3,780 million square miles.[148] We have
at present to consider, however, not so much these observations in
themselves, as the chain of theoretical suggestions by which they
were connected. The distribution of spots, it was pointed out, on
two zones parallel to the equator, showed plainly their intimate
connection with the solar rotation, and indicated as their cause fluid
circulations analogous to those producing the terrestrial trade and
anti-trade winds.
“The spots, in this view of the subject,” he went on to say,[149]
“would come to be assimilated to those regions on the earth’s
surface where, for the moment, hurricanes and tornadoes prevail;
the upper stratum being temporarily carried downwards, displacing
by its impetus the two strata of luminous matter beneath, the upper
of course to a greater extent than the lower, and thus wholly or
partially denuding the opaque surface of the sun below. Such
processes cannot be unaccompanied by vorticose motions, which, left
to themselves, die away by degrees and dissipate, with the peculiarity
that their lower portions come to rest more speedily than their upper,[Pg 59]
by reason of the greater resistance below, as well as the remoteness
from the point of action, which lies in a higher region, so that their
centres (as seen in our waterspouts, which are nothing but small
tornadoes) appear to retreat upwards. Now this agrees perfectly
with what is observed during the obliteration of the solar spots,
which appear as if filled in by the collapse of their sides, the
penumbra closing in upon the spot and disappearing after it.”
But when it comes to be asked whether a cause can be found
by which a diversity of solar temperature might be produced
corresponding with that which sets the currents of the terrestrial
atmosphere in motion, we are forced to reply that we know of no
such cause. For Sir John Herschel’s hypothesis of an increased
retention of heat at the sun’s equator, due to the slightly spheroidal
or bulging form of its outer atmospheric envelope, assuredly gives
no sufficient account of such circulatory movements as he supposed
to exist. Nevertheless, the view that the sun’s rotation is intimately
connected with the formation of spots is so obviously correct, that
we can only wonder it was not thought of sooner, while we are even
now unable to explain with any certainty how it is so connected.
Mere scrutiny of the solar surface, however, is not the only means
of solar observation. We have a satellite, and that satellite from
time to time acts most opportunely as a screen, cutting off a part or
the whole of those dazzling rays in which the master-orb of our
system veils himself from over-curious regards. The importance of
eclipses to the study of the solar surroundings is of comparatively
recent recognition; nevertheless, much of what we know concerning
them has been snatched, as it were, by surprise under favour of the
moon. In former times, the sole astronomical use of such incidents
was the correction of the received theories of the solar and lunar
movements; the precise time of their occurrence was the main fact
to be noted, and subsidiary phenomena received but casual attention.
Now, their significance as a geometrical test of tabular accuracy is
altogether overshadowed by the interest attaching to the physical
observations for which they afford propitious occasions. This change
may be said to date, in its pronounced form, from the great eclipse
of 1842. Although a necessary consequence of the general direction
taken by scientific progress, it remains associated in a special manner
with the name of Francis Baily.
The “philosopher of Newbury” was by profession a London
stockbroker, and a highly successful one. Nevertheless, his services
to science were numerous and invaluable, though not of the brilliant
kind which attract popular notice. Born at Newbury in Berkshire,
April 28, 1774, and placed in the City at the age of fourteen, he
derived from the acquaintance of Dr. Priestley a love of science[Pg 60]
which never afterwards left him. It was, however, no passion such
as flames up in the brain of the destined discoverer, but a regulated
inclination, kept well within the bounds of an actively pursued
commercial career. After travelling for a year or two in what were
then the wilds of North America, he went on the Stock Exchange
in 1799, and earned during twenty-four years of assiduous application
to affairs a high reputation for integrity and ability, to which
corresponded an ample fortune. In the meantime the Astronomical
Society (largely through his co-operation) had been founded; he had
for three years acted as its secretary, and he now felt entitled to
devote himself exclusively to a subject which had long occupied his
leisure hours. He accordingly in 1825 retired from business,
purchased a house in Tavistock Place, and fitted up there a small
observatory. He was, however, by preference a computator rather
than an observer. What Sir John Herschel calls the “archæology
of practical astronomy” found in him an especially zealous student.
He re-edited the star-catalogues of Ptolemy, Ulugh Beigh, Tycho
Brahe, Hevelius, Halley, Flamsteed, Lacaille, and Mayer; calculated
the eclipse of Thales and the eclipse of Agathocles, and vindicated
the memory of the first Astronomer Royal. But he was no less
active in meeting present needs than in revising past performances.
The subject of the reduction of observations, then, as we have already
explained,[150] in a state of deplorable confusion, attracted his most
earnest attention, and he was close on the track of Bessel when made
acquainted with the method of simplification devised at Königsberg.
Anticipated as an inventor, he could still be of eminent use as a
promoter of these valuable improvements; and, carrying them out
on a large scale in the star-catalogue of the Astronomical Society
(published in 1827), “he put” (in the words of Herschel) “the
astronomical world in possession of a power which may be said,
without exaggeration, to have changed the face of sidereal
astronomy.”[151]
His reputation was still further enhanced by his renewal, with
vastly improved apparatus, of the method, first used by Henry
Cavendish in 1797-98, for determining the density of the earth.
From a series of no less than 2,153 delicate and difficult experiments,
conducted at Tavistock Place during the years 1838-42, he concluded
our planet to weigh 5·66 as much as a globe of water of the
same bulk; and this result slightly corrected is still accepted as a
very close approximation of the truth.
What we have thus glanced at is but a fragment of the truly
surprising mass of work accomplished by Baily in the course of a[Pg 61]
variously occupied life. A rare combination of qualities fitted him
for his task. Unvarying health, undisturbed equanimity, methodical
habits, the power of directed and sustained thought, combined to
form in him an intellectual toiler of the surest, though not perhaps
of the highest quality. He was in harness almost to the end. He
was destined scarcely to know the miseries of enforced idleness or
of consciously failing powers. In 1842 he completed the laborious
reduction of Lalande’s great catalogue, undertaken at the request of
the British Association, and was still engaged in seeing it through
the press when he was attacked with what proved his last, as it
was probably his first serious illness. He, however, recovered sufficiently
to attend the Oxford Commemoration of July 2, 1844, where
an honorary degree of D.C.L. was conferred upon him in company
with Airy and Struve; but sank rapidly after the effort, and died on
the 30th of August following, at the age of seventy, lamented and
esteemed by all who knew him.
It is now time to consider his share in the promotion of solar
research. Eclipses of the sun, both ancient and modern, were
a speciality with him, and he was fortunate in those which came
under his observation. Such phenomena are of three kinds—partial,
annular, and total. In a partial eclipse, the moon, instead of passing
directly between us and the sun, slips by, as it were, a little on one
side, thus cutting off from our sight only a portion of his surface.
An annular eclipse, on the other hand, takes place when the moon
is indeed centrally interposed, but falls short of the apparent size
required for the entire concealment of the solar disc, which consequently
remains visible as a bright ring or annulus, even when the
obscuration is at its height. In a total eclipse, on the contrary, the
sun completely disappears behind the dark body of the moon. The
difference of the two latter varieties is due to the fact that the
apparent diameter of the sun and moon are so nearly equal as to
gain alternate preponderance one over the other through the slight
periodical changes in their respective distances from the earth.
Now, on the 15th of May, 1836, an annular eclipse was visible in
the northern parts of Great Britain, and was observed by Baily at
Inch Bonney, near Jedburgh. It was here that he saw the phenomenon
which obtained the name of “Baily’s Beads,” from the
notoriety conferred upon it by his vivid description.
“When the cusps of the sun,” he writes, “were about 40° asunder,
a row of lucid points, like a string of bright beads, irregular in size
and distance from each other, suddenly formed round that part of the
circumference of the moon that was about to enter on the sun’s disc.
Its formation, indeed, was so rapid that it presented the appearance
of having been caused by the ignition of a fine train of gunpowder.[Pg 62]
Finally, as the moon pursued her course, the dark intervening spaces
(which, at their origin, had the appearance of lunar mountains in
high relief, and which still continued attached to the sun’s border)
were stretched out into long, black, thick, parallel lines, joining the
limbs of the sun and moon; when all at once they suddenly gave
way, and left the circumference of the sun and moon in those points,
as in the rest, comparatively smooth and circular, and the moon
perceptibly advanced on the face of the sun.”[152]
These curious appearances were not an absolute novelty. Weber
in 1791, and Von Zach in 1820, had seen the “beads”; Van
Swinden had described the “belts” or “threads.”[153] These last were,
moreover (as Baily clearly perceived), completely analogous to the
“black ligament” which formed so troublesome a feature in the
transits of Venus in 1764 and 1769, and which, to the regret and
confusion, though no longer to the surprise of observers, was renewed
in that of 1874. The phenomenon is largely an effect of what is
called irradiation, by which a bright object seems to encroach upon
a dark one; but under good atmospheric and instrumental conditions
it becomes inconspicuous. The “Beads” must always appear when
the projected lunar edge is serrated with mountains. In Baily’s
observation, they were exaggerated and distorted by an irradiative
clinging together of the limbs of sun and moon.
The immediate result, however, was powerfully to stimulate
attention to solar eclipses in their physical aspect. Never before had
an occurrence of the kind been expected so eagerly or prepared for
so actively as that which was total over Central and Southern
Europe on the 8th of July, 1842. Astronomers hastened from all
quarters to the favoured region. The Astronomer Royal (Airy)
repaired to Turin; Baily to Pavia; Otto Struve threw aside his
work amidst the stars at Pulkowa, and went south as far as Lipeszk;
Schumacher travelled from Altona to Vienna; Arago from Paris to
Perpignan. Nor did their trouble go unrewarded. The expectations
of the most sanguine were outdone by the wonders disclosed.
Baily (to whose narrative we again have recourse) had set up his
Dollond’s achromatic in an upper room of the University of Pavia,
and was eagerly engaged in noting a partial repetition of the singular
appearances seen by him in 1836, when he was “astounded by a
tremendous burst of applause from the streets below, and at the
same moment was electrified at the sight of one of the most brilliant
and splendid phenomena that can well be imagined. For at that
instant the dark body of the moon was suddenly surrounded with
a corona, or kind of bright glory similar in shape and relative
magnitude to that which painters draw round the heads of saints,[Pg 63]
and which by the French is designated an auréole. Pavia contains
many thousand inhabitants, the major part of whom were, at this
early hour, walking about the streets and squares or looking out of
windows, in order to witness this long-talked-of phenomenon; and
when the total obscuration took place, which was instantaneous, there
was a universal shout from every observer, which ‘made the welkin
ring,’ and, for the moment, withdrew my attention from the object
with which I was immediately occupied. I had indeed anticipated
the appearance of a luminous circle round the moon during the time
of total obscurity; but I did not expect, from any of the accounts
of preceding eclipses that I had read, to witness so magnificent an
exhibition as that which took place…. The breadth of the
corona, measured from the circumference of the moon, appeared
to me to be nearly equal to half the moon’s diameter. It had
the appearance of brilliant rays. The light was most dense close
to the border of the moon, and became gradually and uniformly
more attenuate as its distance therefrom increased, assuming the
form of diverging rays in a rectilinear line, which at the extremity
were more divided, and of an unequal length; so that in
no part of the corona could I discover the regular and well-defined
shape of a ring at its outer margin. It appeared to me to have the
sun for its centre, but I had no means of taking any accurate
measures for determining this point. Its colour was quite white,
not pearl-colour, nor yellow, nor red, and the rays had a vivid and
flickering appearance, somewhat like that which a gaslight illumination
might be supposed to assume if formed into a similar shape….
Splendid and astonishing, however, as this remarkable phenomenon
really was, and although it could not fail to call forth the admiration
and applause of every beholder, yet I must confess that there was at
the same time something in its singular and wonderful appearance
that was appalling; and I can readily imagine that uncivilised
nations may occasionally have become alarmed and terrified at such
an object, more especially at times when the true cause of the
occurrence may have been but faintly understood, and the phenomenon
itself wholly unexpected.
“But the most remarkable circumstance attending the phenomenon
was the appearance of three large protuberances apparently emanating
from the circumference of the moon, but evidently forming a portion
of the corona. They had the appearance of mountains of a prodigious
elevation; their colour was red, tinged with lilac or purple;
perhaps the colour of the peach-blossom would more nearly represent
it. They somewhat resembled the snowy tops of the Alpine mountains
when coloured by the rising or setting sun. They resembled
the Alpine mountains also in another respect, inasmuch as their[Pg 64]
light was perfectly steady, and had none of that flickering or sparkling
motion so visible in other parts of the corona. All the three
projections were of the same roseate cast of colour, and very different
from the brilliant vivid white light that formed the corona; but they
differed from each other in magnitude…. The whole of these
three protuberances were visible even to the last moment of total
obscuration; at least, I never lost sight of them when looking in
that direction; and when the first ray of light was admitted from
the sun, they vanished, with the corona, altogether, and daylight
was instantaneously restored.”[154]
Notwithstanding unfavourable weather, the “red flames” were
perceived with little less clearness and no less amazement from the
Superga than at Pavia, and were even discerned by Mr. Airy with
the naked eye. “Their form” (the Astronomer Royal wrote) “was
nearly that of saw-teeth in the position proper for a circular saw
turned round in the same direction in which the hands of a watch
turn…. Their colour was a full lake-red, and their brilliancy
greater than that of any other part of the ring.”[155]
The height of these extraordinary objects was estimated by Arago
at two minutes of arc, representing, at the sun’s distance, an actual
elevation of 54,000 miles. When carefully watched, the rose-flush of
their illumination was perceived to fade through violet to white as
the light returned, the same changes in a reversed order having
accompanied their first appearance. Their forms, however, during
about three minutes of visibility, showed no change, although of so
apparently unstable a character as to suggest to Arago “mountains
on the point of crumbling into ruins” through topheaviness.[156]
The corona, both as to figure and extent, presented very different
appearances at different stations. This was no doubt due to varieties
in atmospheric conditions. At the Superga, for instance, all details
of structure seem to have been effaced by the murky air, only a
comparatively feeble ring of light being seen to encircle the moon.
Elsewhere, a brilliant radiated formation was conspicuous, spreading
at four opposite points into four vast luminous expansions, compared
to feather-plumes or aigrettes.[157] Arago at Perpignan noticed considerable
irregularities in the divergent rays. Some appeared curved
and twisted, a few lay across the others, in a direction almost tangential
to the moon’s limb, the general effect being described as that
of a “hank of thread in disorder.”[158] At Lipeszk, where the sun
stood much higher above the horizon than in Italy or France, the
corona showed with surprising splendour. Its apparent extent was
judged by Struve to be no less than twenty-five minutes (more than[Pg 65]
six times Airy’s estimate), while the great plumes spread their
radiance to three or four degrees from the dark lunar edge. So
dazzling was the light that many well-instructed persons denied
the totality of the eclipse. Nor was the error without precedent,
although the appearances attending respectively a total and an
annular eclipse are in reality wholly dissimilar. In the latter case,
the surviving ring of sunlight becomes so much enlarged by irradiation,
that the interposed dark lunar body is reduced to comparative
insignificance, or even invisibility. Maclaurin tells us[159] that during
an eclipse of this character which he observed at Edinburgh in 1737,
“gentlemen by no means shortsighted declared themselves unable to
discern the moon upon the sun without the aid of a smoked glass;”
and Baily (who, however, was shortsighted) could distinguish, in
1836, with the naked eye, no trace of “the globe of purple velvet”
which the telescope revealed as projected upon the face of the sun.[160]
Moreover, the diminution of light is described by him as “little
more than might be caused by a temporary cloud passing over the
sun”; the birds continued in full song, and “one cock in particular
was crowing with all his might while the annulus was forming.”
Very different were the effects of the eclipse of 1842, as to which
some interesting particulars were collected by Arago.[161] Beasts of
burthen, he tells us, paused in their labour, and could by no amount
of punishment be induced to move until the sun reappeared. Birds
and beasts abandoned their food; linnets were found dead in their
cages; even ants suspended their toil. Diligence-horses, on the
other hand, seemed as insensible to the phenomenon as locomotives.
The convolvulus and some other plants closed their leaves, but those
of the mimosa remained open. The little light that remained was
of a livid hue. One observer described the general coloration as
resembling the lees of wine, but human faces showed pale olive or
greenish. We may, then, rest assured that none of the remarkable
obscurations recorded in history were due to eclipses of the annular
kind.
The existence of the corona is no modern discovery. Indeed, it is
too conspicuous an apparition to escape notice from the least attentive
or least practised observer of a total eclipse. Nevertheless,
explicit references to it are rare in early times. Plutarch, however,
speaks of a “certain splendour” compassing round the hidden edge
of the sun, as a regular feature of total eclipses;[162] and the corona is[Pg 66]
expressly mentioned in a description of an eclipse visible at Corfu in
968 A.D.[163] The first to take the phenomenon into scientific consideration
was Kepler. He showed, from the orbital positions
at the time of the sun and moon, that an eclipse observed by
Clavius at Rome in 1567 could not have been annular,[164] as the
dazzling coronal radiance visible during the obscuration had caused
it to be believed. Although he himself never witnessed a total
eclipse of the sun, he carefully collected and compared the remarks
of those more fortunate, and concluded that the ring of “flame-like
splendour” seen on such occasions was caused by the reflection of
the solar rays from matter condensed in the neighbourhood either of
the sun or moon.[165] To the solar explanation he gave his own decided
preference; but, with one of those curious flashes of half-prophetic
insight characteristic of his genius, declared that “it should be laid
by ready for use, not brought into immediate requisition.”[166] So
literally was his advice acted upon, that the theory, which we now
know to be (broadly speaking) the correct one, only emerged from
the repository of anticipated truths after 236 years of almost
complete retirement, and even then timorously and with hesitation.
The first eclipse of which the attendant phenomena were observed
with tolerable exactness was that which was central in the South of
France, May 12, 1706. Cassini then put forward the view that the
“crown of pale light” seen round the lunar disc was caused by the
illumination of the zodiacal light;[167] but it failed to receive the
attention which, as a step in the right direction, it undoubtedly
merited. Nine years later we meet with Halley’s comments on a
similar event, the first which had occurred in London since March 20,
1140. By nine in the morning of May 3, 1715, the obscuration, he
tells us, “was about ten digits,[168] when the face and colour of the sky
began to change from perfect serene azure blue to a more dusky
livid colour, having an eye of purple intermixt…. A few seconds
before the sun was all hid, there discovered itself round the moon a
luminous ring, about a digit or perhaps a tenth part of the moon’s
diameter in breadth. It was of a pale whiteness, or rather pearl
colour, seeming to be a little tinged with the colours of the iris, and
to be concentric with the moon, whence I concluded it the moon’s
atmosphere. But the great height thereof, far exceeding our earth’s
atmosphere, and the observation of some, who found the breadth
of the ring to increase on the west side of the moon as emersion[Pg 67]
approached, together with the contrary sentiments of those whose
judgment I shall always revere” (Newton is most probably referred
to), “makes me less confident, especially in a matter whereto I
confess I gave not all the attention requisite.” He concludes by
declining to decide whether the “enlightened atmosphere,” which
the appearance “in all respects resembled,” “belonged to sun or
moon.”[169]
A French Academician, who happened to be in London at the
time, was less guarded in expressing an opinion. The Chevalier de
Louville declared emphatically for the lunar atmospheric theory of
the corona,[170] and his authority carried great weight. It was, however,
much discredited by an observation made by Maraldi in 1724,
to the effect that the luminous ring, instead of travelling with the
moon, was traversed by it.[171] This was in reality decisive, though, as
usual, belief lagged far behind demonstration. In 1715 a novel explanation
had been offered by Delisle and Lahire,[172] supported by
experiments regarded at the time as perfectly satisfactory. The
aureola round the eclipsed sun, they argued, is simply a result of
the diffraction, or apparent bending of the sunbeams that graze the
surface of the lunar globe—an effect of the same kind as the coloured
fringes of shadows. And this view prevailed amongst men of science
until (and even after) Brewster showed, with clear and simple
decisiveness, that such an effect could by no possibility be appreciable
at our distance from the moon.[173] Don José Joaquim de
Ferrer, however, who observed a total eclipse of the sun at Kinderhook,
in the State of New York, on June 16, 1806, ignoring this
refined optical rationale, considered two alternative explanations of
the phenomenon as alone possible. The bright ring round the moon
must be due to the illumination either of a lunar or of a solar
atmosphere. If the former, he calculated that it should have a
height fifty times that of the earth’s gaseous envelope. “Such an
atmosphere,” he rightly concluded, “cannot belong to the moon, but
must without any doubt belong to the sun.”[174] But he stood alone in
this unhesitating assertion.
The importance of the problem was first brought fully home to
astronomers by the eclipse of 1842. The brilliant and complex
appearance which on that occasion challenged the attention of so
many observers, demanded and received, no longer the casual attention
hitherto bestowed upon it, but the most earnest study of those[Pg 68]
interested in the progress of science. Nevertheless, it was only by
degrees, and through a process of “exclusions” (to use a Baconian
phrase) that the corona was put in its right place as a solar appendage.
As every other available explanation proved inadmissible and
dropped out of sight, the broad presentation of fact remained, which,
though of sufficiently obvious interpretation, was long and persistently
misconstrued. Nor was it until 1869 that absolutely decisive
evidence on the subject was forthcoming, as we shall see further on.
Sir John Herschel, writing to his venerable aunt, relates that
when the brilliant red flames burst into view behind the dark moon
on the morning of the 8th of July, 1842, the populace of Milan, with
the usual inconsequence of a crowd, raised the shout, “Es leben die
Astronomen!“[175] In reality, none were less prepared for their apparition
than the class to whom the applause due to the magnificent
spectacle was thus adjudged. And in some measure through their
own fault, for many partial hints and some distinct statements
from earlier observers had given unheeded notice that some such
phenomenon might be expected to attend a solar eclipse.
What we now call the “chromosphere” is an envelope of glowing
gases, by which the sun is completely covered, and from which the
“prominences” are emanations, eruptive or flame-like. Now, continual
indications of the presence of this fire-ocean had been detected
during eclipses in the eighteenth and nineteenth centuries. Captain
Stannyan, describing in a letter to Flamsteed an occurrence of the
kind witnessed by him at Berne on May 1 (o.s.), 1706, says that the
sun’s “getting out of the eclipse was preceded by a blood-red streak
of light from its left limb.”[176] A precisely similar appearance was
noted by both Halley and De Louville in 1715; during annular
eclipses by Lord Aberdour in 1737,[177] and by Short in 1748,[178] the tint
of the ruby border being, however, subdued to “brown” or “dusky
red” by the surviving sunlight; while observations identical in
character were made at Amsterdam in 1820,[179] at Edinburgh by
Henderson in 1836, and at New York in 1838.[180]
“Flames” or “prominences,” if more conspicuous, are less constant
in their presence than the glowing stratum from which they spring.
The first to describe them was a Swedish professor named Vassenius,
who observed a total eclipse at Gothenburg, May 2 (o.s.), 1733.[181][Pg 69]
His astonishment equalled his admiration when he perceived, just
outside the edge of the lunar disc, and suspended, as it seemed, in
the coronal atmosphere, three or four reddish spots or clouds, one of
which was so large as to be detected with the naked eye. As to
their nature, he did not even offer a speculation, further than by
tacitly referring them to the moon. The observation was repeated
in 1778 by a Spanish Admiral, but with no better success in directing
efficacious attention to the phenomenon. Don Antonio Ulloa
was on board his ship the Espagne in passage from the Azores to
Cape St. Vincent on the 24th of June in that year, when a total eclipse
of the sun occurred, of which he has left a valuable description. His
notices of the corona are full of interest; but what just now concerns
us is the appearance of “a red luminous point” “near the edge of
the moon,” which gradually increased in size as the moon moved
away from it, and was visible during about a minute and a quarter.[182]
He was satisfied that it belonged to the sun because of its fiery
colour and growth in magnitude, and supposed that it was occasioned
by some crevice or inequality in the moon’s limb, through
which the solar light penetrated.
Allusions less precise, both prior and subsequent, which it is now
easy to refer to similar objects (such as the “slender columns of
smoke” seen by Ferrer)[183] might be detailed; but the evidence
already adduced suffices to show that the prominences viewed with
such amazement in 1842 were no unprecedented or even unusual
phenomenon.
It was more important, however, to decide what was their nature
than whether their appearance might have been anticipated. They
were generally, and not very incorrectly, set down as solar clouds.
Arago believed them to shine by reflected light,[184] but the Abbé
Peytal rightly considered them to be self-luminous. Writing in a
Montpellier paper of July 16, 1842, he declared that we had now
become assured of the existence of a third or outer solar envelope, composed
of a glowing substance of a bright rose tint, forming mountains
of prodigious elevation, analogous in character to the clouds piled
above our horizons.[185] This first distinct recognition of a very
important feature of our great luminary was probably founded on
an observation made by Bérard at Toulon during the then recent
eclipse, “of a very fine red band, irregularly dentelated, or, as it
were, crevassed here and there,”[186] encircling a large arc of the moon’s
circumference. It can hardly, however, be said to have attracted
general notice until July 28, 1851. On that day a total eclipse[Pg 70]
took place, which was observed with considerable success in various
parts of Sweden and Norway by a number of English astronomers.
Mr. Hind saw, on the south limb of the moon, “a long range of
rose-coloured flames,”[187] described by Dawes as “a low ridge of red
prominences, resembling in outline the tops of a very irregular range
of hills.”[188] Airy termed the portion of this “rugged lines of projections”
visible to him the sierra, and was struck with its brilliant
light and “nearly scarlet” colour.[189] Its true character of a continuous
solar envelope was inferred from these data by Grant, Swan,
and Littrow, and was by Father Secchi, after the great eclipse of
1860,[190] formally accepted as established.
Several prominences of remarkable forms, especially one variously
compared to a Turkish scimitar, a sickle, and a boomerang, were
seen in 1851. In connection with them two highly significant
circumstances were pointed out. First, that of the approximate
coincidence between their positions and those of sun-spots previously
observed.[191] Next, that “the moon passed over them, leaving them
behind, and revealing successive portions as she advanced.”[192] This
latter perfectly well-attested fact was justly considered by the
Astronomer Royal and others as affording absolute certainty of the
solar dependence of these singular objects. Nevertheless sceptics
were still found. M. Faye, of the French Academy, inclined to a
lunar origin for them;[193] Feilitsch of Greifswald published in 1852 a
treatise for the express purpose of proving all the luminous phenomena
attendant on solar eclipses—corona, prominences and “sierra”—to
be purely optical appearances.[194] Happily, however, the unanswerable
arguments of the photographic camera were soon to be
made available against such hardy incredulity.
Thus, the virtual discovery of the solar appendages, both coronal
and chromospheric, may be said to have been begun in 1842, and
completed in 1851. The current Herschelian theory of the solar
constitution remained, however, for the time, intact. Difficulties,
indeed, were thickening around it; but their discussion was perhaps
felt to be premature, and they were permitted to accumulate without
debate, until fortified by fresh testimony into unexpected and overwhelming
preponderance.
FOOTNOTES:
[131] Kosmos, Bd. iii., p. 409; Lalande, Bibliographie Astronomique, pp. 179, 202.
[132] R. Wolf, Die Sonne und ihre Flecken, p. 9. Marius himself, however, seems
to have held the Aristotelian terrestrial-exhalation theory of cometary origin.
See his curious little tract, Astronomische und Astrologische Beschreibung der
Cometen, Nürnberg, 1619.
[133] Phil. Trans., vol. xxvii., p. 274. Umbræ (now called penumbræ) are spaces
of half-shadow which usually encircle spots. Faculæ (“little torches,” so named
by Scheiner) are bright streaks or patches closely associated with spots.
[134] Mém. Ac. Sc., 1776 (pub. 1779), p. 507. D. Cassini, however, first put
forward about 1671 the hypothesis alluded to in the text. See Delambre, Hist.
de l’Astr. Mod., t. ii., p. 694; and Kosmos, Bd. iii., p. 410.
[135] Phil. Trans., vol. lxiv., part i., pp. 7-11.
[136] Rosa Ursina, lib. iv., p. 507.
[137] R. Wolf, Die Sonne und ihre Flecken, p. 12.
[138] Schellen, Die Spectralanalyse, Bd. ii., p. 56 (3rd ed.).
[139] Phil. Trans., vol. lxiv., p. 20.
[140] Ibid., vol. lxxxv., 1795, p. 63.
[141] Phil. Trans., vol. xci., 1801, p. 303.
[142] The supposed opaque or protective stratum beneath the photosphere was
named by him “planetary,” from the analogy of terrestrial clouds.
[143] Ibid., p. 305.
[144] Novum Organum, lib. ii. aph. 20.
[145] Brewster’s Life of Newton, vol. ii., p. 103.
[146] Beschäftigungen d. Berl. Ges. Naturforschender Freunde, Bd. ii., p. 233.
[147] Gentleman’s Magazine, 1787, vol. ii., p. 636.
[148] Results, etc., p. 432.
[149] Ibid., p. 434.
[150] See ante, p. 31.
[151] Memoir of Francis Baily, Mem. R. A. S., vol. xv., p. 524.
[152] Mem. R. A. S., vol. x., pp. 5-6.
[153] Ibid., pp. 14-17.
[154] Mem. R. A. S., vol. xv., pp. 4-6.
[155] Ibid., p. 16.
[156] Annuaire, 1846, p. 409.
[157] Ibid., p. 317.
[158] Ibid., p. 322.
[159] Phil. Trans., vol. xl., p. 192.
[160] Mem. R. A. S., vol. x., p. 17.
[161] Ann. du Bureau des Long., 1846, p. 309.
[162] De Facie in Orbe Lunæ, xix., 10. Cf. Grant, Astr. Nach., No. 1838. As
to the phenomenon mentioned by Philostratus in his Life of Apollonius (viii. 23),
see W. T. Lynn, Observatory, vol. ix., p. 128.
[163] Schmidt, Astr. Nach., No. 1832.
[164] Astronomiæ Pars Optica, Op. omnia, t. ii., p. 317.
[165] De Stellâ Novâ, Op., t. ii., pp. 696, 697.
[166] Astr. Pars Op., p. 320.
[167] Mém. de l’Ac. des Sciences, 1706, p. 119.
[168] A digit = 1/12 of the solar diameter.
[169] Phil. Trans., vol. xxix., pp. 247-249.
[170] Mém. de l’Ac. des Sciences, 1715; Histoire, p. 49; Mémoires, pp. 93-98.
[171] Ibid., 1724, p. 178.
[172] Mém. de l’Ac. des Sciences, 1715, pp. 161, 166-169.
[173] Ed. Ency., art. Astronomy, p. 635.
[174] Trans. Am. Phil. Soc., vol. vi., p. 274.
[175] Memoir of Caroline Herschel, p. 327.
[176] Phil. Trans., vol. xxv., p. 2240.
[177] Ibid., vol. xl., p. 182.
[178] Ibid., vol. xlv., p. 586.
[179] Mem. R. A. S., vol. i., pp. 145, 148.
[180] American Journal of Science, vol. xlii., p. 396.
[181] Phil. Trans., vol. xxxviii., p. 134. Father Secchi, however, adverted to a
distinct mention of a prominence observed in 1239 A.D. A description of a total
eclipse of that date includes the remark, “Et quoddam foramen erat ignitum in
circulo solis ex parte inferiore” (Muratori, Rer. It. Scriptores, t. xiv., col. 1097).
The “circulus solis” of course signifies the corona.
[182] Phil. Trans., vol. lxix., p. 114.
[183] Trans. Am. Phil. Soc., vol. vi., 1809, p. 267.
[184] Annuaire, 1846, p. 460.
[185] Ibid., p. 439, note.
[186] Ibid., p. 416.
[187] Mem. R. A. S., vol. xxi., p. 82.
[188] Ibid., p. 90.
[189] Ibid., pp. 7, 8.
[190] Le Soleil, t. i., p. 386.
[191] By Williams and Stanistreet, Mem. R. A. S., vol. xxi., pp. 54, 56. Santini
had made a similar observation at Padua in 1842. Grant, Hist. Astr., p. 401.
[192] Lassell in Month. Not., vol. xii., p. 53.
[193] Comptes Rendus, t. xxxiv., p. 155.
[194] Optische Untersuchungen, and Zeitschrift für populäre Mittheilungen, Bd. i.,
1860, p. 201.
CHAPTER IV
PLANETARY DISCOVERIES
In the course of his early gropings towards a law of the planetary
distances, Kepler tried the experiment of setting a planet, invisible
by reason of its smallness, to revolve in the vast region of seemingly
desert space separating Mars from Jupiter.[195] The disproportionate
magnitude of the same interval was explained by Kant as due to the
overweening size of Jupiter. The zone in which each planet moved
was, according to the philosopher of Königsberg, to be regarded as
the empty storehouse from which its materials had been derived. A
definite relation should thus exist between the planetary masses and
the planetary intervals.[196] Lambert, on the other hand, sportively
suggested that the body or bodies (for it is noticeable that he speaks
of them in the plural) which once bridged this portentous gap in the
solar system, might, in some remote age, have been swept away by
a great comet, and forced to attend its wanderings through space.[197]
These speculations were destined before long to assume a more
definite form. Johann Daniel Titius, a professor at Wittenberg
(where he died in 1796), pointed out in 1772, in a note to a translation
of Bonnet’s Contemplation de la Nature,[198] the existence of a
remarkable symmetry in the disposition of the bodies constituting
the solar system. By a certain series of numbers, increasing in
regular progression,[199] he showed that the distances of the six known
planets from the sun might be represented with a close approach to
accuracy. But with one striking interruption. The term of the[Pg 72]
series succeeding that which corresponded to the orbit of Mars was
without a celestial representative. The orderly flow of the sequence
was thus singularly broken. The space where a planet should—in
fulfilment of the “Law”—have revolved, was, it appeared,
untenanted. Johann Elert Bode, then just about to begin his long
career as leader of astronomical thought and work at Berlin, marked
at once the anomaly, and filled the vacant interval with a hypothetical
planet. The discovery of Uranus, at a distance falling but
slightly short of perfect conformity with the law of Titius, lent
weight to a seemingly hazardous prediction, and Von Zach was
actually at the pains, in 1785, to calculate what he termed
“analogical” elements[200] for this unseen and (by any effect or influence)
unfelt body. The search for it, through confessedly scarcely
less chimerical than that of alchemists for the philosopher’s stone,
he kept steadily in view for fifteen years, and at length (September 21,
1800) succeeded in organising, in combination with five other German
astronomers assembled at Lilienthal, a force of what he jocularly
termed celestial police, for the express purpose of tracking and
intercepting the fugitive subject of the sun. The zodiac was accordingly
divided for purposes of scrutiny into twenty-four zones; their
apportionment to separate observers was in part effected, and the
association was rapidly getting into working order, when news arrived
that the missing planet had been found, through no systematic plan
of search, but by the diligent, though otherwise directed labours of
a distant watcher of the skies.
Giuseppe Piazzi was born at Ponte in the Valtelline, July 16,
1746. He studied at various places and times under Tiraboschi,
Beccaria, Jacquier, and Le Sueur; and having entered the Theatine
order of monks at the age of eighteen, he taught philosophy, science,
and theology in several of the Italian cities, as well as in Malta,
until 1780, when the chair of mathematics in the University of
Palermo was offered to and accepted by him. Prince Caramanico,
then viceroy of Sicily, had scientific leanings, and was easily won
over to the project of building an observatory, a commodious foundation
for which was afforded by one of the towers of the viceregal
palace. This architecturally incongruous addition to an ancient
Saracenic edifice—once the abode of Kelbite and Zirite Emirs—was
completed in February, 1791. Piazzi, meanwhile, had devoted
nearly three years to the assiduous study of his new profession,
acquiring a practical knowledge of Lalande’s methods at the École
Militaire, and of Maskelyne’s at the Royal Observatory; and
returned to Palermo in 1789, bringing with him, in the great five-foot
circle which he had prevailed upon Ramsden to construct,[Pg 73]
the most perfect measuring instrument hitherto employed by an
astronomer.
He had been above nine years at work on his star-catalogue, and
was still profoundly unconscious that a place amongst the Lilienthal
band[201] of astronomical detectives was being held in reserve for him,
when, on the first evening of the nineteenth century, January 1,
1801, he noticed the position of an eighth-magnitude star in a part
of the constellation Taurus to which an error of Wollaston’s had
directed his special attention. Reobserving, according to his custom,
the same set of fifty stars on four consecutive nights, it seemed to
him, on the 2nd, that the one in question had slightly shifted its
position to the west; on the 3rd he assured himself of the fact, and
believed that he had chanced upon a new kind of comet without tail
or coma. The wandering body, whatever its nature, exchanged
retrograde for direct motion on January 14,[202] and was carefully
watched by Piazzi until February 11, when a dangerous illness
interrupted his observations. He had, however, not omitted to give
notice of his discovery; but so precarious were communications in
those unpeaceful times, that his letter to Oriani of January 23 did
not reach Milan until April 5, while a missive of one day later
addressed to Bode came to hand at Berlin, March 20. The delay
just afforded time for the publication, by a young philosopher of
Jena named Hegel, of a “Dissertation” showing, by the clearest
light of reason, that the number of the planets could not exceed
seven, and exposing the folly of certain devotees of induction who
sought a new celestial body merely to fill a gap in a numerical series.[203]
Unabashed by speculative scorn, Bode had scarcely read Piazzi’s
letter when he concluded that it referred to the precise body in
question. The news spread rapidly, and created a profound sensation,
not unmixed with alarm lest this latest addition to the solar
family should have been found only to be again lost. For by that
time Piazzi’s moving star was too near the sun to be any longer
visible, and in order to rediscover it after conjunction a tolerably
accurate knowledge of its path was indispensable. But a planetary
orbit had never before been calculated from such scanty data as
Piazzi’s observation afforded;[204] and the attempts made by nearly
every astronomer of note in Germany to compass the problem were
manifestly inadequate, failing even to account for the positions in
which the body had been actually seen, and à fortiori serving only to[Pg 74]
mislead as to the places where, from September, 1801, it ought once
more to have become discernible. It was in this extremity that the
celebrated mathematician Gauss came to the rescue. He was then
in his twenty-fifth year, and was earning his bread by tuition at
Brunswick, with many possibilities, but no settled career before him.
The news from Palermo may be said to have converted him from an
arithmetician into an astronomer. He was already in possession of
a new and more general method of computing elliptical orbits; and
the system of “least squares,” which he had devised though not
published, enabled him to extract the most probable result from a
given set of observations. Armed with these novel powers, he set
to work; and the communication in November of his elements and
ephemeris for the lost object revived the drooping hopes of the little
band of eager searchers. Their patience, however, was to be still
further tried. Clouds, mist, and sleet seemed to have conspired to
cover the retreat of the fugitive; but on the last night of the year
the sky cleared unexpectedly with the setting in of a hard frost,
and there, in the north-western part of Virgo, nearly in the position
assigned by Gauss to the runaway planet, a strange star was discerned
by Von Zach[205] at Gotha, and on a subsequent evening—the
anniversary of the original discovery—by Olbers at Bremen. The
name of Ceres (as the tutelary goddess of Sicily) was, by Piazzi’s
request, bestowed upon this first known of the numerous, and
probably all but innumerable family of the minor planets.
The recognition of the second followed as the immediate consequence
of the detection of the first. Olbers had made himself so
familiar with the positions of the small stars along the track of the
long-missing body, that he was at once struck (March 28, 1802)
with the presence of an intruder near the spot where he had recently
identified Ceres. He at first believed the new-comer to be a variable
star usually inconspicuous, but just then at its maximum of brightness;
but within two hours he had convinced himself that it was no
fixed star, but a rapidly moving object. The aid of Gauss was again
invoked, and his prompt calculations showed that this fresh celestial
acquaintance (named “Pallas” by Olbers), revolved round the sun at
nearly the same mean distance as Ceres, and was beyond question
of a strictly analogous character.
This result was perplexing in the extreme. The symmetry and
simplicity of the planetary scheme appeared fatally compromised
by the admission of many, where room could, according to old-fashioned
rules, only be found for one. A daring hypothesis of[Pg 75]
Olbers’s invention provided an exit from the difficulty. He supposed
that both Ceres and Pallas were fragments of a primitive trans-Martian
planet, blown to pieces in the remote past, either by the
action of internal forces or by the impact of a comet; and predicted
that many more such fragments would be found to circulate in the
same region. He, moreover, pointed out that these numerous orbits,
however much they might differ in other respects, must all have a
common line of intersection,[206] and that the bodies moving in them
must consequently pass, at each revolution, through two opposite
points of the heavens, one situated in the Whale, the other in the
constellation of the Virgin, where already Pallas had been found and
Ceres recaptured. The intimation that fresh discoveries might be
expected in those particular regions was singularly justified by the
detection of two bodies now known respectively as Juno and Vesta.
The first was found near the predicted spot in Cetus by Harding,
Schröter’s assistant at Lilienthal, September 2, 1804; the second by
Olbers himself in Virgo, after three years of persistent scrutiny,
March 29, 1807.
The theory of an exploded planet now seemed to have everything
in its favour. It required that the mean or average distances of the
newly-discovered bodies should be nearly the same, but admitted a
wide range of variety in the shapes and positions of their orbits,
provided always that they preserved common points of intersection.
These conditions were fulfilled with a striking approach to exactness.
Three of the four “asteroids” (a designation introduced by Sir. W.
Herschel[207]) conformed with very approximate precision to “Bode’s
law” of distances; they all traversed, in their circuits round the
sun, nearly the same parts of Cetus and Virgo; while the eccentricities
and inclinations of their paths departed widely from the
planetary type—that of Pallas, to take an extreme instance, making
with the ecliptic an angle of nearly 35°. The minuteness of these
bodies appeared further to strengthen the imputation of a fragmentary
character. Herschel estimated the diameter of Ceres at 162, that of
Pallas at 147 miles.[208] But these values are now known to be considerably
too small. A suspected variability of brightness in some
of the asteroids, somewhat hazardously explained as due to the
irregularities of figure to be expected in cosmical potsherds (so to[Pg 76]
speak), was added to the confirmatory evidence.[209] The strong point
of the theory, however, lay not in what it explained, but in what it
had predicted. It had been twice confirmed by actual exploration
of the skies, and had produced, in the recognition of Vesta, the
first recorded instance of the premeditated discovery of a heavenly
body.
The view not only commended itself to the facile imagination of
the unlearned, but received the sanction of the highest scientific
authority. The great Lagrange bestowed upon it his analytical
imprimatur, showing that the explosive forces required to produce
the supposed catastrophe came well within the bounds of possibility;
since a velocity of less than twenty times that of a cannon-ball leaving
the gun’s mouth would have sufficed, according to his calculation, to
launch the asteroidal fragments on their respective paths. Indeed,
he was disposed to regard the hypothesis of disruption as more
generally available than its author had designed it to be, and
proposed to supplement with it, as explanatory of the eccentric
orbits of comets, the nebular theory of Laplace, thereby obtaining,
as he said, “a complete view of the origin of the planetary system
more conformable to Nature and mechanical laws than any yet
proposed.”[210]
Nevertheless the hypothesis of Olbers has not held its ground. It
seemed as if all the evidence available for its support had been produced
at once and spontaneously, while the unfavourable items were
elicited slowly, and, as it were, by cross-examination. A more
extended acquaintance with the group of bodies whose peculiarities
it was framed to explain has shown them, after all, as recalcitrant
to any such explanation. Coincidences at the first view significant
and striking have been swamped by contrary examples; and a hasty
general conclusion has, by a not uncommon destiny, at last perished
under the accumulation of particulars. Moreover, as has been
remarked by Professor Newcomb,[211] mutual perturbations would
rapidly efface all traces of a common disruptive origin, and the
catastrophe, to be perceptible in its effects, should have been comparatively
recent.
A new generation of astronomers had arisen before any additions
were made to the little family of the minor planets. Piazzi died in
1826, Harding in 1834, Olbers in 1840; all those who had prepared
or participated in the first discoveries passed away without witnessing
their resumption. In 1830, however, a certain Hencke, ex-postmaster
in the Prussian town of Driessen, set himself to watch for new planets,
and after fifteen long years his patience was rewarded. The asteroid[Pg 77]
found by him, December 8, 1845, received the name of Astræa, and
his further prosecution of the search resulted, July 1, 1847, in the
discovery of Hebe. A few weeks later (August 13), John Russell
Hind (1823-1893), after many months’ exploration from Mr. Bishop’s
observatory in the Regent’s Park, picked up Iris, and October 18,
Flora.[212] The next on the list was Metis, found by Mr. Graham,
April 25, 1848, at Markree, in Ireland.[213] At the close of the period
to which our attention is at present limited, the number of these
small bodies known to astronomy was thirteen; and the course of
discovery has since proceeded far more rapidly and with less
interruption.
Both in itself and in its consequences the recognition of the minor
planets was of the highest importance to science. The traditional
ideas regarding the constitution of the solar system were enlarged
by the admission of a new class of bodies, strongly contrasted, yet
strictly co-ordinate with the old-established planetary order; the
profusion of resource, so conspicuous in the living kingdoms of
Nature, was seen to prevail no less in the celestial spaces; and some
faint preliminary notion was afforded of the indefinite complexity of
relations underlying the apparent simplicity of the majestic scheme
to which our world belongs. Both theoretical and practical
astronomy derived profit from the admission of these apparently
insignificant strangers to the rights of citizenship of the solar system.
The disturbance of their motions by their giant neighbours afforded
a more accurate knowledge of the Jovian mass, which Laplace had
taken about 1/50 too small; the anomalous character of their orbits
presented geometers with highly stimulating problems in the theory
of perturbation; while the exigencies of the first discovery had
produced the Theoria Motus, and won Gauss over to the ranks of
calculating astronomy. Moreover, the sure prospect of further
detections powerfully incited to the exploration of the skies;
observers became more numerous and more zealous in view of the
prizes held out to them; star-maps were diligently constructed, and
the sidereal multitude strewn along the great zodiacal belt acquired
a fresh interest when it was perceived that its least conspicuous
member might be a planetary shred or projectile in the dignified
disguise of a distant sun. Harding’s “Celestial Atlas,” designed
for the special purpose of facilitating asteroidal research, was the
first systematic attempt to represent to the eye the telescopic aspect
of the heavens. It was while engaged on its construction that the
Lilienthal observer successfully intercepted Juno on her passage
through the Whale in 1804; whereupon promoted to Göttingen, he
there completed, in 1822, the arduous task so opportunely entered[Pg 78]
upon a score of years previously. Still more important were the
great star-maps of the Berlin Academy, undertaken at Bessel’s
suggestion, with the same object of distinguishing errant from fixed
stars, and executed, under Encke’s supervision, during the years
1830-59. They have played a noteworthy part in the history of
planetary discovery, nor of the minor kind alone.
We have now to recount an event unique in scientific history.
The discovery of Neptune has been characterised as the result of a
“movement of the age,”[214] and with some justice. It had become
necessary to the integrity of planetary theory. Until it was
accomplished, the phantom of an unexplained anomaly in the
orderly movements of the solar system must have continued to
haunt astronomical consciousness. Moreover, it was prepared by
many, suggested as possible by not a few, and actually achieved,
simultaneously, independently, and completely, by two investigators.
The position of the planet Uranus was recorded as that of a fixed
star no less than twenty times between 1690 and the epoch of its
final detection by Herschel. But these early observations, far from
affording the expected facilities for the calculation of its orbit, proved
a source of grievous perplexity. The utmost ingenuity of geometers
failed to combine them satisfactorily with the later Uranian places,
and it became evident, either that they were widely erroneous, or
that the revolving body was wandering from its ancient track. The
simplest course was to reject them altogether, and this was done in
the new Tables published in 1821 by Alexis Bouvard, the indefatigable
computating partner of Laplace. But the trouble was not
thus to be got rid of. After a few years fresh irregularities began
to appear, and continued to increase until absolutely “intolerable.”
It may be stated as illustrative of the perfection to which astronomy
had been brought, that divergencies regarded as menacing the very
foundation of its theories never entered the range of unaided vision.
In other words, if the theoretical and the real Uranus had been
placed side by side in the sky, they would have seemed, to the
sharpest eye, to form a single body.[215]
The idea that these enigmatical disturbances were due to the
attraction of an unknown exterior body was a tolerably obvious
one; and we accordingly find it suggested in many different quarters.
Bouvard himself was perhaps the first to conceive it. He kept the[Pg 79]
possibility continually in view, and bequeathed to his nephew’s
diligence the inquiry into its reality when he felt that his own span
was drawing to a close; but before any progress had been made
with it, he had already (June 7, 1843) “ceased to breathe and to
calculate.” The Rev. T. J. Hussey actually entertained in 1834 the
notion, but found his powers inadequate to the task, of assigning an
approximate place to the disturbing body; and Bessel, in 1840, laid
his plans for an assault in form upon the Uranian difficulty, the
triumphant exit from which fatal illness frustrated his hopes of
effecting or even witnessing.
The problem was practically untouched when, in 1841, an undergraduate
of St. John’s College, Cambridge, formed the resolution of
grappling with it. The projected task was an arduous one. There
were no guiding precedents for its conduct. Analytical obstacles
had to be encountered so formidable as to appear invincible even to
such a mathematician as Airy. John Couch Adams, however, had
no sooner taken his degree, which he did as senior wrangler in
January, 1843, than he set resolutely to work, and on October 21,
1845, was able to communicate to the Astronomer Royal numerical
estimates of the elements and mass of the unknown planet, together
with an indication of its actual place in the heavens. These results,
it has been well said,[216] gave “the final and inexorable proof” of the
validity of Newton’s Law. The date October 21, 1845, “may therefore
be regarded as marking a distinct epoch in the history of
gravitational astronomy.”
Sir George Biddell Airy had begun in 1835 his long and energetic
administration of the Royal Observatory, and was already in possession
of data vitally important to the momentous inquiry then on
foot. At his suggestion, and under his superintendence, the reduction
of all the planetary observations made at Greenwich from 1750
onwards had been undertaken in 1833. The results, published in
1846, constituted a permanent and universal stock of materials for
the correction of planetary theory. But in the meantime, investigators,
both native and foreign, were freely supplied with the
“places and errors,” which, clearly exhibiting the discrepancies
between observation and calculation—between what was and what
was expected—formed the very groundwork of future improvements.
Mr. Adams had no reason to complain of official discourtesy. His
labours received due and indispensable aid; but their purpose was
regarded as chimerical. “I have always,” Sir George Airy wrote,[217]
“considered the correctness of a distant mathematical result to be a
subject rather of moral than of mathematical evidence.” And that[Pg 80]
actually before him seemed, from its very novelty, to incur a
suspicion of unlikelihood. No problem in planetary disturbance had
heretofore been attacked, so to speak, from the rear. The inverse
method was untried, and might well be deemed impracticable. For
the difficulty of determining the perturbations produced by a given
planet is small compared with the difficulty of finding a planet by
its resulting perturbations. Laplace might have quailed before it;
yet it was now grappled with as a first essay in celestial dynamics.
Moreover, Adams unaccountably neglected to answer until too late
a question regarded by Airy in the light of an experimentum crucis
as to the soundness of the new theory. Nor did he himself take
any steps to obtain a publicity which he was more anxious to merit
than to secure. The investigation consequently remained buried in
obscurity. It is now known that had a search been instituted in the
autumn of 1845 for the remote body whose existence had been so
marvellously foretold, it would have been found within three and a
half lunar diameters (1° 49′) of the spot assigned to it by Adams.
A competitor, however, equally daring and more fortunate—audax
fortunâ adjutus, as Gauss said of him—was even then entering
the field. Urbain Jean Joseph Leverrier, the son of a small Government
employé in Normandy, was born at Saint-Lô, March 11, 1811.
He studied with brilliant success at the École Polytechnique, accepted
the post of astronomical teacher there in 1837, and, “docile to
circumstance,” immediately concentrated the whole of his vast,
though as yet undeveloped powers upon the formidable problems,
of celestial mechanics. He lost no time in proving to the mathematical
world that the race of giants was not extinct. Two papers
on the stability of the solar system, presented to the Academy of
Sciences, September 16 and October 14, 1839, showed him to be
the worthy successor of Lagrange and Laplace, and encouraged
hopes destined to be abundantly realised. His attention was directed
by Arago to the Uranian difficulty in 1845, when he cheerfully put
aside certain intricate cometary researches upon which he happened
to be engaged, in order to obey with dutiful promptitude the
summons of the astronomical chief of France. In his first memoir
on the subject (communicated to the Academy, November 10, 1845),
he proved the inadequacy of all known causes of disturbance to
account for the vagaries of Uranus; in a second (June 1, 1848),
he demonstrated that only an exterior body, occupying at a certain
date a determinate position in the zodiac, could produce the observed
effects; in a third (August 31, 1846), he assigned the orbit of the
disturbing body, and announced its visibility as an object with a
sensible disc about as bright as a star of the eighth magnitude.
The question was now visibly approaching an issue. On September[Pg 81]
10, Sir John Herschel declared to the British Association respecting
the hypothetical new planet: “We see it as Columbus saw
America from the coast of Spain. Its movements have been felt,
trembling along the far-reaching line of our analysis with a certainty
hardly inferior to that of ocular demonstration.” Less than a fortnight
later, September 23, Professor Galle, of the Berlin Observatory,
received a letter from Leverrier requesting his aid in the telescopic
part of the inquiry already analytically completed. He directed
his refractor to the heavens that same night, and perceived, within
less than a degree of the spot indicated, an object with a measurable
disc nearly three seconds in diameter. Its absence from
Bremiker’s recently-completed map of that region of the sky
showed it to be no star, and its movement in the predicted direction
confirmed without delay the strong persuasion of its planetary
nature.[218]
In this remarkable manner the existence of the remote member
of our system known as “Neptune” was ascertained. But the discovery,
which faithfully reflected the duplicate character of the
investigation which led to it, had been already secured at Cambridge
before it was announced from Berlin. Sir George Airy’s incredulity
vanished in the face of the striking coincidence between the position
assigned by Leverrier to the unknown planet in June, and that laid
down by Adams in the previous October; and on the 9th of July he
wrote to Professor Challis, director of the Cambridge Observatory,
recommending a search with the great Northumberland equatoreal.
Had a good star-map been at hand, the process would have been
a simple one; but of Bremiker’s “Hora XXI.” no news had yet
reached England, and there was no other sufficiently comprehensive
to be available for an inquiry which, in the absence of such aid,
promised to be both long and laborious. As the event proved, it
might have been neither. “After four days of observing,” Challis
wrote, October 12, 1846, to Airy, “the planet was in my grasp if
only I had examined or mapped the observations.”[219] Had he done
so, the first honours in the discovery, both theoretical and optical,
would have fallen to the University of Cambridge. But Professor
Challis had other astronomical avocations to attend to, and, moreover,
his faith in the precision of the indications furnished to him
was, by his own confession, a very feeble one. For both reasons he
postponed to a later stage of the proceedings the discussion and
comparison of the data nightly furnished to him by his telescope,
and thus allowed to lie, as it were, latent in his observations the
[Pg 82]
momentous result which his diligence had insured, but which his
delay suffered to be anticipated.[220]
Nevertheless, it should not be forgotten that the Berlin astronomer
had two circumstances in his favour apart from which his swift
success could hardly have been achieved. The first was the possession
of a good star-map; the second was the clear and confident
nature of Leverrier’s instructions. “Look where I tell you,” he
seemed authoritatively to say, “and you will see an object such as
I describe.”[221] And in fact, not only Galle on the 23rd of September,
but also Challis on the 29th, immediately after reading the French
geometer’s lucid and impressive treatise, picked out from among
the stellar points strewing the zodiac, a small planetary disc, which
eventually proved to be that of the precise body he had been in
search of during two months.
The controversy that ensued had its ignominious side; but it was
entered into by neither of the parties principally concerned. Adams
bore the disappointment, which the dilatory proceedings at Greenwich
and Cambridge had inflicted upon him, with quiet heroism.
His silence on the subject of what another man would have called
his wrongs remained unbroken to the end of his life;[222] and he took
every opportunity of testifying his admiration for the genius of
Leverrier.
Personal questions, however, vanish in the magnitude of the event
they relate to. By it the last lingering doubts as to the absolute
exactness of the Newtonian Law were dissipated. Recondite
analytical methods received a confirmation brilliant and intelligible
even to the minds of the vulgar, and emerged from the patient
solitude of the study to enjoy an hour of clamorous triumph. For
ever invisible to the unaided eye of man, a sister-globe to our earth
was shown to circulate, in perpetual frozen exile, at thirty times its
distance from the sun. Nay, the possibility was made apparent
that the limits of our system were not even thus reached, but that
yet profounder abysses of space might shelter obedient, though little
favoured, members of the solar family, by future astronomers to be
recognised through the sympathetic thrillings of Neptune, even as
Neptune himself was recognised through the tell-tale deviations of
Uranus.
It is curious to find that the fruit of Adams’s and Leverrier’s[Pg 83]
laborious investigations had been accidentally all but snatched half
a century before it was ripe to be gathered. On the 8th, and again
on the 10th of May, 1795, Lalande noted the position of Neptune
as that of a fixed star, but perceiving that the two observations did
not agree, he suppressed the first as erroneous, and pursued the
inquiry no further. An immortality which he would have been the
last to despise hung in the balance; the feather-weight of his carelessness,
however, kicked the beam, and the discovery was reserved
to be more hardly won by later comers.
Bode’s Law did good service in the quest for a trans-Uranian
planet by affording ground for a probable assumption as to its
distance. A starting-point for approximation was provided by it;
but it was soon found to be considerably at fault. Even Uranus is
about 36 millions of miles nearer to the sun than the order of progression
requires; and Neptune’s vast distance of 2,800 million
should be increased by no less than 800 million miles, and its period
of 165 lengthened out to 225 years,[223] in order to bring it into conformity
with the curious and unexplained rule which planetary
discoveries have alternately tended to confirm and to invalidate.
Within seventeen days of its identification with the Berlin
achromatic, Neptune was found to be attended by a satellite. This
discovery was the first notable performance of the celebrated two-foot
reflector[224] erected by Mr. Lassell at his suggestively named
residence of Starfield, near Liverpool. William Lassell was a brewer
by profession, but by inclination an astronomer. Born at Bolton in
Lancashire, June 18, 1799, he closed a life of eminent usefulness to
science, October 5, 1818, thus spanning with his well-spent years
four-fifths of the momentous period which we have undertaken to
traverse. At the age of twenty-one, being without the means to
purchase, he undertook to construct telescopes, and naturally turned
his attention to the reflecting sort, as favouring amateur efforts by
the comparative simplicity of its structure. His native ingenuity
was remarkable, and was developed by the hourly exigencies of his
successive enterprises. Their uniform success encouraged him to
enlarge his aims, and in 1844 he visited Birr Castle for the purpose
of inspecting the machine used in polishing the giant speculum of
Parsonstown. In the construction of his new instrument, however,
he eventually discarded the model there obtained, and worked on a
method of his own, assisted by the supreme mechanical skill of
James Nasmyth. The result was a Newtonian of exquisite definition,
with an aperture of two, and a focal length of twenty feet,[Pg 84]
provided by a novel artifice with the equatoreal mounting, previously
regarded as available only for refractors.
This beautiful instrument afforded to its maker, October 10, 1846,
a cursory view of a Neptunian attendant. But the planet was then
approaching the sun, and it was not until the following July that
the observation could be verified, which it was completely, first by
Lassell himself, and somewhat later by Otto Stuve and Bond of
Cambridge (U.S.). When it is considered that this remote object
shines by reflecting sunlight reduced by distance to 1/900th of the
intensity with which it illuminates our moon, the fact of its visibility,
even in the most perfect telescopes, is a somewhat surprising one.
It can only, indeed, be accounted for by attributing to it dimensions
very considerable for a body of the secondary order. It shares with
the moons of Uranus the peculiarity of retrograde motion; that is
to say, its revolutions, running counter to the grand current of
movement in the solar system, are performed from east to west, in
a plane inclined at an angle of 35° to that of the ecliptic. Their
swiftness serves to measure the mass of the globe round which they
are performed. For while our moon takes twenty-seven days and
nearly eight hours to complete its circuit of the earth, the satellite
of Neptune, at a distance not greatly inferior, sweeps round its
primary in five days and twenty-one hours, showing (according to a
very simple principle of computation) that it is urged by a force
seventeen times greater than the terrestrial pull upon the lunar
orb. Combining this result with those of Professor Barnard’s[225]
and Dr. See’s[226] recent measurements of the small telescopic disc of
this farthest known planet, it is found that while in mass Neptune
equals seventeen, in bulk it is equivalent to forty-nine earths. This
is as much as to say that it is composed of relatively very light
materials, or more probably of materials distended by internal heat,
as yet unwasted by radiation into space, to about five times the
volume they would occupy in the interior of our globe. The fact,
at any rate, is fairly well ascertained, that the average density of
Neptune is about twice that of water.
We must now turn from this late-recognised member of our
system to bestow some brief attention upon the still fruitful field
of discovery offered by one of the immemorial five. The family of
Saturn, unlike that of its brilliant neighbour, has been gradually
introduced to the notice of astronomers. Titan, the sixth Saturnian
moon in order of distance, led the way, being detected by Huygens,
March 25, 1655; Cassini made the acquaintance of four more
between 1671 and 1684; while Mimas and Enceladus, the two innermost,
were caught by Herschel in 1789, as they threaded their lucid[Pg 85]
way along the edge of the almost vanished ring. In the distances
of these seven revolving bodies from their primary, an order of progression
analogous to that pointed out by Titius in the planetary
intervals was found to prevail; but with one conspicuous interruption,
similar to that which had first suggested the search for new
members of the solar system. Between Titan and Japetus—the
sixth and seventh reckoning outwards—there was obviously room
for another satellite. It was discovered on both sides of the Atlantic
simultaneously, on the 19th of September, 1848. Mr. W. C. Bond,
employing the splendid 15-inch refractor of the Harvard Observatory,
noticed, September 16, a minute star situated in the plane of Saturn’s
rings. The same object was discerned by Mr. Lassell on the 18th.
On the following evening, both observers perceived that the
problematical speck of light kept up with, instead of being left
behind by the planet as it moved, and hence inferred its true
character.[227] Hyperion, the seventh by distance and eighth by
recognition of Saturn’s attendant train, is of so insignificant a size
when compared with some of its fellow-moons (Titan is but little
inferior to the planet Mars), as to have suggested to Sir John
Herschel[228] the idea that it might be only one of several bodies
revolving very close together—in fact, an asteroidal satellite; but the
conjecture has, so far, not been verified.
The coincidence of its duplicate discovery was singularly paralleled
two years later. Galileo’s amazement when his “optic glass” revealed
to him the “triple” form of Saturn—planeta tergeminus—has
proved to be, like the laughter of the gods, “inextinguishable.” It
must revive in every one who contemplates anew the unique arrangements
of that world apart known to us as the Saturnian system.
The resolution of the so-called ansæ, or “handles,” into one encircling
ring by Huygens in 1655, the discovery by Cassini in 1675
of the division of that ring into two concentric ones, together with
Laplace’s investigation of the conditions of stability of such a formation,
constituted, with some minor observations, the sum of the
knowledge obtained, up to the middle of the last century, on
the subject of this remarkable formation. The first place in
the discovery now about to be related belongs to an American
astronomer.
William Cranch Bond, born in 1789 at Portland, in the State of
Maine, was a watchmaker, whom the solar eclipse of 1806 attracted
to study the wonders of the heavens. When, in 1815, the erection
of an observatory in connection with Harvard College, Cambridge,
was first contemplated, he undertook a mission to England for the
purpose of studying the working of similar institutions there, and on[Pg 86]
his return erected a private observatory at Dorchester, where he
worked diligently for many years. Then at last, in 1843, the long-postponed
design of the Harvard authorities was resumed, and on
the completion of the new establishment, Bond, who had been from
1838 officially connected with the College and had carried on his
scientific labours within its precincts, was offered and accepted the
post of its director. Placed in 1847 in possession of one of the finest
instruments in the world—a masterpiece of Merz and Mahler—he
headed the now long list of distinguished Transatlantic observers.
Like the elder Struve, he left an heir to his office and to his
eminence, but George Bond unfortunately died in 1865, at the early
age of thirty-nine, having survived his father but six years.
On the night of November 15, 1850—the air, remarkably enough,
being so hazy that only the brightest stars could be perceived with the
naked eye—William Bond discerned a dusky ring, extending about
halfway between the inner brighter one and the globe of Saturn.
A fortnight later, but before the observation had been announced in
England, the same appearance was seen by the Rev. W. R. Dawes
with the comparatively small refractor of his observatory at
Wateringbury, and on December 3 was described by Mr. Lassell
(then on a visit to him) as “something like a crape veil covering a
part of the sky within the inner ring.”[229] Next morning the Times
containing the report of Bond’s discovery reached Wateringbury.
The most surprising circumstance in the matter was that the novel
appendage had remained so long unrecognised. As the rings opened
out to their full extent, it became obvious with very moderate
optical assistance; yet some of the most acute observers who have
ever lived, using instruments of vast power, had heretofore failed to
detect its presence. It soon appeared, however, that Galle of Berlin[230]
had noticed, June 10, 1838, a veil-like extension of the lucid ring
across half the dark space separating it from the planet; but the
observation, although communicated at the time to the Berlin
Academy of Sciences, had remained barren. Traces of the dark
ring, moreover, were found in drawings executed by Campani in
1664[231] and by Hooke in 1666;[232] while Picard (June 15, 1673),[233] Hadley
(spring of 1720),[234] and Herschel,[235] had all undoubtedly seen it under
the aspect of a dark bar or belt crossing the Saturnian globe. It
was, then, of no recent origin; but there seemed reason to think
that it had lately gained considerably in brightness. The full[Pg 87]
meaning of this suspected change it was reserved for later investigations
to develop.
What we may, in a certain sense, call the closing result of the
race for discovery, in which several observers seemed at that time
to be engaged, was the establishment, on a satisfactory footing, of
our acquaintance with the dependent system of Uranus. Sir William
Herschel, whose researches formed, in so many distinct lines of
astronomical inquiry, the starting-points of future knowledge,
detected, January 11, 1787,[236] two Uranian moons, since called
Oberon and Titania, and ascertained the curious circumstance of
their motion in a plane almost at right angles to the ecliptic, in a
direction contrary to that of all previously known denizens (other
than cometary) of the solar kingdom. He believed that he caught
occasional glimpses of four more, but never succeeded in assuring
himself of their substantial existence. Even the two first remained
unseen save by himself until 1828, when his son re-observed them
with a 20-foot reflector, similar to that with which they had been
originally discovered. Thenceforward they were kept fairly within
view, but their four questionable companions, in spite of some false
alarms of detection, remained in the dubious condition in which
Herschel had left them. At last, on October 24, 1851,[237] after some
years of fruitless watching, Lassell espied “Ariel” and “Umbriel,”
two Uranian attendants, interior to Oberon and Titania, and of
about half their brightness; so that their disclosure is still reckoned
amongst the very highest proofs of instrumental power and perfection.
In all probability they were then for the first time seen; for
although Professor Holden[238] made out a plausible case in favour of
the fitful visibility to Herschel of each of them in turn, Lassell’s
argument[239] that the glare of the planet in Herschel’s great specula
must have rendered almost impossible the perception of objects so
minute and so close to its disc, appears tolerably decisive to the
contrary. Uranus is thus attended by four moons, and, so far as
present knowledge extends, by no more. Among the most important
of the “negative results”[240] secured by Lassell’s observations at
Malta during the years 1852-53 and 1861-65, were the convincing
evidence afforded by them that, without great increase of optical
power, no further Neptunian or Uranian satellites can be perceived,
and the consequent relegation of Herschel’s baffling quartette, notwithstanding
the unquestioned place long assigned to them in
astronomical text-books, to the Nirvana of non-existence.
FOOTNOTES:
[195] Op., t. i., p. 107. He interposed, but tentatively only, another similar body
between Mercury and Venus.
[196] Allgemeine Naturgeschichte (ed. 1798), pp. 118, 119.
[197] Cosmologische Briefe, No. 1 (quoted by Von Zach, Monat. Corr., vol. iii.,
p. 592).
[198] Second ed., p. 7. See Bode, Von dem neuen Hauptplaneten, p. 43, note.
[199] The representative numbers are obtained by adding 1 to the following series
(irregular, it will be observed, in its first member, which should be 1/2 instead
of 0); 0, 3, 6, 12, 24, 48, etc. The formula is a purely empirical one, and is,
moreover, completely at fault as regards the distance of Neptune.
[200] Monat. Corr., vol. iii., p. 596.
[201] Wolf, Geschichte der Astronomie, p. 648.
[202] Such reversals of direction in the apparent movements of the planets are a
consequence of the earth’s revolution in its orbit.
[203] Dissertatio Philosophica de Orbitis Planetarum, 1801. See Wolf, Gesch.
d. Astr., p. 685.
[204] Observations on Uranus, as a supposed fixed star, went back to 1690.
[205] He had caught a glimpse of it on December 7, but was prevented by bad
weather from verifying his suspicion. Monat. Corr., vol. v., p. 171.
[206] Planetary fragments, hurled in any direction, and with any velocity short of
that which would for ever release them from the solar sway, would continue
to describe elliptic orbits round the sun, all passing through the scene of the
explosion, and thus possessing a common line of intersection.
[207] Phil. Trans., vol. xcii., part ii., p. 228.
[208] Ibid., p. 218. In a letter to Von Zach of June 24, 1802, he speaks of Pallas
as “almost incredibly small,” and makes it only seventy English miles in
diameter. Monat. Corr., vol. vi., pp. 89, 90.
[209] Olbers, Monat. Corr., vol. vi., p. 88.
[210] Conn. d. Tems for 1814, p. 218.
[211] Popular Astronomy, p. 327.
[212] Month. Not., vol. vii., p. 299; vol. viii., p. 1.
[213] Ibid., p. 146.
[214] Airy, Mem. R. A. S., vol. xvi., p. 386.
[215] See Newcomb’s Pop. Astr., p. 359. The error of Uranus amounted, in
1844, to 2′; but even the tailor of Breslau, whose extraordinary powers of vision
Humboldt commemorates (Kosmos, Bd. ii., p. 112), could only see Jupiter’s first
satellite at its greatest elongation, 2′ 15′. He might, however, possibly have
distinguished two objects of equal lustre at a lesser interval.
[216] J. W. L. Glaisher, Observatory, vol. xv., p. 177.
[217] Mem. R. A. S., vol. xvi., p. 399.
[218] For an account of D’Arrest’s share in the detection see Copernicus, vol. ii.,
pp. 63, 96.
[219] Mem. R. A. S., vol. xvi., p. 412.
[220] He had recorded the places of 3,150 stars (three of which were different
positions of the planet), and was preparing to map them, when, October 1, news
of the discovery arrived from Berlin. Prof. Challis’s Report, quoted in Obituary
Notice, Month. Not., Feb., 1883, p. 170.
[221] See Airy in Mem. R. A. S., vol. xvi., p. 411.
[222] He died January 21, 1892, in his 71st year.
[223] Ledger, The Sun, its Planets and their Satellites, p. 414.
[224] Presented by the Misses Lassell, after their father’s death, to the Greenwich
Observatory.
[225] Astr. Jour., No. 508.
[226] Report of U.S. Naval Observatory for 1900, p. 15.
[227] Grant, Hist. of Astr., p. 271.
[228] Month. Not., vol. ix., p. 91.
[229] Month. Not., vol. xi., p. 21.
[230] Astr. Nach., No. 756 (May 2, 1851).
[231] Phil. Trans., vol. i., p. 246. See H. T. Vivian, Engl. Mech., April 20, 1894.
[232] Secchi, Month. Not., vol. xiii., p. 248.
[233] Hind, ibid., vol. xv., p. 32.
[234] Lynn, Observatory, Oct. 1, 1883; Hadley, Phil. Trans., vol. xxxii., p. 385.
[235] Proctor, Saturn and its System, p. 64.
[236] Phil. Trans., vol. lxxvii., p. 125.
[237] Month. Not., vol. xi., p. 248.
[238] Ibid., vol. xxxv., pp. 16-22.
[239] Ibid., p. 26.
[240] Ibid., vol. xli., p. 190.
CHAPTER V
COMETS
Newton showed that the bodies known as “comets,” or hirsute stars,
obey the law of gravitation; but it was by no means certain that the
individual of the species observed by him in 1680 formed a permanent
member of the solar system. The velocity, in fact, of its rush round
the sun was quite possibly sufficient to carry it off for ever into the
depths of space, there to wander, a celestial casual, from star to star.
With another comet, however, which appeared two years later, the
case was different. Edmund Halley, who afterwards succeeded
Flamsteed as Astronomer Royal, calculated the elements of its orbit
on Newton’s principles, and found them to resemble so closely those
similarly arrived at for comets observed by Peter Apian in 1531,
and by Kepler in 1607, as almost to compel the inference that all
three were apparitions of a single body. This implied its revolution
in a period of about seventy-six years, and Halley accordingly fixed
its return for 1758-9. So fully alive was he to the importance of
the announcement that he appealed to a “candid posterity,” in the
event of its verification, to acknowledge that the discovery was due
to an Englishman. The prediction was one of the test-questions
put by Science to Nature, on the replies to which largely depend
both the development of knowledge and the conviction of its reality.
In the present instance, the answer afforded may be said to have
laid the foundation of this branch of astronomy. Halley’s comet
punctually reappeared on Christmas Day, 1758, and effected its
perihelion passage on the 12th of March following, thus proving
beyond dispute that some at least of these erratic bodies are
domesticated within our system, and strictly conform, if not to its
unwritten customs (so to speak), at any rate to its fundamental
laws. Their movements, in short, were demonstrated by the most
unanswerable of all arguments—that of verified calculation—to be
calculable, and their investigation was erected into a legitimate
department of astronomical science.[Pg 89]
This notable advance was the chief result obtained in the field of
inquiry just now under consideration during the eighteenth century.
But before it closed, its cultivation had received a powerful stimulus
through the invention of an improved method. The name of Olbers
has already been brought prominently before our readers in connection
with asteroidal discoveries; these, however, were but chance
excursions from the path of cometary research which he steadily
pursued through life. An early predilection for the heavens was
fixed in this particular direction by one of the happy inspirations of
genius. As he was watching, one night in the year 1779, by the
sick-bed of a fellow-student in medicine at Göttingen, an important
simplification in the mode of computing the paths of comets occurred
to him. Although not made public until 1797, “Olbers’s method”
was then universally adopted, and is still regarded as the most
expeditious and convenient in cases where absolute rigour is not
required. By its introduction, not only many a toilsome and thankless
hour was spared, but workers were multiplied, and encouraged
in the prosecution of labours more useful than attractive.
The career of Heinrich Olbers is a brilliant example of what may
be done by an amateur in astronomy. He at no time did regular
work in an observatory; he was never the possessor of a transit or
any other fixed instrument; moreover, all the best years of his life
were absorbed in the assiduous exercise of a toilsome profession.
Born in 1758 at the village of Arbergen, where his father was pastor,
he settled in 1781 as a physician in the neighbouring town of Bremen,
and continued in active practice there for over forty years. It was
thus only the hours which his robust constitution enabled him to
spare from sleep that were available for his intellectual pleasures.
Yet his recreation was, as Von Zach remarked,[241] no less prolific of
useful results than the severest work of other men. The upper part
of his house in the Sandgasse was fitted up with such instruments
and appliances as restrictions of space permitted, and there, night
after night during half a century and upwards, he discovered,
calculated, or observed the cometary visitants of northern skies.
Almost as effective in promoting the interests of science as the
valuable work actually done by him, was the influence of his genial
personality. He engaged confidence by his ready and discerning
sympathy; he inspired affection by his benevolent disinterestedness;
he quickened thought and awakened zeal by the suggestions of a
lively and inventive spirit, animated with the warmest enthusiasm
for the advancement of knowledge. Nearly every astronomer in
Germany enjoyed the benefits of a frequently active correspondence
with him, and his communications to the scientific periodicals of the[Pg 90]
time were numerous and striking. The motive power of his mind
was thus widely felt and continually in action. Nor did it wholly
cease to be exerted even when the advance of age and the progress
of infirmity rendered him incapable of active occupation. He was,
in fact, alive even to the last day of his long life of eighty-one years;
and his death, which occurred March 2, 1840, left vacant a position
which a rare combination of moral and intellectual qualities had
conspired to render unique.
Amongst the many younger men who were attracted and stimulated
by intercourse with him was Johann Franz Encke. But while Olbers
became a mathematician because he was an astronomer, Encke became
an astronomer because he was a mathematician. A born geometer,
he was naturally sent to Göttingen and placed under the tuition
of Gauss. But geometers are men; and the contagion of patriotic
fervour which swept over Germany after the battle of Leipsic did
not spare Gauss’s promising pupil. He took up arms in the Hanseatic
Legion, and marched and fought until the oppressor of his country
was safely ensconced behind the ocean-walls of St. Helena. In the
course of his campaigning he met Lindenau, the militant director of
the Seeberg Observatory, and by his influence was appointed his
assistant, and eventually, in 1822, became his successor. Thence he
was promoted in 1825 to Berlin, where he superintended the building
of the new observatory, so actively promoted by Humboldt, and
remained at its head until within some eighteen months of his death
in August, 1865.
On the 26th of November, 1818, Pons of Marseilles discovered a
comet, whose inconspicuous appearance gave little promise of its
becoming one of the most interesting objects in our system. Encke
at once took the calculation of its elements in hand, and brought
out the unexpected result that it revolved round the sun in a period
of about 3-1/3 years.[242] He, moreover, detected its identity with comets
seen by Méchain in 1786, by Caroline Herschel in 1795, by Pons,
Huth, and Bouvard in 1805, and after six laborious weeks of
research into the disturbances experienced by it from the planets
during the entire interval since its first ascertained appearance, he
fixed May 24, 1822, as the date of its next return to perihelion.
Although on that occasion, owing to the position of the earth,
invisible in the northern hemisphere, Sir Thomas Brisbane’s observatory
at Paramatta was fortunately ready equipped for its recapture,
which Rümker effected quite close to the spot indicated by Encke’s
ephemeris.
The importance of this event can be better understood when it is[Pg 91]
remembered that it was only the second instance of the recognised
return of a comet (that of Halley’s, sixty-three years previously,
having, as already stated, been the first); and that it, moreover,
established the existence of a new class of celestial objects, somewhat
loosely distinguished as “comets of short period.” These bodies (of
which about thirty have been found to circulate within the orbit of
Saturn) are remarkable as showing certain planetary affinities in the
manners of their motions not at all perceptible in the wider travelling
members of their order. They revolve, without exception, in the
same direction as the planets—from west to east; they exhibit a
marked tendency to conform to the zodiacal track which limits
planetary excursions north and south; and their paths round the
sun, although much more eccentric than the approximately circular
planetary orbits, are far less so than the extravagantly long ellipses
in which comets comparatively untrained (as it were) in the habits
of the solar system ordinarily perform their revolutions.
No great comet is of the “planetary” kind. These are, indeed,
only by exception visible to the naked eye; they possess extremely
feeble tail-producing powers, and give small signs of central condensation.
Thin wisps of cosmical cloud, they flit across the telescopic
field of view without sensibly obscuring the smallest star. Their
appearance, in short, suggests—what some notable facts in their
history will presently be shown to confirm—that they are bodies
already effete, and verging towards dissolution. If it be asked what
possible connection can be shown to exist between the shortness of
period by which they are essentially characterised, and what we
may call their superannuated condition, we are not altogether at a
loss for an answer. Kepler’s remark,[243] that comets are consumed by
their own emissions, has undoubtedly a measure of truth in it. The
substance ejected into the tail must, in overwhelmingly large proportion,
be for ever lost to the central mass from which it issues.
True, it is of a nature inconceivably tenuous; but unrepaired waste,
however small in amount, cannot be persisted in with impunity.
The incitement to such self-spoliation proceeds from the sun; it
accordingly progresses more rapidly the more numerous are the
returns to the solar vicinity. Comets of short period may thus
reasonably be expected to wear out quickly.
They are, moreover, subject to many adventures and vicissitudes.
Their aphelia—or the farthest points of their orbits from the sun—are
usually, if not invariably, situated so near to the path either
of Jupiter or of Saturn, as to permit these giant planets to act as
secondary rulers of their destinies. By their influence they were, in[Pg 92]
all likelihood, originally fixed in their present tracks; and by their
influence, exerted in an opposite sense, they may, in some cases, be
eventually ejected from them. Careers so varied, as can easily be
imagined, are apt to prove instructive, and astronomers have not
been backward in extracting from them the lessons they are fitted to
convey. Encke’s comet, above all, has served as an index to much
curious information, and it may be hoped that its function in that
respect is by no means at an end. The great extent of the solar
system traversed by its eccentric path makes it peculiarly useful for
the determination of the planetary masses. At perihelion it penetrates
within the orbit of Mercury; it considerably transcends at
aphelion the farthest excursion of Pallas. Its vicinity to the former
planet in August, 1835, offered the first convenient opportunity
of placing that body in the astronomical balance. Its weight or
mass had previously been assumed, not ascertained; and the comparatively
slight deviation from its regular course impressed upon
the comet by its attractive power showed that it had been assumed
nearly twice too great.[244] That fundamental datum of planetary
astronomy—the mass of Jupiter—was corrected by similar means;
and it was reassuring to find the correction in satisfactory accord
with that already introduced from observations of the asteroidal
movements.
The fact that comets contract in approaching the sun had been
noticed by Hevelius; Pingré admitted it with hesitating perplexity;[245]
the example of Encke’s comet rendered it conspicuous and undeniable.
On the 28th of October, 1828, the diameter of the
nebulous matter composing this body was estimated at 312,000
miles. It was then about one and a half times further from the
sun than the earth is at the time of the equinox. On the 24th of
December following, its distance being reduced by nearly two-thirds,
it was found to be only 14,000 miles across.[246] That is to say, it had
shrunk during those two months of approach to 1/11000th part of its
original volume! Yet it had still seventeen days’ journey to make
before reaching perihelion. The same curious circumstance was even
more markedly apparent at its return in 1838. Its bulk, or the
actual space occupied by it, appeared to be reduced, as it drew near
the hearth of our system, in the enormous proportion of 800,000[Pg 93]
to 1. A corresponding expansion accompanied on each occasion its
retirement from the sphere of observation. Similar changes of
volume, though rarely to the same astounding extent, have been
perceived in other comets. They still remain unexplained; but it
can scarcely be doubted that they are due to the action of the same
energetic internal forces which reveal themselves in so many splendid
and surprising cometary phenomena.
Another question of singular interest was raised by Encke’s
acute inquiries into the movements and disturbances of the first
known “comet of short period.” He found from the first that
its revolutions were subject to some influence besides that of
gravity. After every possible allowance had been made for the
pulls, now backward, now forward, exerted upon it by the several
planets, there was still a surplus of acceleration left unaccounted
for. Each return to perihelion took place about two and a half
hours sooner than received theories warranted. Here, then, was
a “residual phenomenon” of the utmost promise for the disclosure
of novel truths. Encke (in accordance with the opinion
of Olbers) explained it as due to the presence in space of some
such “subtle matter” as was long ago invoked by Euler[247] to be
the agent of eventual destruction for the fair scheme of planetary
creation. The apparent anomaly of accounting for an accelerative
effect by a retarding cause disappears when it is considered that any
check to the motion of bodies revolving round a centre of attraction
causes them to draw closer to it, thus shortening their periods and
quickening their circulation. If space were filled with a resisting
medium capable of impeding, even in the most infinitesimal degree,
the swift course of the planets, their orbits should necessarily be,
not ellipses, but very close elliptical spirals along which they would
slowly, but inevitably, descend into the burning lap of the sun.
The circumstance that no such tendency can be traced in their
revolutions by no means sets the question at rest. For it might
well be that an effect totally imperceptible until after the lapse of
countless ages, as regards the solid orbs of our system, might be
obvious in the movements of bodies like comets of small mass and
great bulk; just as a feather or a gauze veil at once yields its motion
to the resistance of the air, while a cannon-ball cuts its way through
with comparatively slight loss of velocity.
It will thus be seen that issues of the most momentous character
hang on the time-keeping of comets; for plainly all must in some
degree suffer the same kind of hindrance as Encke’s, if the
cause of that hindrance be the one suggested. None of its
congeners, however, show any trace of similar symptoms. True,[Pg 94]
the late Professor Oppolzer announced,[248] in 1880, that a comet,
first seen by Pons in 1819, and rediscovered by Winnecke in 1858,
having a period of 2,052 days (5·6 years), was accelerated at each
revolution precisely in the manner required by Encke’s theory.
But M. von Haerdtl’s subsequent investigation, the materials for
which included numerous observations of the body in question at its
return to the sun in 1886, decisively negatived the presence of any
such effect.[249] Moreover, the researches of Von Asten and Backlund[250]
into the movements of Encke’s comet revealed a perplexing circumstance.
They confirmed Encke’s results for the period covered by
them, but exhibited the acceleration as having suddenly diminished
by nearly one-half in 1868. The reality and permanence of this
change were fully established by observations of the ensuing
return in March, 1885. Some physical alteration of the retarded
body seems indicated; but visual evidence countenances no such
assumption. In aspect the comet is no less thin and diffuse than in
1795 or in 1848.
The character of the supposed resistance in inter-planetary space
has, it may be remarked, been often misapprehended. What Encke
stipulated for was not a medium equally diffused throughout the
visible universe, such as the ethereal vehicle of the vibrations of
light, but a rare fluid, rapidly increasing in density towards the sun.[251]
This cannot be a solar atmosphere, since it is mathematically certain,
as Laplace has shown,[252] that no envelope partaking of the sun’s axial
rotation can extend farther from his surface than nine-tenths of the
mean distance of Mercury; while physical evidence assures us that
the actual depth of the solar atmosphere bears a very minute proportion
to the possible depth theoretically assigned to it. That matter,
however, not atmospheric in its nature—that is, neither forming one
body with the sun nor altogether aëriform—exists in its neighbourhood,
can admit of no reasonable doubt. The great lens-shaped
mass of the zodiacal light, stretching out at times far beyond the
earth’s orbit, may indeed be regarded as an extension of the corona,
the streamers of which themselves mark the wide diffusion, all
round the solar globe, of granular or gaseous materials. Yet comets
have been known to penetrate the sphere occupied by them without
perceptible loss of velocity. The hypothesis, then, of a resisting
medium receives at present no countenance from the movements of
comets, whether of short or of long periods.
Although Encke’s comet has made thirty-five complete rounds of
its orbit since its first detection in 1786, it shows no certain signs[Pg 95]
of decay. Variations in its brightness are, it is true, conspicuous,
but they do not proceed continuously.[253]
The history of the next known planet-like comet has proved of
even more curious interest than that of the first. It was discovered
by an Austrian officer named Wilhelm von Biela at Josephstadt in
Bohemia, February 27, 1826, and ten days later by the French
astronomer Gambart at Marseilles. Both observers computed its
orbit, showed its remarkable similarity to that traversed by comets
visible in 1772 and 1805, and connected them together as previous
appearances of the body just detected by assigning to its revolutions
a period of between six and seven years. The two brief letters conveying
these strikingly similar inferences were printed side by side
in the same number of the Astronomische Nachrichten (No. 94); but
Biela’s priority in the discovery of the comet was justly recognised
by the bestowal upon it of his name.
The object in question was at no time, subsequently to 1805,
visible to the naked eye. Its aspect in Sir John Herschel’s great
reflector on the 23rd of September, 1832, was described by him as
that of a “conspicuous nebula,” nearly 3 minutes in diameter. No
trace of a tail was discernible. While he was engaged in watching
it, a small knot of minute stars was directly traversed by it, “and
when on the cluster,” he tells us,[254] it “presented the appearance of a
nebula resolvable and partly resolved into stars, the stars of the
cluster being visible through the comet.” Yet the depth of cometary
matter through which such faint stellar rays penetrated undimmed,
was, near the central parts of the globe, not less than 50,000 miles.
It is curious to find that this seemingly harmless, and we may
perhaps add effete body, gave occasion to the first (and not the last)
cometary “scare” of an enlightened century. Its orbit, at the
descending node, may be said to have intersected that of the earth;
since, according as it bulged in or out under the disturbing influence
of the planets, the passage of the comet was affected inside or outside
the terrestrial track. Now, certain calculations published by Olbers
in 1828[255] showed that, on October 29, 1832, a considerable portion
of its nebulous surroundings would actually sweep over the spot
which, a month later, would be occupied by our planet. It needed
no more to set the popular imagination in a ferment. Astronomers,
after all, could not, by an alarmed public, be held to be infallible.
Their computations, it was averred, which a trifling oversight would
suffice to vitiate, exhibited clearly enough the danger, but afforded
no guarantee of safety from a collision, with all the terrific consequences[Pg 96]
frigidly enumerated by Laplace. Nor did the panic subside
until Arago formally demonstrated that the earth and the comet
could by no possibility approach within less than fifty millions of
miles.[256]
The return of the same body in 1845-46 was marked by an extraordinary
circumstance. When first seen, November 28, it wore
its usual aspect of a faint round patch of cosmical fog; but on
December 19, Mr. Hind noticed that it had become distorted somewhat
into the form of a pear; and ten days later, it had divided
into two separate objects. This singular duplication was first
perceived at New Haven in America, December 29,[257] by Messrs.
Herrick and Bradley, and by Lieutenant Maury at Washington,
January 13, 1846. The earliest British observer of the phenomenon
(noticed by Wichmann the same evening at Königsberg) was
Professor Challis. “I see two comets!” he exclaimed, putting his
eye to the great equatoreal of the Cambridge Observatory on the
night of January 15; then, distrustful of what his senses had told
him, he called in his judgment to correct their improbable report by
resolving one of the dubious objects into a hazy star.[258] On the 23rd,
however, both were again seen by him in unmistakable cometary
shape, and until far on in March (Otto Struve caught a final glimpse
of the pair on the 16th of April),[259] continued to be watched with
equal curiosity and amazement by astronomers in every part of the
northern hemisphere. What Seneca reproved Ephorus for supposing
to have taken place in 373 b.c.—what Pingré blamed Kepler
for conjecturing in 1618—had then actually occurred under the
attentive eyes of science in the middle of the nineteenth century!
At a distance from each other of about two-thirds the distance of
the moon from the earth, the twin comets meantime moved on
tranquilly, so far, at least, as their course through the heaven was
concerned. Their extreme lightness, or the small amount of matter
contained in each, could not have received a more signal illustration
than by the fact that their revolutions round the sun were performed
independently; that is to say, they travelled side by side without
experiencing any appreciable mutual disturbance, thus plainly showing[Pg 97]
that at an interval of only 157,250 miles their attractive power
was virtually inoperative. Signs of internal agitation, however,
were not wanting. Each fragment threw out a short tail in a direction
perpendicular to the line joining their centres, and each
developed a bright nucleus, although the original comet had
exhibited neither of these signs of cometary vitality. A singular
interchange of brilliancy was, besides, observed to take place between
the coupled objects, each of which alternately outshone and was outshone
by the other, while an arc of light, apparently proceeding from
the more lustrous, at times bridged the intervening space. Obviously,
the gravitational tie, rendered powerless by exiguity of matter, was
here replaced by some other form of mutual action, the nature of
which can as yet be dealt with only by conjecture.
Once more, in August, 1852, the double comet returned to the
neighbourhood of the sun, but under circumstances not the most
advantageous for observation. Indeed, the companion was not
detected until September 16, when Father Secchi at Rome perceived
it to have increased its distance from the originating body to a
million and a quarter of miles, or about eight times the average
interval at the former appearance. Both vanished shortly afterwards,
and have never since been seen, notwithstanding the eager
watch kept for objects of such singular interest, and the accurate
knowledge of their track supplied by Santini’s investigations. A
dangerously near approach to Jupiter in 1841 is believed to have
occasioned their disruption, and the disaggregating process thus
started was likely to continue. We can scarcely doubt that
the fate has overtaken them which Newton assigned as the end
of all cometary existence. Diffundi tandem et spargi per cœlos
universos.[260]
Biela’s is not the only vanished comet. Brorsen’s, discovered at
Kiel in 1846, and observed at four subsequent returns, failed unaccountably
to become visible in 1890.[261] Yet numerous sentinels
were on the alert to surprise its approach along a well-ascertained
track, traversed in five and a half years. The object presented from
the first a somewhat time-worn aspect. It was devoid of tail, or
any other kind of appendage; and the rapid loss of the light
acquired during perihelion passage was accompanied by inordinate
expansion of an already tenuous globular mass. Another lost or
mislaid comet is one found by De Vico at Rome, August 22, 1844.
It was expected to return early in 1850, but did not, and has never[Pg 98]
since been seen; unless its re-appearance as E. Swift’s comet of 1894
should be ratified by closer inquiry.[262]
A telescopic comet with a period of 7-1/2 years, discovered November
22, 1843, by M. Faye of the Paris Observatory, formed the subject
of a characteristically patient and profound inquiry on the part of
Leverrier, designed to test its suggested identity with Lexell’s comet
of 1770. The result was decisive against the hypothesis of Valz,
the divergences between the orbits of the two bodies being found to
increase instead of to diminish, as the history of the new-comer was
traced backward into the previous century.[263] Faye’s comet pursues
the most nearly circular path of any similar known object; even
at its nearest approach to the sun it remains farther off than
Mars when he is most distant from it; and it was proved by the
admirable researches of Professor Axel Möller,[264] director of the
Swedish observatory of Lund, to exhibit no trace of the action of
a resisting medium.
Periodical comets are evidently bodies which have each lived
through a chapter of accidents, and a significant hint as to the
nature of their adventures can be gathered from the fact that their
aphelia are pretty closely grouped about the tracks of the major
planets. Halley’s, and five other comets are thus related to
Neptune; three connect themselves with Uranus, two with Saturn,
above a score with Jupiter. Some form of dependence is plainly
indicated, and the researches of Tisserand,[265] Callandreau,[266] and
Newton[267] of Yale College, leave scarcely a doubt that the “capture-theory”
represents the essential truth in the matter. The original
parabolic paths of these comets were then changed into ellipses
by the backward pull of a planet, whose sphere of attraction they
chanced to enter when approaching the sun from outer space.
Moreover, since a body thus affected should necessarily return at
each revolution to the scene of encounter, the same process of
retardation may, in some cases, have been repeated many times,
until the more restricted cometary orbits were reduced to their
present dimensions. The prevalence, too, among periodical comets,
of direct motion, is shown to be inevitable by M. Callandreau’s
demonstration that those travelling in a retrograde direction would,
by planetary action, be thrown outside the probable range of
terrestrial observation. The scarcity of hyperbolic comets can be[Pg 99]
similarly explained. They would be created whenever the attractive
influence of the disturbing planet was exerted in a forward or
accelerative sense, but could come only by a rare exception to our
notice. The inner planets, including the earth, have also unquestionably
played their parts in modifying cometary orbits; and
Mr. Plummer suggests, with some show of reason, that the capture
of Encke’s comet may be a feat due to Mercury.[268]
No great comet appeared between the “star” which presided at
the birth of Napoleon and the “vintage” comet of 1811. The
latter was first described by Flaugergues at Viviers, March 26, 1811;
Wisniewski, at Neu-Tscherkask in Southern Russia, caught a final
glimpse of it, August 17, 1812. Two disappearances in the solar
rays as the earth moved round in its orbit, and two reappearances
after conjunction, were included in this unprecedentedly long period
of visibility of 510 days. This relative permanence (so far as the
inhabitants of Europe were concerned) was due to the high northern
latitude attained near perihelion, combined with a certain leisureliness
of movement along a path everywhere external to that of the
earth. The magnificent luminous train of this body, on October 15,
the day of its nearest terrestrial approach, covered an arc of the
heavens 23-1/2 degrees in length, corresponding to a real extension of
one hundred millions of miles. Its form was described by Sir
William Herschel as that of “an inverted hollow cone,” and its
colour as yellowish, strongly contrasted with the bluish-green tint
of the “head,” round which it was flung like a transparent veil.
The planetary disc of the head, 127,000 miles across, appeared to
be composed of strongly-condensed nebulous matter; but somewhat
eccentrically situated within it was a star-like nucleus of a reddish
tinge, which Herschel presumed to be solid, and ascertained, with
his usual care, to have a diameter of 428 miles. From the total
absence of phases, as well as from the vivacity of its radiance, he
confidently inferred that its light was not borrowed, but inherent.[269]
This remarkable apparition formed the subject of a memoir by
Olbers,[270] the striking yet steadily reasoned out suggestions contained
in which there was at that time no means of following up with
profit. Only of late has the “electrical theory,” of which Zöllner[271]
regarded Olbers as the founder, assumed a definite and measurable
form, capable of being tested by the touchstone of fact, as knowledge
makes its slow inroads on the fundamental mystery of the physical
universe.
The paraboloidal shape of the bright envelope separated by a dark
interval from the head of the great comet of 1811, and constituting,
as it were, the root of its tail, seemed to the astronomer of Bremen
to reveal the presence of a double repulsion; the expelled vapours
accumulating where the two forces, solar and cometary, balanced
each other, and being then swept backwards in a huge train. He
accordingly distinguished three classes of these bodies:—First,
comets which develop no matter subject to solar repulsion. These
have no tails, and are probably mere nebulosities, without solid
nuclei. Secondly, comets which are acted upon by solar repulsion
only, and consequently throw out no emanations towards the sun.
Of this kind was a bright comet visible in 1807.[272] Thirdly, comets
like that of 1811, giving evidence of action of both kinds. These
are distinguished by a dark hoop encompassing the head and dividing
it from the luminous envelope, as well as by an obscure caudal axis,
resulting from the hollow, cone-like structure of the tail.
Again, the ingenious view subsequently propounded by M. Brédikhine
as to the connection between the form of these appendages and the
kind of matter composing them, was very clearly anticipated by
Olbers. The amount of tail-curvature, he pointed out, depends in
each case upon the proportion borne by the velocity of the ascending
particles to that of the comet in its orbit; the swifter the outrush,
the straighter the resulting tail. But the velocity of the ascending
particles varies with the energy of their repulsion by the sun, and
this again, it may be presumed, with their quality. Thus multiple
tails are developed when the same comet throws off, as it approaches
perihelion, specifically distinct substances. The long, straight ray
which proceeded from the comet of 1807, for example, was doubtless
made up of particles subject to a much more vigorous solar repulsion
than those formed into the shorter curved emanation issuing from
it nearly in the same direction. In the comet of 1811 he calculated
that the particles expelled from the head travelled to the remote
extremity of the tail in eleven minutes, indicating by this enormous
rapidity of movement (comparable to that of the transmission of
light) the action of a force much more powerful than the opposing
one of gravity. The not uncommon phenomena of multiple envelopes,
on the other hand, he explained as due to the varying amounts of
repulsion exercised by the nucleus itself on the different kinds of
matter developed from it.
The movements and perturbations of the comet of 1811 were
no less profoundly studied by Argelander than its physical
constitution by Olbers. The orbit which he assigned to it is of
such vast dimensions as to require no less that 3,065 years for[Pg 101]
the completion of its circuit; and to carry the body describing
it at each revolution to fourteen times the distance from the
sun of the frigid Neptune. Thus, when it last visited our
neighbourhood, Achilles may have gazed on its imposing train
as he lay on the sands all night bewailing the loss of Patroclus;
and when it returns, it will perhaps be to shine upon the ruins of
empires and civilizations still deep buried among the secrets of the
coming time.[273]
On the 26th of June, 1819, while the head of a comet passed
across the face of the sun, the earth was in all probability involved
in its tail. But of this remarkable double event nothing was
known until more than a month later, when the fact of its past
occurrence emerged from the calculations of Olbers.[274] Nor had the
comet itself been generally visible previous to the first days of July.
Several observers, however, on the publication of these results,
brought forward accounts of singular spots perceived by them upon
the sun at the time of the transit, and an original drawing of one
of them, by Pastorff of Buchholtz, has been preserved. This undoubtedly
authentic delineation[275] represents a round nebulous object
with a bright spot in the centre, of decidedly cometary aspect, and
not in the least like an ordinary solar “macula.” Mr. Hind,[276] nevertheless,
showed its position on the sun to be irreconcilable with that
which the comet must have occupied; and Mr. Ranyard’s discovery
of a similar smaller drawing by the same author, dated May 26,
1828,[277] reduces to evanescence the probability of its connection
with that body. Indeed, recent experience renders very doubtful
the possibility of such an observation.
The return of Halley’s comet in 1835 was looked forward to
as an opportunity for testing the truth of floating cometary theories,
and did not altogether disappoint expectation. As early as 1817,
its movements and disturbances since 1759 were proposed by the
Turin Academy of Sciences as the subject of a prize ultimately
awarded to Baron Damoiseau. Pontécoulant was adjudged a similar
distinction by the Paris Academy in 1829; while Rosenberger’s
calculations were rewarded with the gold medal of the Royal
Astronomical Society.[278]
They were verified by the detection at Rome, August 6, 1835, of[Pg 102]
a nearly circular misty object not far from the predicted place of
the comet. It was not, however, until the middle of September
that it began to throw out a tail, which by the 15th of October had
attained a length of about 24 degrees (on the 19th, at Madras, it extended
to fully 30),[279] the head showing to the naked eye as a reddish
star rather brighter than Aldebaran or Antares.[280] Some curious
phenomena accompanied the process of tail-formation. An outrush
of luminous matter, resembling in shape a partially opened fan,
issued from the nucleus towards the sun, and at a certain point, like
smoke driven before a high wind, was vehemently swept backwards
in a prolonged train. The appearance of the comet at this time was
compared by Bessel,[281] who watched it with minute attention, to that
of a blazing rocket. He made the singular observation that this
fan of light, which seemed the source of supply for the tail,
oscillated like a pendulum to and fro across a line joining the sun
and nucleus, in a period of 4-3/5 days; and he was unable to escape
from the conclusion[282] that a repulsive force, about twice as powerful
as the attractive force of gravity, was concerned in the production
of these remarkable effects. Nor did he hesitate to recur to the
analogy of magnetic polarity, or to declare, still more emphatically
than Olbers, “the emission of the tail to be a purely electrical
phenomenon.”[283]
The transformations undergone by this body were almost as
strange and complete as those which affected the brigands in Dante’s
Inferno. When first seen, it wore the aspect of a nebula; later it
put on the distinctive garb of a comet; it next appeared as a star;
finally, it dilated, first in a spherical, then in a paraboloidal form,
until May 5, 1836, when it vanished from Herschel’s observation at
Feldhausen as if by melting into adjacent space from the excessive
diffusion of its light. A very uncommon circumstance in its development
was that it lost all trace of tail previous to its arrival at perihelion
on the 16th of November. Nor did it begin to recover
its elongated shape for more than two months afterwards. On the
23rd of January, Boguslawski perceived it as a star of the
sixth magnitude, without measurable disc.[284] Only two nights later,
Maclear, director of the Cape Observatory, found the head to be 131
seconds across.[285] And so rapidly did the augmentation of size
progress, that Sir John Herschel estimated the actual bulk of this[Pg 103]
singular object to have increased forty-fold in the ensuing week.
“I can hardly doubt,” he remarks, “that the comet was fairly
evaporated in perihelio by the heat, and resolved into transparent
vapour, and is now in process of rapid condensation and re-precipitation
on the nucleus.”[286] A plausible, but no longer admissible, interpretation
of this still unexplained phenomenon. The next return
of this body, which will be considerably accelerated by Jupiter’s
influence, is expected to take place in 1910.[287]
By means of an instrument devised to test the quality of light,
Arago obtained decisive evidence that some at least of the radiance
proceeding from Halley’s comet was derived by reflection from the
sun.[288] Indications of the same kind had been afforded[289] by the
comet which suddenly appeared above the north-western horizon of
Paris, July 3, 1819, after having enveloped (as already stated) our
terrestrial abode in its filmy appendages; but the “polariscope” had
not then reached the perfection subsequently given to it, and its
testimony was accordingly far less reliable than in 1835. Such experiments,
however, are in reality more beautiful and ingenious than
instructive, since ignited as well as obscure bodies possess the power
of throwing back light incident upon them, and will consequently
transmit to us from the neighbourhood of the sun rays partly direct,
partly reflected, of which a certain proportion will exhibit the
peculiarity known as polarisation.
The most brilliant comets of the century were suddenly rivalled
if not surpassed by the extraordinary object which blazed out beside
the sun, February 28, 1843. It was simultaneously perceived in
Mexico and the United States, in Southern Europe, and at sea off
the Cape of Good Hope, where the passengers on board the Owen
Glendower were amazed by the sight of a “short, dagger-like object,”
closely following the sun towards the western horizon.[290] At Florence,
Amici found its distance from the sun’s centre at noon to be only
1° 23′; and spectators at Parma were able, when sheltered from
the direct glare of mid-day, to trace the tail to a length of four or
five degrees. The full dimensions of this astonishing appurtenance
began to be disclosed a few days later. On the 3rd of March it
measured 25°, and on the 11th, at Calcutta, Mr. Clerihew observed
a second streamer, nearly twice as long as the first, and making an
angle with it of 18°, to have been emitted in a single day. This
rapidity of projection, Sir John Herschel remarked, “conveys an
astounding impression of the intensity of the forces at work.” “It
[Pg 104]is clear,” he continued, “that if we have to deal here with matter, such
as we conceive it—viz., possessing inertia—at all, it must be under the
dominion of forces incomparably more energetic than gravitation,
and quite of a different nature.”[291]
On the 17th of March a silvery ray, some 40° long and slightly
curved at its extremity, shone out above the sunset clouds in this
country. No previous intimation had been received of the possibility
of such an apparition, and even astronomers—no lightning
messages across the seas being as yet possible—were perplexed.
The nature of the phenomenon, indeed, soon became evident, but
the wonder of it did not diminish with the study of its attendant
circumstances. Never before, within astronomical memory, had our
system been traversed by a body pursuing such an adventurous
career. The closest analogy was offered by the great comet of 1680
(Newton’s), which rushed past the sun at a distance of only
144,000 miles; but even this—on the cosmical scale—scarcely perceptible
interval was reduced nearly one-half in the case we are now
concerned with. The centre of the comet of 1843 approached the
formidable luminary within 78,000 miles, leaving, it is estimated, a
clear space of not more than 32,000 between the surfaces of the
bodies brought into such perilous proximity. The escape of the
wanderer was, however, secured by the extraordinary rapidity of its
flight. It swept past perihelion at a rate—366 miles a second—which,
if continued, would have carried it right round the sun in
two hours; and in only eleven minutes more than that short period
it actually described half the curvature of its orbit—an arc of 180°—although
in travelling over the remaining half many hundreds of
sluggish years will doubtless be consumed.
The behaviour of this comet may be regarded as an experimentum
crucis as to the nature of tails. For clearly no fixed appendage
many millions of miles in length could be whirled like a brandished
sabre from one side of the sun to the other in 131 minutes. Cometary
trains are then, as Olbers rightly conceived them to be, emanations,
not appendages—inconceivably rapid outflows of highly rarefied
matter, the greater part, if not all, of which becomes permanently
detached from the nucleus.
That of the comet of 1843 reached, about the time that it became
visible in this country, the extravagant length of 200 millions of
miles.[292] It was narrow, and bounded by nearly parallel and nearly
rectilinear lines, resembling—to borrow a comparison of Aristotle’s—a
“road” through the constellations; and after the 3rd of March
showed no trace of hollowness, the axis being, in fact, rather brighter[Pg 105]
than the edges. Distinctly perceptible in it were those singular
aurora-like coruscations which gave to the “tresses” of Charles V.’s
comet the appearance—as Cardan described it—of “a torch agitated
by the wind,” and have not unfrequently been observed to characterise
other similar objects. A consideration first adverted to by
Olbers proves these to originate in our own atmosphere. For owing
to the great difference in the distances from the earth of the origin
and extremity of such vast effluxes, the light proceeding from their
various parts is transmitted to our eyes in notably different intervals
of time. Consequently a luminous undulation, even though propagated
instantaneously from end to end of a comet’s tail, would
appear to us to occupy many minutes in its progress. But the
coruscations in question pass as swiftly as a falling star. They are,
then, of terrestrial production.
Periods of the utmost variety were by different computators
assigned to the body, which arrived at perihelion, February 27, 1843,
at 9.47 p.m. Professor Hubbard of Washington found that it
required 533 years to complete a revolution; MM. Laugier and
Mauvais of Paris considered the true term to be 35;[293] Clausen
looked for its return at the end of between six and seven. A recent
discussion[294] by Professor Kreutz of all the available data gives a probable
period of 512 years for this body, and precludes its hypothetical
identity with the comet of 1668, known as the “Spina” of Cassini.
It may now be asked, what were the conclusions regarding the
nature of comets drawn by astronomers from the considerable amount
of novel experience accumulated during the first half of this century?
The first and best assured was that the matter composing them is
in a state of extreme tenuity. Numerous and trustworthy observations
showed that the feeblest rays of light might traverse some
hundreds of thousands of miles of their substance, even where it
was apparently most condensed, without being perceptibly weakened.
Nay, instances were recorded in which stars were said to have
gained in brightness from the process![295] On the 24th of June, 1825,
Olbers[296] saw the comet then visible all but obliterated by the central
passage of a star too small to be distinguished with the naked eye,
its own light remaining wholly unchanged. A similar effect was
noted December 1, 1811, when the great comet of that year
approached so close to Altair, the lucida of the Eagle, that the star
seemed to be transformed into the nucleus of the comet.[297] Even the[Pg 106]
central blaze of Halley’s comet in 1835 was powerless to impede the
passage of stellar rays. Struve[298] observed at Dorpat, on September
17, an all but central occultation; Glaisher[299] one (so far as he could
ascertain) absolutely so eight days later at Cambridge. In neither
case was there any appreciable diminution of the star’s light. Again,
on the 11th of October, 1847, Mr. Dawes,[300] an exceptionally keen
observer, distinctly saw a star of the tenth magnitude through the
exact centre of a comet discovered on the first of that month by
Maria Mitchell of Nantucket.
Examples, on the other hand, are not wanting of the diminution
of stellar light under similar circumstances;[301] and we meet two
alleged instances of the vanishing of a star behind a comet.
Wartmann of Geneva observed the first, November 28, 1828;[302] but
his instrument was defective, and the eclipsing body, Encke’s comet,
has shown itself otherwise perfectly translucent. The second case
of occultation occurred September 13, 1890, when an eleventh
magnitude star was stated to have completely disappeared during
the transit over it of Denning’s comet.[303]
From the failure to detect any effects of refraction in the light of
stars occulted by comets, it was inferred (though, as we know now,
erroneously) that their composition is rather that of dust than that
of vapour; that they consist not of any continuous substance, but
of discrete solid particles, very finely divided and widely scattered.
In conformity with this view was the known smallness of their
masses. Laplace had shown that if the amount of matter forming
Lexell’s comet had been as much as 1/5000 of that contained in our
globe, the effect of its attraction, on the occasion of its approach
within 1,438,000 miles of the earth, July 1, 1770, must have been
apparent in the lengthening of the year. And that some comets, at
any rate, possess masses immeasurably below this maximum value
was clearly proved by the undisturbed parallel march of the two
fragments of Biela’s in 1846.
But the discovery in this branch most distinctive of the period
under review is that of “short period” comets, of which four[304] were
known in 1850. These, by the character of their movements, serve
as a link between the planetary and cometary worlds, and by the[Pg 107]
nature of their construction, seem to mark a stage in cometary
decay. For that comets are rather transitory agglomerations, than
permanent products of cosmical manufacture, appeared to be demonstrated
by the division and disappearance of one amongst their
number, as well as by the singular and rapid changes in appearance
undergone by many, and the seemingly irrevocable diffusion of
their substance visible in nearly all. They might then be defined,
according to the ideas respecting them prevalent fifty years ago, as
bodies unconnected by origin with the solar system, but encountered,
and to some extent appropriated, by it in its progress through
space, owing their visibility in great part, if not altogether, to light
reflected from the sun, and their singular and striking forms to the
action of repulsive forces emanating from him, the penalty of their
evanescent splendour being paid in gradual waste and final dissipation
and extinction.[Pg 108]
FOOTNOTES:
[241] Allgemeine Geographische Ephemeriden, vol. iv., p. 287.
[242] Astr. Jahrbuch, 1823, p. 217. The period (1,208 days) of this body is
considerably shorter than that of any other known comet.
[243] “Sicut bombyces filo fundendo, sic cometas cauda exspiranda consumi et
denique mori.”—De Cometis, Op., vol. vii., p. 110.
[244] Considerable uncertainty, however, still prevails on the point. The inverse
relation assumed by Lagrange to exist between distance from the sun and density
brought out the Mercurian mass 1/2025810 that of the sun (Laplace, Exposition du
Syst. du Monde, t. ii., p. 50, ed. 1824). Von Asten deduced from the movements
of Encke’s comet, 1818-48, a value of 1/7636440; while Backlund from its
seven returns, 1871-1891, derived 1/9647000 (Comptes Rendus, Oct. 1, 1894).
[245] Arago, Annuaire (1832), p. 218.
[246] Hind, The Comets, p. 20.
[247] Phil. Trans., vol. xlvi., p. 204.
[248] Astr. Nach., No. 2,134.
[249] Comptes Rendus, t. cvii., p. 588.
[250] Mém. de St. Pétersbourg, t. xxxii., No. 3, 1884; Astr. Nach., No. 2,727.
[251] Month. Not., vol. xix., p. 72.
[252] Mécanique Céleste, t. ii., p. 197.
[253] See Berberich, Astr. Nach., Nos. 2,836-7, 3,125; Deichmüller, Ibid.,
No. 3,123.
[254] Month. Not., vol. ii., p. 117.
[255] Astr. Nach., No. 128.
[256] Annuaire, 1832, p. 186.
[257] Am. Journ. of Science, vol. i. (2nd series), p. 293. Prof. Hubbard’s calculations
indicated a probability that the definitive separation of the two nuclei
occurred as early as September 30, 1884. Astronomical Journal (Gould’s),
vol. iv., p. 5. See also, on the subject of this comet, W. T. Lynn, Intellectual
Observer, vol. xi., p. 208; E. Ledger, Observatory, August, 1883, p. 244; and
H. A. Newton, Am. Journ. of Science, vol. xxxi., p. 81, February, 1886.
[258] Month. Not., vol. vii., p. 73.
[259] Bulletin Ac. Imp. de St. Pétersbourg, t. vi., col. 77. The latest observation
of the parent nucleus was that of Argelander, April 27, at Bonn.
[260] D’Arrest, Astr. Nach., No. 1,624.
[261] Der Brorsen’sche Comet. Von Dr. E. Lamp, Kiel, 1892; Plummer, Knowledge,
vol. xix., p. 41.
[262] Schulhof, Astr. Nach., No. 3,267; Observatory, vol. xviii., p. 64; F. H.
Seares, Astr. Nach., Nos. 3,606-7; Plummer, Knowledge, vol. xix., p. 156.
[263] Comptes Rendus, t. xxv., p. 570.
[264] Month. Not., vol. xii., p. 248.
[265] Bull. Astr., t. vi., pp. 241, 289.
[266] Étude sur la Théorie des Comètes périodiques. Annales de l’Observatoíre, t. xx.,
Paris, 1891.
[267] Amer. Journ. of Science, vol. xlii., pp. 183, 482, 1891.
[268] Observatory, vol. xiv., p. 194.
[269] Phil. Trans., vol. cii., pp. 118-124.
[270] Ueber den Schweif des grossen Cometen von 1811, Monat. Corr., vol. xxv.,
pp. 3-22. Reprinted by Zöllner. Ueber die Natur der Cometen, pp. 3-15.
[271] Natur der Cometen, p. 148.
[272] The subject of a classical memoir by Bessel, published in 1810.
[273] A fresh investigation of its orbit has been published by N. Herz of Vienna.
See Bull. Astr., t. ix., p. 427.
[274] Astr. Jahrbuch (Bode’s), 1823, p. 134.
[275] Reproduced in Webb’s Celestial Objects, 4th ed.
[276] Month. Not., vol. xxxvi., p. 309.
[277] Celestial Objects, p. 40, note.
[278] See Airy’s Address, Mem. R. A. S., vol. x., p. 376. Rosenberger calculated
no more, though he lived until 1890. W. T. Lynn, Observatory, vol. xiii.,
p. 125.
[279] Hind, The Comets, p. 47.
[280] Arago, Annuaire, 1836, p. 228.
[281] Astr. Nach., No. 300.
[282] It deserves to be recorded that Robert Hooke drew a very similar inference
from his observations of the comets of 1680 and 1682. Month. Not., vol. xiv.,
pp. 77-83.
[283] Briefwechsel zwischen Olbers und Bessel, Bd. ii., p. 390.
[284] Herschel, Results, p. 405.
[285] Mem. R. A. S., vol. x., p. 92,
[286] Results, p. 401.
[287] Pontécoulant, Comptes Rendus, t. lviii., p. 825.
[288] Annuaire, 1836, p. 233.
[289] Cosmos, vol. i., p. 90, note (Otté’s trans.).
[290] Herschel, Outlines of Astronomy, p. 399, 9th ed.
[291] Outlines, p. 398.
[292] Boguslawski calculated that it extended on the 21st of March to 581
millions.—Report. Brit. Ass., 1845, p. 89.
[293] Comptes Rendus, t. xvi., p. 919.
[294] Observatory, vol. xxiv., p. 167; Astr. Nach., No. 3,320.
[295] Piazzi noticed a considerable increase of lustre in a very faint star of the
twelfth magnitude viewed through a comet. Mädler, Reden, etc., p. 248, note.
[296] Astr. Jahrbuch, 1828, p. 151.
[297] Mädler, Gesch. d. Astr., Bd. ii., p. 412.
[298] Recueil de l’Ac. Imp. de St. Pétersbourg, 1835, p. 143.
[299] Guillemin’s World of Comets, trans, by J. Glaisher, p. 294, note.
[300] Month. Not., vol. viii., p. 9.
[301] A real, though only partial stoppage of light seems indicated by Herschel’s
observations on the comet of 1807. Stars seen through the tail, October 18, lost
much of their lustre. One near the head was only faintly visible by glimpses.
Phil. Trans., vol. xcvii., p. 153.
[302] Arago, Annuaire, 1832, p. 205.
[303] Ibid., 1891, p. 290.
[304] Viz., Encke’s, Biela’s, Faye’s, and Brorsen’s.
CHAPTER VI
INSTRUMENTAL ADVANCES
It is impossible to follow with intelligent interest the course of
astronomical discovery without feeling some curiosity as to the
means by which such surpassing results have been secured.
Indeed, the bare acquaintance with what has been achieved, without
any corresponding knowledge of how it has been achieved, supplies
food for barren wonder rather than for fruitful and profitable
thought. Ideas advance most readily along the solid ground of
practical reality, and often find true sublimity while laying aside
empty marvels. Progress is the result, not so much of sudden
flights of genius, as of sustained, patient, often commonplace endeavour;
and the true lesson of scientific history lies in the close
connection which it discloses between the most brilliant developments
of knowledge and the faithful accomplishment of his daily task by
each individual thinker and worker.
It would be easy to fill a volume with the detailed account of
the long succession of optical and mechanical improvements by
means of which the observation of the heavens has been brought
to its present degree of perfection; but we must here content
ourselves with a summary sketch of the chief amongst them.
The first place in our consideration is naturally claimed by the
telescope.
This marvellous instrument, we need hardly remind our readers,
is of two distinct kinds—that in which light is gathered together
into a focus by refraction, and that in which the same end is attained
by reflection. The image formed is in each case viewed through a
magnifying lens, or combination of lenses, called the eye-piece. Not
for above a century after the “optic glasses” invented or stumbled
upon by the spectacle-maker of Middelburg (1608) had become
diffused over Europe, did the reflecting telescope come, even in
England, the place of its birth, into general use. Its principle (a
sufficiently obvious one) had indeed been suggested by Mersenne as[Pg 109]
early as 1639;[305] James Gregory in 1663[306] described in detail a mode
of embodying that principle in a practical shape; and Newton,
adopting an original system of construction, actually produced in
1668 a tiny speculum, one inch across, by means of which the
apparent distance of objects was reduced thirty-nine times. Nevertheless,
the exorbitantly long tubeless refractors, introduced by
Huygens, maintained their reputation until Hadley exhibited to the
Royal Society, January 12, 1721,[307] a reflector of six inches aperture,
and sixty-two in focal length, which rivalled in performance, and of
course indefinitely surpassed in manageability, one of the “aerial”
kind of 123 feet.
The concave-mirror system now gained a decided ascendant,
and was brought to unexampled perfection by James Short of
Edinburgh during the years 1732-68. Its resources were, however,
first fully developed by William Herschel. The energy and inventiveness
of this extraordinary man marked an epoch wherever
they were applied. His ardent desire to measure and gauge the
stupendous array of worlds which his specula revealed to him, made
him continually intent upon adding to their “space-penetrating
power” by increasing their light-gathering surface. These, as he
was the first to explain,[308] are in a constant proportion one to the
other. For a telescope with twice the linear aperture of another
will collect four times as much light, and will consequently
disclose an object four times as faint as could be seen with the
first, or, what comes to the same, an object equally bright at twice
the distance. In other words, it will possess double the space-penetrating
power of the smaller instrument. Herschel’s great
mirrors—the first examples of the giant telescopes of modern
times—were then primarily engines for extending the bounds of
the visible universe; and from the sublimity of this “final cause”
was derived the vivid enthusiasm which animated his efforts to
success.
It seems probable that the seven-foot telescope constructed by
him in 1775—that is within little more than a year after his
experiments in shaping and polishing metal had begun—already
exceeded in effective power any work by an earlier optician;
and both his skill and his ambition rapidly developed. His
efforts culminated, after mirrors of ten, twenty, and thirty feet
focal length had successively left his hands, in the gigantic
forty-foot, completed August 28, 1789. It was the first reflector
in which only a single mirror was employed. In the
“Gregorian” form, the focussed rays are, by a second reflection[Pg 110]
from a small concave[309] mirror, thrown straight back through a
central aperture in the larger one, behind which the eye-piece is
fixed. The object under examination is thus seen in the natural
direction. The “Newtonian,” on the other hand, shows the object in
a line of sight at right angles to the true one, the light collected by
the speculum being diverted to one side of the tube by the interposition
of a small plane mirror, situated at an angle of 45° to the
axis of the instrument. Upon these two systems Herschel worked
until 1787, when, becoming convinced of the supreme importance of
economising light (necessarily wasted by the second reflection), he
laid aside the small mirror of his forty-foot then in course of construction,
and turned it into a “front-view” reflector. This was done—according
to the plan proposed by Lemaire in 1732—by slightly
inclining the speculum so as to enable the image formed by it to be
viewed with an eye-glass fixed at the upper margin of the tube.
The observer thus stood with his back turned to the object he was
engaged in scrutinising.
The advantages of the increased brilliancy afforded by this
modification were strikingly illustrated by the discovery, August 28
and September 17, 1789, of the two Saturnian satellites nearest
the ring. Nevertheless, the monster telescope of Slough cannot be
said to have realised the sanguine expectations of its constructor.
The occasions on which it could be usefully employed were found
to be extremely rare. It was injuriously affected by every change
of temperature. The great weight (25 cwt.) of a speculum four feet
in diameter rendered it peculiarly liable to distortion. With all
imaginable care, the delicate lustre of its surface could not be
preserved longer than two years,[310] when the difficult process of
repolishing had to be undertaken. It was accordingly never used
after 1811, when, having gone blind from damp, it lapsed by degrees
into the condition of a museum inmate.
The exceedingly high magnifying powers employed by Herschel
constituted a novelty in optical astronomy, to which he attached
great importance. The work of ordinary observation would,
however, be hindered rather than helped by them. The attempt to
increase in this manner the efficacy of the telescope is speedily
checked by atmospheric, to say nothing of other difficulties.
Precisely in the same proportion as an object is magnified, the
disturbances of the medium through which it is seen are magnified[Pg 111]
also. Even on the clearest and most tranquil nights, the air is never
for a moment really still. The rays of light traversing it are continually
broken by minute fluctuations of refractive power caused
by changes of temperature and pressure, and the currents which
these engender. With such luminous quiverings and waverings the
astronomer has always more or less to reckon; their absence is
simply a question of degree; if sufficiently magnified, they are at
all times capable of rendering observation impossible.
Thus, such powers as 3,000, 4,000, 5,000, even 6,652,[311] which
Herschel now and again applied to his great telescopes, must, save
on the rarest occasions, prove an impediment rather than an aid to
vision. They were, however, used by him only for special purposes,
experimentally, not systematically, and with the clearest discrimination
of their advantages and drawbacks. It is obvious that perfectly
different ends are subserved by increasing the aperture and by increasing
the power of a telescope. In the one case, a larger quantity
of light is captured and concentrated; in the other, the same amount
is distributed over a wider area. A diminution of brilliancy in the
image accordingly attends, cœteris paribus, upon each augmentation of
its apparent size. For this reason, such faint objects as nebulæ are
most successfully observed with moderate powers applied to instruments
of a great capacity for light, the details of their structure
actually disappearing when highly magnified. With stellar groups
the reverse is the case. Stars cannot be magnified, simply because
they are too remote to have any sensible dimensions; but the space
between them can. It was thus for the purpose of dividing very
close double stars that Herschel increased to such an unprecedented
extent the magnifying capabilities of his instruments; and to this
improvement incidentally the discovery of Uranus, March 13, 1781,[312]
was due. For by the examination with strong lenses of an object
which, even with a power of 227, presented a suspicious appearance,
he was able at once to pronounce its disc to be real, not merely
“spurious,” and so to distinguish it unerringly from the crowd of
stars amidst which it was moving.
While the reflecting telescope was astonishing the world by its
rapid development in the hands of Herschel, its unpretending rival
was slowly making its way towards the position which the future
had in store for it. The great obstacle which long stood in the way
of the improvement of refractors was the defect known as “chromatic
aberration.” This is due to no other cause than that which produces[Pg 112]
the rainbow and the spectrum—the separation, or “dispersion” in
their passage through a refracting medium, of the variously coloured
rays composing a beam of white light. In an ordinary lens there
is no common point of concentration; each colour has its own
separate focus; and the resulting image, formed by the superposition
of as many images as there are hues in the spectrum, is indefinitely
terminated with a tinted border, eminently baffling to
exactness of observation.
The extravagantly long telescopes of the seventeenth century
were designed to avoid this evil (as well as another source of indistinct
vision in the spherical shape of lenses); but no attempt to
remedy it was made until an Essex gentleman succeeded, in 1733, in
so combining lenses of flint and crown glass as to produce refraction
without colour.[313] Mr. Chester More Hall was, however, equally
indifferent to fame and profit, and took no pains to make his invention
public. The effective discovery of the achromatic telescope was,
accordingly, reserved for John Dollond, whose method of correcting
at the same time chromatic and spherical aberration was laid before
the Royal Society in 1758. Modern astronomy may be said to have
been thereby rendered possible. Refractors have always been found
better suited than reflectors to the ordinary work of observatories.
They are, so to speak, of a more robust, as well as of a more plastic
nature. They suffer less from vicissitudes of temperature and
climate. They retain their efficiency with fewer precautions and
under more trying circumstances. Above all, they co-operate more
readily with mechanical appliances, and lend themselves with far
greater facility to purposes of exact measurement.
A practical difficulty, however, impeded the realisation of the
brilliant prospects held out by Dollond’s invention. It was found
impossible to procure flint-glass, such as was needed for optical use—that
is, of perfectly homogeneous quality—except in fragments of
insignificant size. Discs of more than two or three inches in
diameter were of extreme rarity; and the crushing excise duty
imposed upon the article by the financial unwisdom of the Government,
both limited its production, and, by rendering experiments
too costly for repetition, barred its improvement.
Up to this time, Great Britain had left foreign competitors far
behind in the instrumental department of astronomy. The quadrants
and circles of Bird, Cary and Ramsden were unapproached abroad.
The reflecting telescope came into existence and reached maturity
on British soil. The refracting telescope was cured of its inherent[Pg 113]
vices by British ingenuity. But with the opening of the nineteenth
century, the almost unbroken monopoly of skill and contrivance
which our countrymen had succeeded in establishing was invaded,
and British workmen had to be content to exchange a position of
supremacy for one of at least partial temporary inferiority.
Somewhat about the time that Herschel set about polishing his
first speculum, Pierre Louis Guinand, a Swiss artisan, living near
Chaux-de-Fonds, in the canton of Neuchâtel, began to grind spectacles
for his own use, and was thence led on to the rude construction of
telescopes by fixing lenses in pasteboard tubes. The sight of an
England achromatic stirred a higher ambition, and he took the first
opportunity of procuring some flint glass from England (then the
only source of supply), with the design of imitating an instrument
the full capabilities of which he was destined to be the humble
means of developing. The English glass proving of inferior quality,
he conceived the possibility, unaided and ignorant of the art as he
was, of himself making better, and spent seven years (1784-90) in
fruitless experiments directed to that end. Failure only stimulated
him to enlarge their scale. He bought some land near Les Brenets,
constructed upon it a furnace capable of melting two quintals of
glass, and reducing himself and his family to the barest necessaries
of life, he poured his earnings (he at this time made bells for
repeaters) unstintingly into his crucibles.[314] His undaunted resolution
triumphed. In 1799 he carried to Paris and there showed to
Lalande several discs of flawless crystal four to six inches in diameter.
Lalande advised him to keep his secret, but in 1805 he was induced
to remove to Munich, where he became the instructor of the immortal
Fraunhofer. His return to Les Brenets in 1814 was
signalised by the discovery of an ingenious mode of removing
striated portions of glass by breaking and re-soldering the product
of each melting, and he eventually attained to the manufacture of
perfect discs up to 18 inches in diameter. An object-glass for which
he had furnished the material to Cauchoix, procured him, in 1823, a
royal invitation to settle in Paris; but he was no longer equal to the
change, and died at the scene of his labours, February 13 following.
This same lens (12 inches across) was afterwards purchased by
Sir James South, and the first observation made with it, February
13, 1830, disclosed to Sir John Herschel the sixth minute star in
the central group of the Orion nebula, known as the “trapezium.”[315]
Bequeathed by South to Trinity College, Dublin, it was employed
at the Dunsink Observatory by Brünnow and Ball in their investigations
of stellar parallax. A still larger objective (of nearly 14
inches) made of Guinand’s glass was secured in Paris, about the same[Pg 114]
time, by Mr. Edward Cooper of Markree Castle, Ireland. The
peculiarity of the method discovered at Les Brenets resided in the
manipulation, not in the quality of the ingredients; the secret, that
is to say, was not chemical, but mechanical.[316] It was communicated
by Henry Guinand (a son of the inventor) to Bontemps, one of the
directors of the glassworks at Choisy-le-Roi, and by him transmitted
to Messrs. Chance of Birmingham, with whom he entered into
partnership when the revolutionary troubles of 1848 obliged him to
quit his native country. The celebrated American opticians, Alvan
Clark & Sons, derived from the Birmingham firm the materials for
some of their early telescopes, notably the 19-inch Chicago and
26-inch Washington equatoreals; but the discs for the great Lick
refractor, and others shaped by them in recent years, have been
supplied by Feil of Paris.
Two distinguished amateurs, meanwhile, were preparing to reassert
on behalf of reflecting instruments their claim to the place
of honour in the van of astronomical discovery. Of Mr. Lassell’s
specula something has already been said.[317] They were composed of
an alloy of copper and tin, with a minute proportion of arsenic
(after the example of Newton[318]), and were remarkable for perfection
of figure and brilliancy of surface.
The capabilities of the Newtonian plan were developed still more
fully—it might almost be said to the uttermost—by the enterprise
of an Irish nobleman. William Parsons, known as Lord Oxmantown
until 1841, when, on his father’s death, he succeeded to the title of
Earl of Rosse, was born at York, June 17, 1800. His public duties
began before his education was completed. He was returned to
Parliament as member for King’s County while still an undergraduate
at Oxford, and continued to represent the same constituency
for thirteen years (1821-34). From 1845 until his death,
which took place, October 31, 1867, he sat, silent but assiduous, in
the House of Lords as an Irish representative peer; he held the not
unlaborious post of President of the Royal Society from 1849 to
1854; presided over the meeting of the British Association at Cork
in 1843, and was elected Vice-Chancellor of Dublin University in
1862. In addition to these extensive demands upon his time and
thoughts, were those derived from his position as practically the
feudal chief of a large body of tenantry in times of great and
anxious responsibility, to say nothing of the more genial claims of
an unstinted hospitality. Yet, while neglecting no public or private
duty, this model nobleman found leisure to render to science services
so conspicuous as to entitle his name to a lasting place in its annals.[Pg 115]
He early formed the design of reaching the limits of the attainable
in enlarging the powers of the telescope, and the qualities of his
mind conspired with the circumstances of his fortune to render the
design a feasible one. From refractors it was obvious that no such
vast and rapid advance could be expected. English glass-manufacture
was still in a backward state. So late as 1839, Simms (successor to
the distinguished instrumentalist Edward Troughton) reported a
specimen of crystal scarcely 7-1/2 inches in diameter, and perfect only
over six, to be unique in the history of English glass-making.[319] Yet
at that time the fifteen-inch achromatic of Pulkowa had already left
the workshop of Fraunhofer’s successors at Munich. It was not
indeed until 1845, when the impost which had so long hampered
their efforts was removed, that the optical artists of these islands
were able to compete on equal terms with their rivals on the
Continent. In the case of reflectors, however, there seemed no
insurmountable obstacle to an almost unlimited increase of light-gathering
capacity; and it was here, after some unproductive experiments
with fluid lenses, that Lord Oxmantown concentrated his
energies.
He had to rely entirely on his own invention, and to earn his own
experience. James Short had solved the problem of giving to
metallic surfaces a perfect parabolic figure (the only one by which
parallel incident rays can be brought to an exact focus); but so jealous
was he of his secret, that he caused all his tools to be burnt before his
death;[320] nor was anything known of the processes by which Herschel
had achieved his astonishing results. Moreover, Lord Oxmantown
had no skilled workmen to assist him. His implements, both animate
and inanimate, had to be formed by himself. Peasants taken from
the plough were educated by him into efficient mechanics and
engineers. The delicate and complex machinery needed in operations
of such hairbreadth nicety as his enterprise involved, the
steam-engine which was to set it in motion, at times the very
crucibles in which his specula were cast, issued from his own workshops.
In 1827 experiments on the composition of speculum-metal were
set on foot, and the first polishing-machine ever driven by steam-power
was contrived in 1828. But twelve arduous years of
struggle with recurring difficulties passed before success began
to dawn. A material less tractable than the alloy selected, of
four chemical equivalents of copper to one of tin,[321] can scarcely be
conceived. It is harder than steel, yet brittle as glass, crumbling[Pg 116]
into fragments with the slightest inadvertence of handling or treatment;[322]
and the precision of figure requisite to secure good definition
is almost beyond the power of language to convey. The quantities
involved are so small as not alone to elude sight, but to confound
imagination. Sir John Herschel tells us that “the total thickness
to be abraded from the edge of a spherical speculum 48 inches in
diameter and 40 feet focus, to convert it into a paraboloid, is only
1/21333 of an inch;”[323] yet upon this minute difference of form depends
the clearness of the image, and, as a consequence, the entire efficiency
of the instrument. “Almost infinite,” indeed (in the phrase of the
late Dr. Robinson), must be the exactitude of the operation adapted
to bring about so delicate a result.
At length, in 1839, two specula, each three feet in diameter,
were turned out in such perfection as to prompt a still bolder
experiment. The various processes needed to insure success
were now ascertained and under control; all that was necessary was
to repeat them on a larger scale. A gigantic mirror, six feet
across and fifty-four in focal length, was accordingly cast on the
13th of April, 1842; in two months it was ground down to figure
by abrasion with emery and water, and daintily polished with
rouge; and by the month of February, 1845, the “leviathan of
Parsonstown” was available for the examination of the heavens.
The suitable mounting of this vast machine was a problem
scarcely less difficult than its construction. The shape of a
speculum needs to be maintained with an elaborate care equal to
that used in imparting it. In fact, one of the most formidable
obstacles to increasing the size of such reflecting surfaces consists in
their liability to bend under their own weight. That of the great
Rosse speculum was no less than four tons. Yet, although six
inches in thickness, and composed of a material only a degree
inferior in rigidity to wrought iron, the strong pressure of a man’s
hand at its back produced sufficient flexure to distort perceptibly
the image of a star reflected in it.[324] Thus the delicacy of its form
was perishable equally by the stress of its own gravity, and by the
slightest irregularity in the means taken to counteract that stress.
The problem of affording a perfectly equable support in all possible
positions was solved by resting the speculum upon twenty-seven
platforms of cast iron, felt-covered, and carefully fitted to the shape
of the areas they were to carry, which platforms were themselves[Pg 117]
borne by a complex system of triangles and levers, ingeniously
adapted to distribute the weight with complete uniformity.[325]
A tube which resembled, when erect, one of the ancient round
towers of Ireland,[326] served as the habitation of the great mirror. It
was constructed of deal staves bound together with iron hoops, was
fifty-eight feet long (including the speculum-box), and seven in
diameter. A reasonably tall man may walk through it (as Dean Peacock
once did) with umbrella uplifted. Two piers of solid masonry,
about fifty feet high, seventy long, and twenty-three apart, flanked
the huge engine on either side. Its lower extremity rested on a
universal joint of cast iron; above, it was slung in chains, and even
in a gale of wind remained perfectly steady. The weight of the
entire, although amounting to fifteen tons, was so skilfully counterpoised,
that the tube could with ease be raised or depressed by two
men working a windlass. Its horizontal range was limited by the
lofty walls erected for its support to about ten degrees on each side
of the meridian; but it moved vertically from near the horizon
through the zenith as far as the pole. Its construction was of the
Newtonian kind, the observer looking into the side of the tube near
its upper end, which a series of galleries and sliding stages enabled
him to reach in any position. It has also, though rarely, been used
without a second mirror, as a “Herschelian” reflector.
The splendour of the celestial objects as viewed with this vast
“light-grasper” surpassed all expectation. “Never in my life,”
exclaimed Sir James South, “did I see such glorious sidereal
pictures.”[327] The orb of Jupiter produced an effect compared to
that of the introduction of a coach-lamp into the telescope;[328] and
certain star-clusters exhibited an appearance (we again quote Sir
James South) “such as man before had never seen, and which for
its magnificence baffles all description.” But it was in the examination
of the nebulæ that the superiority of the new instrument was
most strikingly displayed. A large number of these misty objects,
which the utmost powers of Herschel’s specula had failed to resolve
into stars, yielded at once to the Parsonstown reflector; while many
others showed under entirely changed forms through the disclosure
of previously unseen details of structure.
One extremely curious result of the increase of light was the
abolition of any sharp distinction between the two classes of
“annular” and “planetary” nebulæ. Up to that time, only four
ring-shaped systems—two in the northern and two in the southern[Pg 118]
hemisphere—were known to astronomers; they were now reinforced
by five of the planetary kind, the discs of which were observed to be
centrally perforated; while the definite margins visible in weaker
instruments were replaced by ragged edges or filamentous fringes.
Still more striking was the discovery of an entirely new and
most remarkable species of nebulæ. These were termed “spiral,”
from the more or less regular convolutions, resembling the whorls of
a shell, in which the matter composing them appeared to be distributed.
The first and most conspicuous specimen of this class was
met with in April, 1845; it is situated in Canes Venatici, close to
the tail of the Great Bear, and wore, in Sir J. Herschel’s instruments,
the aspect of a split ring encompassing a bright nucleus, thus
presenting, as he supposed, a complete analogue to the system of the
Milky Way. In the Rosse mirror it shone out as a vast whirlpool
of light—a stupendous witness to the presence of cosmical activities
on the grandest scale, yet regulated by laws as to the nature of
which we are profoundly ignorant. Professor Stephen Alexander
of New Jersey, however, concluded, from an investigation (necessarily
founded on highly precarious data) of the mechanical condition of
these extraordinary agglomerations, that we see in them “the
partially scattered fragments of enormous masses once rotating in a
state of dynamical equilibrium.” He further suggested “that the
separation of these fragments may still be in progress,”[329] and traced
back their origin to the disruption, through its own continually
accelerated rotation, of a “primitive spheroid” of inconceivably vast
dimensions. Such also, it was added (the curvilinear form of
certain outliers of the Milky Way giving evidence of a spiral
structure), is probably the history of our own cluster; the stars
composing which, no longer held together in a delicately adjusted
system like that of the sun and planets, are advancing through a
period of seeming confusion towards an appointed goal of higher
order and more perfect and harmonious adaptation.[330]
The class of spiral nebulæ included, in 1850, fourteen members,
besides several in which the characteristic arrangement seemed
partial or dubious.[331] A tendency in the exterior stars of other
clusters to gather into curved branches (as in our Galaxy) was likewise
noted; and the existence of unsuspected analogies was proclaimed
by the significant combination in the “Owl” nebula (a
large planetary in Ursa Major)[332] of the twisted forms of a spiral with
the perforated effect distinctive of an annular nebula.[Pg 119]
Once more, by the achievements of the Parsonstown reflector, the
supposition of a “shining fluid” filling vast regions of space was
brought into (as it has since proved) undeserved discredit. Although
Lord Rosse himself rejected the inference, that because many nebulæ
had been resolved, all were resolvable, very few imitated his truly
scientific caution; and the results of Bond’s investigations[333] with the
Harvard College refractor quickened and strengthened the current
of prevalent opinion. It is now certain that the evidence furnished
on both sides of the Atlantic as to the stellar composition of some
conspicuous objects of this class (notably the Orion and “Dumb-bell”
nebulæ) was delusive; but the spectroscope alone was capable of
meeting it with a categorical denial. Meanwhile there seemed good
ground for the persuasion, which now, for the last time, gained the
upper hand, that nebulæ are, without exception, true “island-universes,”
or assemblages of distant suns.
Lord Rosse’s telescope possesses a nominal power of 6,000—that
is, it shows the moon as if viewed with the naked eye at a distance
of forty miles. But this seeming advantage is neutralised by the
weakening of the available light through excessive diffusion, as well
as by the troubles of the surging sea of air through which the observation
must necessarily be made. Professor Newcomb, in fact,
doubts whether with any telescope our satellite has ever been seen
to such advantage as it would be if brought within 500 miles of the
unarmed eye.[334]
The French opticians’ rule of doubling the number of millimetres
contained in the aperture of an instrument to find the highest
magnifying power usually applicable to it, would give 3,600 as the
maximum for the leviathan of Birr Castle; but in a climate like that
of Ireland the occasions must be rare when even that limit can be
reached. Indeed, the experience acquired by its use plainly shows
that atmospheric rather than mechanical difficulties impede a still
further increase of telescopic power. Its construction may accordingly
be said to mark the ne plus ultra of effort in one direction,
and the beginning of its conversion towards another. It became
thenceforward more and more obvious that the conditions of observation
must be ameliorated before any added efficacy could be given
to it. The full effect of an uncertain climate in nullifying optical
improvements was recognised, and the attention of astronomers
began to be turned towards the advantages offered by more tranquil
and more translucent skies.
Scarcely less important for the practical uses of astronomy than
the optical qualities of the telescope is the manner of its mounting.[Pg 120]
The most admirable performance of the optician can render but unsatisfactory
service if its mechanical accessories are ill-arranged or
inconvenient. Thus the astronomer is ultimately dependent upon
the mechanician; and so excellently have his needs been served, that
the history of the ingenious contrivances by which discoveries have
been prepared would supply a subject (here barely glanced at) not
far inferior in extent and instruction to the history of those discoveries
themselves.
There are two chief modes of using the telescope, to which all
others may be considered subordinate.[335] Either it may be invariably
directed towards the south, with no motion save in the
plane of the meridian, so as to intercept the heavenly bodies at
the moment of transit across that plain; or it may be arranged
so as to follow the daily revolution of the sky, thus keeping the
object viewed permanently in sight instead of simply noting the
instant of its flitting across the telescopic field. The first plan
is that of the “transit instrument,” the second that of the
“equatoreal.” Both were, by a remarkable coincidence, introduced
about 1690[336] by Olaus Römer, the brilliant Danish astronomer
who first measured the velocity of light.
The uses of each are entirely different. With the transit, the
really fundamental task of astronomy—the determination of the
movements of the heavenly bodies—is mainly accomplished; while
the investigation of their nature and peculiarities is best conducted
with the equatoreal. One is the instrument of mathematical, the
other of descriptive astronomy. One furnishes the materials with
which theories are constructed and the tests by which they are
corrected; the other registers new facts, takes note of new appearances,
sounds the depths and peers into every nook of the heavens.
The great improvement of giving to a telescope equatoreally
mounted an automatic movement by connecting it with clockwork,
was proposed in 1674 by Robert Hooke. Bradley in 1721 actually
observed Mars with a telescope “moved by a machine that made it
keep pace with the stars;”[337] and Von Zach relates[338] that he had once[Pg 121]
followed Sirius for twelve hours with a “heliostat” of Ramsden’s
construction. But these eighteenth-century attempts were of no
practical effect. Movement by clockwork was virtually a complete
novelty when it was adopted by Fraunhofer in 1824 to the Dorpat
refractor. By simply giving to an axis unvaryingly directed towards
the celestial pole an equable rotation with a period of twenty-four
hours, a telescope attached to it, and pointed in any direction, will trace
out on the sky a parallel of declination, thus necessarily accompanying
the movement of any star upon which it may be fixed. It
accordingly forms part of the large sum of Fraunhofer’s merits to
have secured this inestimable advantage to observers.
Sir John Herschel considered that Lassell’s application of equatoreal
mounting to a nine-inch Newtonian in 1840 made an epoch
in the history of “that eminently British instrument, the reflecting
telescope.”[339] Nearly a century earlier,[340] it is true, Short had
fitted one of his Gregorians to a complicated system of circles
in such a manner that, by moving a handle, it could be made to
follow the revolution of the sky; but the arrangement did not
obtain, nor did it deserve, general adoption. Lassell’s plan was a
totally different one; he employed the crossed axes of the true
equatoreal, and his success removed, to a great extent, the fatal
objection of inconvenience in use, until then unanswerably urged
against reflectors. The very largest of these can now be mounted
equatoreally; even the Rosse, within its limited range, has been for
some years provided with a movement by clockwork along declination-parallels.
The art of accurately dividing circular arcs into the minute
equal parts which serve as the units of astronomical measurement,
remained, during the whole of the eighteenth century, almost exclusively
in English hands. It was brought to a high degree of perfection
by Graham, Bird and Ramsden, all of whom, however, gave
the preference to the old-fashioned mural quadrant and zenith-sector
over the entire circle, which Römer had already found the
advantage of employing. The five-foot vertical circle, which Piazzi
with some difficulty induced Ramsden to complete for him in 1789,
was the first divided instrument constructed in what may be called
the modern style. It was provided with magnifiers for reading off
the divisions (one of the neglected improvements of Römer), and
was set up above a smaller horizontal circle, forming an “altitude
and azimuth” combination (again Römer’s invention), by which both
the elevation of a celestial object above the horizon and its position
as referred to the horizon could be measured. In the same year,
Borda invented the “repeating circle” (the principle of which had[Pg 122]
been suggested by Tobias Mayer in 1756[341]), a device for exterminating,
so far as possible, errors of graduation by repeating an observation
with different parts of the limb. This was perhaps the earliest
systematic effort to correct the imperfections of instruments by the
manner of their use.
The manufacture of astronomical circles was brought to a very
refined state of excellence early in the nineteenth century by
Reichenbach at Munich, and after 1818 by Repsold at Hamburg.
Bessel states[342] that the “reading-off” on an instrument of the kind
by the latter artist was accurate to about 1/80th of a human hair.
Meanwhile the traditional reputation of the English school was fully
sustained; and Sir George Airy did not hesitate to express his
opinion that the new method of graduating circles, published by
Troughton in 1809,[343] was the “greatest improvement ever made in
the art of instrument-making.”[344] But a more secure road to improvement
than that of mere mechanical exactness was pointed out by
Bessel. His introduction of a regular theory of instrumental errors
might almost be said to have created a new art of observation.
Every instrument, he declared in memorable words,[345] must be twice
made—once by the artist, and again by the observer. Knowledge
is power. Defects that are ascertained and can be allowed for are
as good as non-existent. Thus the truism that the best instrument
is worthless in the hands of a careless or clumsy observer, became
supplemented by the converse maxim, that defective appliances may,
through skilful use, be made to yield valuable results. The Königsberg
observations—of which the first instalment was published in
1815—set the example of regular “reduction” for instrumental
errors. Since then, it has become an elementary part of an astronomer’s
duty to study the idiosyncrasy of each one of the mechanical
contrivances at his disposal, in order that its inevitable, but now
certified deviations from ideal accuracy may be included amongst
the numerous corrections by which the pure essence of even
approximate truth is distilled from the rude impressions of sense.
Nor is this enough; for the casual circumstances attending each
observation have to be taken into account with no less care than the
inherent or constitutional peculiarities of the instrument with which
it is made. There is no “once for all” in astronomy. Vigilance
can never sleep; patience can never tire. Variable as well as constant
sources of error must be anxiously heeded; one infinitesimal
inaccuracy must be weighed against another; all the forces and
vicissitudes of nature—frosts, dews, winds, the interchanges of heat,[Pg 123]
the disturbing effects of gravity, the shiverings of the air, the
tremors of the earth, the weight and vital warmth of the observer’s
own body, nay, the rate at which his brain receives and transmits
its impressions, must all enter into his calculations, and be sifted out
from his results.
It was in 1823 that Bessel drew attention to discrepancies in the
times of transits given by different astronomers.[346] The quantities
involved were far from insignificant. He was himself nearly a
second in advance of all his contemporaries, Argelander lagging
behind him as much as a second and a quarter. Each individual, in
fact, was found to have a certain definite rate of perception, which,
under the name of “personal equation,” now forms so important an
element in the correction of observations that a special instrument
for accurately determining its amount in each case is in actual use
at Greenwich.
Such are the refinements upon which modern astronomy depends
for its progress. It is a science of hairbreadths and fractions of a
second. It exists only by the rigid enforcement of arduous accuracy
and unwearying diligence. Whatever secrets the universe still has
in store for man will only be communicated on these terms. They
are, it must be acknowledged, difficult to comply with. They
involve an unceasing struggle against the infirmities of his nature
and the instabilities of his position. But the end is not unworthy
the sacrifices demanded. One additional ray of light thrown on the
marvels of creation—a single, minutest encroachment upon the
strongholds of ignorance—is recompense enough for a lifetime of
toil. Or rather, the toil is its own reward, if pursued in the lofty
spirit which alone becomes it. For it leads through the abysses of
space and the unending vistas of time to the very threshold of that
infinity and eternity of which the disclosure is reserved for a life to
come.
FOOTNOTES:
[305] Grant, Hist. Astr., p. 527.
[306] Optica Promota, p. 93.
[307] Phil. Trans., vol. xxxii., p. 383.
[308] Ibid., vol. xc., p. 65.
[309] Cassegrain, a Frenchman, substituted in 1672 a convex for a concave secondary
speculum. The tube was thereby enabled to be shortened by twice the focal
length of the mirror in question. The great Melbourne reflector (four feet
aperture, by Grubb) is constructed upon this plan.
[310] Phil. Trans., vol. civ., p. 275, note.
[311] Phil. Trans., vol. xc., p. 70. With the forty-foot, however, only very
moderate powers seemed to have been employed, whence Dr. Robinson argued
a deficiency of defining power. Proc. Roy. Irish Ac., vol. ii., p. 11.
[312] Phil. Trans., vol. lxxi., p. 492.
[313] It is remarkable that, as early as 1695, the possibility of an achromatic
combination was inferred by David Gregory from the structure of the human
eye. See his Catoptricæ et Dioptricæ Sphericæ Elementa, p. 98.
[314] Wolf, Biographien, Bd. ii., p. 301.
[315] Month. Not., vol. i., p. 153. note.
[316] Henrivaux, Encyclopédie Chimique, t. v., fasc. 5, p. 363.
[317] See ante, p. 83.
[318] Phil. Trans., vol. vii., p. 4007.
[319] J. Herschel, The Telescope, p. 39.
[320] Month. Not., vol. xxix., p. 125.
[321] A slight excess of copper renders the metal easier to work, but liable to
tarnish. Robinson, Proc. Roy. Irish Ac., vol. ii., p. 4.
[322] Brit. Ass., 1843, Dr. Robinson’s closing Address. Athenæum, Sept. 23,
p. 866.
[323] The Telescope, p. 82.
[324] Lord Rosse in Phil. Trans., vol. cxl., p. 302.
[325] This method is the same in principle with that applied by Grubb in 1834 to a
15-inch speculum for the observatory of Armagh. Phil. Trans., vol. clix., p. 145.
[326] Robinson, Proc. Roy. Ir. Ac., vol. iii., p. 120.
[327] Astr. Nach., No. 536.
[328] Airy, Month. Not., vol. ix., p. 120.
[329] Astronomical Journal (Gould’s), vol. ii., p. 97.
[330] Ibid., p. 160.
[331] Lord Rosse in Phil. Trans., vol. cxl., p. 505.
[332] No. 2343 of Herschel’s (1864) Catalogue. Before 1850 a star was visible in
each of the two larger openings by which it is pierced; since then, one only.
Webb, Celestial Objects (4th ed.), p. 409.
[333] Mem. Am. Ac., vol. iii., p. 87; Astr. Nach., No. 611.
[334] Pop. Astr., p. 145.
[335] This statement must be taken in the most general sense. Supplementary
observations of great value are now made at Greenwich with the altitude and
azimuth instrument, which likewise served Piazzi to determine the places of his
stars; while a “prime vertical instrument” is prominent at Pulkowa.
[336] As early as 1620, according to R. Wolf (Ges. der Astr., p. 587), Father
Scheiner made the experiment of connecting a telescope with an axis directed to
the pole, while Chinese “equatoreal armillæ,” dating from the thirteenth century,
existed at Pekin until 1900, when they were carried off as “loot” to Berlin.
J. L. E. Dreyer, Copernicus, vol. i., p. 134.
[337] Miscellaneous Works, p. 350.
[338] Astr. Jahrbuch, 1799 (published 1796), p. 115.
[339] Month. Not., vol. xli., p. 189.
[340] Phil. Trans., vol. xlvi., p. 242.
[341] Grant, Hist. of Astr., p. 487.
[342] Pop. Vorl., p. 546.
[343] Phil. Trans., vol. xcix., p. 105.
[344] Report Brit. Ass., 1832, p. 132.
[345] Pop. Vorl., p. 432.
[346] C. T. Anger, Grundzüge der neucren astronomischen Beobachtungs-Kunst,
p. 3.
PART II
RECENT PROGRESS OF ASTRONOMY
CHAPTER I
FOUNDATION OF ASTRONOMICAL PHYSICS
In the year 1826, Heinrich Schwabe of Dessau, elated with the hope
of speedily delivering himself from the hereditary incubus of an
apothecary’s shop,[347] obtained from Munich a small telescope and
began to observe the sun. His choice of an object for his researches
was instigated by his friend Harding of Göttingen. It was a
peculiarly happy one. The changes visible in the solar surface were
then generally regarded as no less capricious than the changes in the
skies of our temperate regions. Consequently, the reckoning and
registering of sun-spots was a task hardly more inviting to an
astronomer than the reckoning and registering of summer clouds.
Cassini, Keill, Lemonnier, Lalande, were unanimous in declaring
that no trace of regularity could be detected in their appearances
or effacements.[348] Delambre pronounced them “more curious than
really useful.”[349] Even Herschel, profoundly as he studied them, and
intimately as he was convinced of their importance as symptoms of
solar activity, saw no reason to suspect that their abundance and
scarcity were subject to orderly alternation. One man alone in the
eighteenth century, Christian Horrebow of Copenhagen, divined
their periodical character, and foresaw the time when the effects of
the sun’s vicissitudes upon the globes revolving round him might be
investigated with success; but this prophetic utterance was of the
nature of a soliloquy rather than of a communication, and remained
hidden away in an unpublished journal until 1859, when it was
brought to light in a general ransacking of archives.[350]
Indeed, Schwabe himself was far from anticipating the discovery
which fell to his share. He compared his fortune to that of Saul,
who, seeking his father’s asses, found a kingdom.[351] For the hope
which inspired his early resolution lay in quite another direction.
His patient ambush was laid for a possible intramercurial planet,
which, he thought, must sooner or later betray its existence in
crossing the face of the sun. He took, however, the most effectual
measures to secure whatever new knowledge might be accessible.
During forty-three years his “imperturbable telescope”[352] never
failed, weather and health permitting, to bring in its daily report
as to how many, or if any, spots were visible on the sun’s disc, the
information obtained being day by day recorded on a simple and
unvarying system. In 1843 he made his first announcement of a
probable decennial period,[353] but it met with no general attention;
although Julius Schmidt of Bonn (afterwards director of the Athens
Observatory) and Gautier of Geneva were impressed with his
figures, and Littrow had himself, in 1836,[354] hinted at the likelihood
of some kind of regular recurrence. Schwabe, however, worked on,
gathering each year fresh evidence of a law such as he had indicated;
and when Humboldt published in 1851, in the third volume of his
Kosmos,[355] a table of the sun-spot statistics collected by him from 1826
downwards, the strength of his case was perceived with, so to speak,
a start of surprise; the reality and importance of the discovery were
simultaneously recognised, and the persevering Hofrath of Dessau
found himself famous among astronomers. His merit—recognised
by the bestowal of the Astronomical Society’s Gold Medal in 1857—consisted
in his choice of an original and appropriate line of work,
and in the admirable tenacity of purpose with which he pursued it.
His resources and acquirements were those of an ordinary amateur;
he was distinguished solely by the unfortunately rare power of
turning both to the best account. He died where he was born and
had lived, April 11, 1875, at the ripe age of eighty-six.
Meanwhile an investigation of a totally different character, and
conducted by totally different means, had been prosecuted to a very
similar conclusion. Two years after Schwabe began his solitary
observations, Humboldt gave the first impulse, at the Scientific
Congress of Berlin in 1828, to a great international movement for
attacking simultaneously, in various parts of the globe, the complex
problem of terrestrial magnetism. Through the genius and
energy of Gauss, Göttingen became its centre. Thence new[Pg 127]
apparatus, and a new system for its employment, issued; there, in
1833, the first regular magnetic observatory was founded, whilst at
Göttingen was fixed the universal time-standard for magnetic
observations. A letter addressed by Humboldt in April, 1836, to the
Duke of Sussex as President of the Royal Society, enlisted the co-operation
of England. A network of magnetic stations was spread
all over the British dominions, from Canada to Van Diemen’s Land;
measures were concerted with foreign authorities, and an expedition
was fitted out, under the able command of Captain (afterwards Sir
James) Clark Ross, for the special purpose of bringing intelligence
on the subject from the dismal neighbourhood of the South Pole. In
1841, the elaborate organisation created by the disinterested efforts of
scientific “agitators” was complete; Gauss’s “magnetometers” were
vibrating under the view of attentive observers in five continents,
and simultaneous results began to be recorded.
Ten years later, in September, 1851, Dr. John Lamont, the
Scotch director of the Munich Observatory, in reviewing the
magnetic observations made at Göttingen and Munich from
1835 to 1850, perceived with some surprise that they gave
unmistakable indications of a period which he estimated at 10-1/3
years.[356] The manner in which this periodicity manifested itself
requires a word of explanation. The observations in question
referred to what is called the “declination” of the magnetic needle—that
is, to the position assumed by it with reference to the points
of the compass when moving freely in a horizontal plane. Now this
position—as was discovered by Graham in 1722—is subject to a
small daily fluctuation, attaining its maximum towards the east
about 8 A.M., and its maximum towards the west shortly before
2 P.M. In other words, the direction of the needle approaches (in
these countries at the present time) nearest to the true north some
four hours before noon, and departs farthest from it between one
and two hours after noon. It was the range of this daily variation
that Lamont found to increase and diminish once in every 10-1/3 years.
In the following winter, Sir Edward Sabine, ignorant as yet of
Lamont’s conclusion, undertook to examine a totally different set of
observations. The materials in his hands had been collected at the
British colonial stations of Toronto and Hobarton from 1843 to
1848, and had reference, not to the regular diurnal swing of the
needle, but to those curious spasmodic vibrations, the inquiry into
the laws of which was the primary object of the vast organisation
set on foot by Humboldt and Gauss. Yet the upshot was practically
the same. Once in about ten years, magnetic disturbances (termed
by Humboldt “storms”) were perceived to reach a maximum of[Pg 128]
violence and frequency. Sabine was the first to note the coincidence
between this unlooked-for result and Schwabe’s sun-spot period.
He showed that, so far as observation had yet gone, the two cycles
of change agreed perfectly both in duration and phase, maximum
corresponding to maximum, minimum to minimum. What the
nature of the connection could be that bound together by a common
law effects so dissimilar as the rents in the luminous garment of the
sun, and the swayings to and fro of the magnetic needle, was and
still remains beyond the reach of well-founded theory; but the fact
was from the first undeniable.
The memoir containing this remarkable disclosure was presented
to the Royal Society, March 18, and read May 6, 1852.[357]
On the 31st of July following, Rudolf Wolf at Berne,[358] and on the
18th of August, Alfred Gautier at Sion,[359] announced, separately
and independently, perfectly similar conclusions. This triple event
is perhaps the most striking instance of the successful employment
of the Baconian method of co-operation in discovery, by which
“particulars” are amassed by one set of investigators—corresponding
to the “Depredators” and “Inoculators” of Solomon’s House—while
inductions are drawn from them by another and a higher
class—the “Interpreters of Nature.” Yet even here the convergence
of two distinct lines of research was wholly fortuitous, and
skilful combination owed the most brilliant part of its success to the
unsought bounty of what we call Fortune.
The exactness of the coincidence thus brought to light was fully
confirmed by further inquiries. A diligent search through the
scattered records of sun-spot observations, from the time of Galileo
and Scheiner onwards, put Wolf[360] in possession of materials by which
he was enabled to correct Schwabe’s loosely-indicated decennial
period to one of slightly over eleven (11.11) years; and he further
showed that this fell in with the ebb and flow of magnetic change
even better than Lamont’s 10-1/3 year cycle. The analogy was also
pointed out between the “light-curve,” or zig-zagged line representing
on paper the varying intensity in the lustre of certain stars, and
the similar delineation of spot-frequency; the ascent from minimum
to maximum being, in both cases, usually steeper than the descent
from maximum to minimum; while an additional point of resemblance
was furnished by the irregularities in height of the various
maxima. In other words, both the number of spots on the sun and
the brightness of variable stars increase, as a rule, more rapidly than[Pg 129]
they decrease; nor does the amount of that increase, in either
instance, show any approach to uniformity.
The endeavour, suggested by the very nature of the phenomenon,
to connect sun-spots with weather was less successful. The first
attempt of the kind was made by Sir William Herschel in 1801, and
a very notable one it was. Meteorological statistics, save of the
scantiest and most casual kind, did not then exist; but the price of
corn from year to year was on record, and this, with full recognition
of its inadequacy, he adopted as his criterion. Nor was he much
better off for information respecting the solar condition. What
little he could obtain, however, served, as he believed, to confirm
his surmise that a copious emission of light and heat accompanies
an abundant formation of “openings” in the dazzling substance
whence our supply of those indispensable commodities is derived.[361]
He gathered, in short, from his inquiries very much what he had
expected to gather, namely, that the price of wheat was high when
the sun showed an unsullied surface, and that food and spots became
plentiful together.[362]
Yet this plausible inference was scarcely borne out by a more
exact collocation of facts. Schwabe failed to detect any reflection
of the sun-spot period in his meteorological register. Gautier[363]
reached a provisional conclusion the reverse—though not markedly
the reverse—of Herschel’s. Wolf, in 1852, derived from an
examination of Vogel’s collection of Zürich Chronicles (1000-1800
A.D.) evidence showing (as he thought) that minimum years were
usually wet and stormy, maximum years dry and genial;[364] but a
subsequent review of the subject in 1859 convinced him that no
relation of any kind between the two kinds of effects was traceable.[365]
With the singular affection of our atmosphere known as the
Aurora Borealis (more properly Aurora Polaris) the case was
different. Here the Zürich Chronicles set Wolf on the right track
in leading him to associate such luminous manifestations with a
disturbed condition of the sun; since subsequent detailed observation
has exhibited the curve of auroral frequency as following with such
fidelity the jagged lines figuring to the eye the fluctuations of solar[Pg 130]
and magnetic activity, as to leave no reasonable doubt that all three
rise and sink together under the influence of a common cause. As
long ago as 1716,[366] Halley had conjectured that the Northern Lights
were due to magnetic “effluvia,” but there was no evidence on the
subject forthcoming until Hiorter observed at Upsala in 1741 their
agitating influence upon the magnetic needle. That the effect was
no casual one was made superabundantly clear by Arago’s researches
in 1819 and subsequent years. Now both were perceived to be
swayed by the same obscure power of cosmical disturbance.
The sun is not the only one of the heavenly bodies by which the
magnetism of the earth is affected. Proofs of a similar kind of lunar
action were laid by Kreil in 1841 before the Bohemian Society of
Sciences, and with minor corrections were fully substantiated by
Sabine’s more extended researches. It was thus ascertained that
each lunar day, or the interval of twenty-four hours and about
fifty-four minutes between two successive meridian passages of our
satellite, is marked by a perceptible, though very small, double
oscillation of the needle—two progressive movements from east to
west, and two returns from west to east.[367] Moreover, the lunar, like
the solar influence (as was proved in each case by Sabine’s analysis
of the Hobarton and Toronto observations), extends to all three
“magnetic elements,” affecting not only the position of the horizontal
or declination needle, but also the dip and intensity. It seems
not unreasonable to attribute some portion of the same subtle power
to the planets and even to the stars, though with effects rendered
imperceptible by distance.
We have now to speak of the discovery and application to the
heavenly bodies of a totally new method of investigation. Spectrum
analysis may be shortly described as a mode of distinguishing the
various species of matter by the kind of light proceeding from each.
This definition at once explains how it is that, unlike every other
system of chemical analysis, it has proved available in astronomy.
Light, so far as quality is concerned, ignores distance. No intrinsic
change, that we yet know of, is produced in it by a journey from
the farthest bounds of the visible universe; so that, provided only
that in quantity it remain sufficient for the purpose, its peculiarities
can be equally well studied whether the source of its vibrations be
one foot or a hundred billion miles distant. Now the most obvious
distinction between one kind of light and another resides in colour.
But of this distinction the eye takes cognisance in an æsthetic, not
in a scientific sense. It finds gladness in the “thousand tints” of
nature, but can neither analyse nor define them. Here the refracting[Pg 131]
prism—or the combination of prisms known as the “spectroscope”—comes
to its aid, teaching it to measure as well as to
perceive. It furnishes, in a word, an accurate scale of colour. The
various rays which, entering the eye together in a confused crowd,
produce a compound impression made up of undistinguishable
elements, are, by the mere passage through a triangular piece of
glass, separated one from the other, and ranged side by side in
orderly succession, so that it becomes possible to tell at a glance
what kinds of light are present, and what absent. Thus, if we could
only be assured that the various chemical substances when made to
glow by heat, emit characteristic rays—rays, that is, occupying a
place in the spectrum reserved for them, and for them only—we
should at once be in possession of a mode of identifying such substances
with the utmost readiness and certainty. This assurance,
which forms the solid basis of spectrum analysis, was obtained slowly
and with difficulty.
The first to employ the prism in the examination of various
flames (for it is only in a state of vapour that matter emits distinctive
light) was a young Scotchman named Thomas Melvill, who died
in 1753, at the age of twenty-seven. He studied the spectrum of
burning spirits, into which were successively introduced sal ammoniac,
potash, alum, nitre, and sea-salt, and observed the singular
predominance, under almost all circumstances, of a particular shade
of yellow light, perfectly definite in its degree of refrangibility[368]—in
other words, taking up a perfectly definite position in the spectrum.
His experiments were repeated by Morgan,[369] Wollaston, and—with
far superior precision and diligence—by Fraunhofer.[370] The
great Munich optician, whose work was completely original, rediscovered
Melvill’s deep yellow ray and measured its place in the
colour-scale. It has since become well known as the “sodium line,”
and has played a very important part in the history of spectrum
analysis. Nevertheless, its ubiquity and conspicuousness long
impeded progress. It was elicited by the combustion of a surprising
variety of substances—sulphur, alcohol, ivory, wood, paper; its persistent
visibility suggesting the accomplishment of some universal
process of nature rather than the presence of one individual kind
of matter. But if spectrum analysis were to exist as a science at
all, it could only be by attaining certainty as to the unvarying
association of one special substance with each special quality of light.
Thus perplexed, Fox Talbot[371] hesitated in 1826 to enounce this[Pg 132]
fundamental principle. He was inclined to believe that the presence
in the spectrum of any individual ray told unerringly of the
volatilisation in the flame under scrutiny of some body as whose
badge or distinctive symbol that ray might be regarded; but the
continual prominence of the yellow beam staggered him. It appeared,
indeed, without fail where sodium was; but it also appeared
where it might be thought only reasonable to conclude that sodium
was not. Nor was it until thirty years later that William Swan,[372] by
pointing out the extreme delicacy of the spectral test, and the
singularly wide dispersion of sodium, made it appear probable (but
even then only probable) that the questionable yellow line was
really due invariably to that substance. Common salt (chloride of
sodium) is, in fact, the most diffusive of solids. It floats in the air;
it flows with water; every grain of dust has its attendant particle;
its absolute exclusion approaches the impossible. And withal, the
light that it gives in burning is so intense and concentrated, that if
a single grain be divided into 180 million parts, and one alone of
such inconceivably minute fragments be present in a source of light,
the spectroscope will show unmistakably its characteristic beam.
Amongst the pioneers of knowledge in this direction were Sir
John Herschel[373]—who, however, applied himself to the subject in the
interests of optics, not of chemistry—W. A. Miller,[374] and Wheatstone.
The last especially made a notable advance when, in the
course of his studies on the “prismatic decomposition” of the electric
light, he reached the significant conclusion that the rays visible in
its spectrum were different for each kind of metal employed as
“electrodes.”[375] Thus indications of a wider principle were to be
found in several quarters, but no positive certainty on any single
point was obtained, until, in 1859, Gustav Kirchhoff, professor of
physics in the University of Heidelberg, and his colleague, the
eminent chemist Robert Bunsen, took the matter in hand. By
them the general question as to the necessary and invariable connection
of certain rays in the spectrum with certain kinds of matter,
was first resolutely confronted, and first definitely answered. It
was answered affirmatively—else there could have been no science
of spectrum analysis—as the result of experiments more numerous,
more stringent, and more precise than had previously been[Pg 133]
undertaken.[376] And the assurance of their conclusion was rendered doubly
sure by the discovery, through the peculiarities of their light alone,
of two new metals, named from the blue and red rays by which
they were respectively distinguished, “cæsium,” and “rubidium.”[377]
Both were immediately afterwards actually obtained in small
quantities by evaporation of the Durckheim mineral waters.
The link connecting this important result with astronomy may
now be indicated. In the year 1802 it occurred to William Hyde
Wollaston to substitute for the round hole used by Newton and his
successors for the admittance of light to be examined with the
prism, an elongated “crevice” 1/20th of an inch in width. He thereupon
perceived that the spectrum, thus formed of light, as it were,
purified by the abolition of overlapping images, was traversed by
seven dark lines. These he took to be natural boundaries of the
various colours,[378] and satisfied with this quasi-explanation, allowed
the subject to drop. It was independently taken up after twelve
years by a man of higher genius. In the course of experiments on
light, directed towards the perfecting of his achromatic lenses,
Fraunhofer, by means of a slit and a telescope, made the surprising
discovery that the solar spectrum is crossed, not by seven,
but by thousands of obscure transverse streaks.[379] Of these he
counted some 600, and carefully mapped 324, while a few of the
most conspicuous he set up (if we may be permitted the expression)
as landmarks, measuring their distances apart with a theodolite, and
affixing to them the letters of the alphabet, by which they are still
universally known. Nor did he stop here. The same system of
examination applied to the rest of the heavenly bodies showed the
mild effulgence of the moon and planets to be deficient in precisely
the same rays as sunlight; while in the stars it disclosed the differences
in likeness which are always an earnest of increased knowledge.
The spectra of Sirius and Castor, instead of being delicately ruled
crosswise throughout, like that of the sun, were seen to be interrupted
by three massive bars of darkness—two in the blue and one
in the green;[380] the light of Pollux, on the other hand, seemed precisely
similar to sunlight attenuated by distance or reflection, and
that of Capella, Betelgeux, and Procyon to share some of its
peculiarities. One solar line especially—that marked in his map
with the letter D—proved common to all the four last-mentioned
stars; and it was remarkable that it exactly coincided in position
with the conspicuous yellow beam (afterwards, as we have said,
identified with the light of glowing sodium) which he had already[Pg 134]
found to accompany most kinds of combustion. Moreover, both the
dark solar and the bright terrestrial “D lines” were displayed by the
refined Munich appliances as double.
In this striking correspondence, discovered by Fraunhofer in 1815,
was contained the very essence of solar chemistry; but its true
significance did not become apparent until long afterwards. Fraunhofer
was by profession, not a physicist, but a practical optician.
Time pressed; he could not and would not deviate from his
appointed track; all that was possible to him was to indicate the road
to discovery, and exhort others to follow it.[381]
Partially and inconclusively at first this was done. The “fixed
lines” (as they were called) of the solar spectrum took up the
position of a standing problem, to the solution of which no approach
seemed possible. Conjectures as to their origin were indeed rife.
An explanation put forward by Zantedeschi[382] and others, and
dubiously favoured by Sir David Brewster and Dr. J. H. Gladstone,[383]
was that they resulted from “interference”—that is, a destruction
of the motion producing in our eyes the sensation of light, by the
superposition of two light-waves in such a manner that the crests of
one exactly fill up the hollows of the other. This effect was supposed
to be brought about by imperfections in the optical apparatus
employed.
A more plausible view was that the atmosphere of the earth was
the agent by which sunlight was deprived of its missing beams.
For a few of them this is actually the case. Brewster found in 1832
that certain dark lines, which were invisible when the sun stood high
in the heavens, became increasingly conspicuous as he approached
the horizon.[384] These are the well-known “atmospheric lines;” but
the immense majority of their companions in the spectrum remain
quite unaffected by the thickness of the stratum of air traversed by
the sunlight containing them. They are then obviously due to
another cause.
There remained the true interpretation—absorption in the sun’s
atmosphere; and this, too, was extensively canvassed. But a
remarkable observation made by Professor Forbes of Edinburgh[385] on
the occasion of the annular eclipse of May 15, 1836, appeared to
throw discredit upon it. If the problematical dark lines were really
occasioned by the stoppage of certain rays through the action of a
vaporous envelope surrounding the sun, they ought, it seemed, to be[Pg 135]
strongest in light proceeding from his edges, which, cutting that
envelope obliquely, passed through a much greater depth of it. But
the circle of light left by the interposing moon, and of course
derived entirely from the rim of the solar disc, yielded to Forbes’s
examination precisely the same spectrum as light coming from its
central parts. This circumstance helped to baffle inquirers, already
sufficiently perplexed. It still remains an anomaly, of which no
satisfactory explanation has been offered.
Convincing evidence as to the true nature of the solar lines was
however at length, in the autumn of 1859, brought forward at
Heidelburg. Kirchhoff’s experimentum crucis in the matter was a
very simple one. He threw bright sunshine across a space occupied
by vapour of sodium, and perceived with astonishment that the dark
Fraunhofer line D, instead of being effaced by flame giving a
luminous ray of the same refrangibility, was deepened and thickened
by the superposition.
He tried the same experiment, substituting for sunbeams light
from a Drummond lamp, and with similar result. A dark furrow,
corresponding in every respect to the solar D-line, was instantly
seen to interrupt the otherwise unbroken radiance of its spectrum.
The inference was irresistible, that the effect thus produced
artificially was brought about naturally in the same way, and that
sodium formed an ingredient in the glowing atmosphere of the sun.[386]
This first discovery was quickly followed up by the identification of
numerous bright rays in the spectra of other metallic bodies with
others of the hitherto mysterious Fraunhofer lines. Kirchhoff was
thus led to the conclusion that (besides sodium) iron, magnesium,
calcium, and chromium, are certainly solar constituents, and that
copper, zinc, barium, and nickel are also present, though in smaller
quantities.[387] As to cobalt, he hesitated to pronounce, but its
existence in the sun has since been established.
These memorable results were founded upon a general principle
first enunciated by Kirchhoff in a communication to the Berlin
Academy, December 15, 1859, and afterwards more fully developed
by him.[388] It may be expressed as follows: Substances of every kind
are opaque to the precise rays which they emit at the same
temperature; that is to say, they stop the kinds of light or heat
which they are then actually in a condition to radiate. But it does[Pg 136]
not follow that cool bodies absorb the rays which they would give
out if sufficiently heated. Hydrogen at ordinary temperatures, for
instance, is almost perfectly transparent, but if raised to the glowing
point—as by the passage of electricity—it then becomes capable of
arresting, and at the same time of displaying in its own spectrum
light of four distinct colours.
This principle is fundamental to solar chemistry. It gives the
key to the hieroglyphics of the Fraunhofer lines. The identical
characters which are written bright in terrestrial spectra are written
dark in the unrolled sheaf of sun-rays; the meaning remains unchanged.
It must, however, be remembered that they are only
relatively dark. The substances stopping those particular tints in
the neighbourhood of the sun are at the same time vividly glowing
with the very same. Remove the dazzling solar background, by
contrast with which they show as obscure, and they will be seen,
and, at critical moments, actually have been seen, in all their native
splendour. It is because the atmosphere of the sun is cooler than
the globe it envelops that the different kinds of vapour constituting
that atmosphere take more than they give, absorb more light than
they are capable of emitting; raise them to the same temperature as
the sun itself, and their powers of emission and absorption being
brought exactly to the same level, the thousands of dusky rays in
the solar spectrum will be at once obliterated.
The establishment of the terrestrial science of spectrum analysis
was due, as we have seen, equally to Kirchhoff and Bunsen, but its
celestial application to Kirchhoff alone. He effected this object of
the aspirations, more or less dim, of many other thinkers and
workers, by the union of two separate, though closely related lines
of research—the study of the different kinds of light emitted by
various bodies, and the study of the different kinds of light absorbed
by them. The latter branch appears to have been first entered upon
by Dr. Thomas Young in 1803;[389] it was pursued by the younger
Herschel,[390] by William Allen Miller, Brewster, and Gladstone.
Brewster indeed made, in 1833,[391] a formal attempt to found what
might be called an inverse system of analysis with the prism based
upon absorption; and his efforts were repeated, just a quarter of a
century later, by Gladstone.[392] But no general point of view was
attained; nor, it may be added, was it by this path attainable.
Kirchhoff’s map of the solar spectrum, drawn to scale with exquisite
accuracy, and printed in three shades of ink to convey the
graduated obscurity of the lines, was published in the Transactions[Pg 137]
of the Berlin Academy for 1861 and 1862.[393] Representations of the
principal lines belonging to various elementary bodies formed, as
it were, a series of marginal notes accompanying the great solar
scroll, enabling the veriest tiro in the new science to decipher its
meaning at a glance. Where the dark solar and bright metallic rays
agreed in position, it might safely be inferred that the metal emitting
them was a solar constituent; and such coincidences were numerous.
In the case of iron alone, no less than sixty occurred in one-half of
the spectral area, rendering the chances[394] absolutely overwhelming
against mere casual conjunction. The preparation of this elaborate
picture proved so trying to the eyes that Kirchhoff was compelled
by failing vision to resign the latter half of the task to his pupil
Hofmann. The complete map measured nearly eight feet in length.
The conclusions reached by Kirchhoff were no sooner announced
than they took their place, with scarcely a dissenting voice, among
the established truths of science. The broad result, that the dark
lines in the spectrum of the sun afford an index to its chemical composition
no less reliable than any of the tests used in the laboratory,
was equally captivating to the imagination of the vulgar, and
authentic in the judgment of the learned; and, like all genuine
advances in the knowledge of Nature, it stimulated curiosity far
more than it gratified it. Now the history of how discoveries were
missed is often quite as instructive as the history of how they were
made; it may then be worth while to expend a few words on the
thoughts and trials by which, in the present case, the actual event
was heralded.
Three times it seemed on the verge of being anticipated. The
experiment, which in Kirchhoff’s hands proved decisive, of passing
sunlight through glowing vapours and examining the superposed
spectra, was performed by Professor W. A. Miller of King’s College
in 1845.[395] Nay, more, it was performed with express reference to
the question, then already (as has been noted) in debate, of the
possible production of Fraunhofer’s lines by absorption in a solar
atmosphere. Yet it led to nothing.
Again, at Paris in 1849, with a view to testing the asserted coincidence
between the solar D-line and the bright yellow beam in the
spectrum of the electric arc (really due to the unsuspected presence
of sodium), Léon Foucault threw a ray of sunshine across the arc and
observed its spectrum.[396] He was surprised to see that the D-line[Pg 138]
was rendered more intensely dark by the combination of lights.
To assure himself still further, he substituted a reflected image of
one of the white-hot carbon-points for the sunbeam, with an identical
result. The same ray was missing. It needed but another step to
have generalised this result, and thus laid hold of a natural truth
of the highest importance; but that step was not taken. Foucault,
keen and brilliant though he was, rested satisfied with the information
that the voltaic arc had the power of stopping the kind of
light emitted by it; he asked no further question, and was consequently
the bearer of no further intelligence on the subject.
The truth conveyed by this remarkable experiment was, however,
divined by one eminent man. Professor Stokes of Cambridge stated
to Sir William Thomson (now Lord Kelvin), shortly after it had
been made, his conviction that an absorbing atmosphere of sodium
surrounded the sun. And so forcibly was his hearer impressed with
the weight of the argument based upon the absolute agreement of the
D-line in the solar spectrum with the yellow ray of burning sodium
(then freshly certified by W. H. Miller), combined with Foucault’s
“reversal” of that ray, that he regularly inculcated, in his public
lectures on natural philosophy at Glasgow, five or six years before
Kirchhoff’s discovery, not only the fact of the presence of sodium in
the solar neighbourhood, but also the principle of the study of solar
and stellar chemistry in the spectra of flames.[397] Yet it does not
appear to have occurred to either of these two distinguished professors—themselves
among the foremost of their time in the successful
search for new truths—to verify practically a sagacious
conjecture in which was contained the possibility of a scientific
revolution. It is just to add, that Kirchhoff was unacquainted,
when he undertook his investigation, either with the experiment of
Foucault or the speculation of Stokes.
For C. J. Ångström, on the other hand, perhaps somewhat too
much has been claimed in the way of anticipation. His Optical
Researches appeared at Upsala in 1853, and in their English garb
two years later.[398] They were undoubtedly pregnant with suggestion,
yet made no epoch in discovery. The old perplexities continued to
prevail after, as before their publication. To Ångström, indeed,
belongs the great merit of having revived Euler’s principle of the
equivalence of emission and absorption; but he revived it in its
original crude form, and without the qualifying proviso which alone
gave it value as a clue to new truths. According to his statement,
a body absorbs all the series of vibrations it is, under any
circumstances, capable of emitting, as well as those connected with[Pg 139]
them by simple harmonic relations. This is far too wide. To
render it either true or useful, it had to be reduced to the cautious
terms employed by Kirchhoff. Radiation strictly and necessarily
corresponds with absorption only when the temperature is the same.
In point of fact, Ångström was still, in 1853, divided between
adsorption and interference as the mode of origin of the Fraunhofer
dark rays. Very important, however, was his demonstration of the
compound nature of the spark-spectrum, which he showed to be
made up of the spectrum of the metallic electrodes superposed upon
that of the gas or gases across which the discharge passed.
It may here be useful—since without some clear ideas on the
subject no proper understanding of recent astronomical progress is
possible—to take a cursory view of the elementary principles of
spectrum analysis. To many of our readers they are doubtless
already familiar; but it is better to appear trite to some than
obscure even to a few.
The spectrum, then, of a body is simply the light proceeding from
it spread out by refraction[399] into a brilliant variegated band, passing
from brownish-red through crimson, orange, yellow, green, and azure
into dusky violet. The reason of this spreading-out or “dispersion”
is that the various colours have different wave-lengths, and consequently
meet with different degrees of retardation in traversing
the denser medium of the prism. The shortest and quickest vibrations
(producing the sensation we call “violet”) are thrown farthest
away from their original path—in other words, suffer the widest
“deviation;” the longest and slowest (the red) travel much nearer
to it. Thus the sheaf of rays which would otherwise combine into
a patch of white light are separated through the divergence of their
tracks after refraction by a prism, so as to form a tinted riband.
This visible spectrum is prolonged invisibly at both ends by a long
range of vibrations, either too rapid or too sluggish to affect the eye
as light, but recognisable through their chemical and heating effects.
Now all incandescent solid or liquid substances, and even gases
ignited under great pressure, give what is called a “continuous
spectrum;” that is to say, the light derived from them is of every
conceivable hue. Sorted out with the prism, its tints merge imperceptibly
one into the other, uninterrupted by any dark spaces. No
colours, in short, are missing. But gases and vapours rendered
luminous by heat emit rays of only a few tints, which accordingly
form an interrupted spectrum, usually designated as one of lines or
bands. And since these rays are perfectly definite and characteristic—not
being the same for any two substances—it is easy to tell[Pg 140]
what kind of matter is concerned in producing them. We may
suppose that the inconceivably minute particles which by their
rapid thrilling agitate the ethereal medium so as to produce
light, are free to give out their peculiar tone of vibration only when
floating apart from each other in gaseous form; but when crowded
together into a condensed mass, the clear ring of the distinctive note
is drowned, so to speak, in a universal molecular clang. Thus
prismatic analysis has no power to identify individual kinds of
matter, except when they present themselves as glowing vapours.
A spectrum is said to be “reversed” when lines previously seen
bright on a dark background appear dark on a bright background.
In this form it is equally characteristic of chemical composition with
the “direct” spectrum, being due to absorption, as the latter is to
emission. And absorption and emission are, by Kirchhoff’s law,
strictly correlative. This is easily understood by the analogy of
sound. For just as a tuning-fork responds to sound-waves of its
own pitch, but remains indifferent to those of any other, so those
particles of matter whose nature it is, when set swinging by heat,
to vibrate a certain number of times in a second, thus giving rise to
light of a particular shade of colour, appropriate those same vibrations,
and those only, when transmitted past them,—or, phrasing
it otherwise, are opaque to them, and transparent to all others.
It should further be explained that the shape of the bright or
dark spaces in the spectrum has nothing whatever to do with the
nature of the phenomena. The “lines” and “bands” so frequently
spoken of are seen as such for no other reason than because the
light forming them is admitted through a narrow, straight opening.
Change that opening into a fine crescent or a sinuous curve, and
the “lines” will at once appear as crescents or curves.
Resuming in a sentence what has been already explained, we
find that the prismatic analysis of the heavenly bodies was
founded upon three classes of facts: First, the unmistakable
character of the light given by each different kind of glowing
vapour; secondly, the identity of the light absorbed with the light
emitted by each; thirdly, the coincidence observed between rays
missing from the solar spectrum and rays absorbed by various
terrestrial substances. Thus, a realm of knowledge, pronounced by
Morinus[400] in the seventeenth century, and no less dogmatically by
Auguste Comte[401] in the nineteenth, hopelessly out of reach of the
human intellect, was thrown freely open, and the chemistry of the
sun and stars took at once a leading place among the experimental
sciences.
The immediate increase of knowledge was not the chief result of
Kirchhoff’s labours; still more important was the change in the
scope and methods of astronomy, which, set on foot in 1852 by the
detection of a common period affecting at once the spots on the sun
and the magnetism of the earth, was extended and accelerated by
the discovery of spectrum analysis. The nature of that change is
concisely indicated by the heading of the present chapter; we would
now ask our readers to endeavour to realise somewhat distinctly
what is implied by the “foundation of astronomical physics.”
Just three centuries ago, Kepler drew a forecast of what he
called a “physical astronomy”—a science treating of the efficient
causes of planetary motion, and holding the “key to the inner
astronomy.”[402] What Kepler dreamed of and groped after, Newton
realized. He showed the beautiful and symmetrical revolutions of
the solar system to be governed by a uniformly acting cause, and
that cause no other than the familiar force of gravity, which gives
stability to all our terrestrial surroundings. The world under our
feet was thus for the first time brought into physical connection
with the worlds peopling space, and a very tangible relationship was
demonstrated as existing between what used to be called the “corruptible”
matter of the earth and the “incorruptible” matter of the
heavens.
This process of unification of the cosmos—this levelling of the
celestial with the sublunary—was carried no farther until the fact
unexpectedly emerged from a vast and complicated mass of observations,
that the magnetism of the earth is subject to subtle influences,
emanating, certainly from some, and presumably from all of the
heavenly bodies; the inference being thus rendered at least
plausible, that a force not less universal than gravity itself, but with
whose modes of action we are as yet unacquainted, pervades the
universe, and forms, it might be said, an intangible bond of sympathy
between its parts. Now for the investigation of this influence two
roads are open. It may be pursued by observation either of the
bodies from which it proceeds, or of the effects which it produces—that
is to say, either by the astronomer or by the physicist, or,
better still, by both concurrently. Their acquisitions are mutually
profitable; nor can either be considered as independent of the other.
Any important accession to knowledge respecting the sun, for
example, may be expected to cast a reflected light on the still
obscure subject of terrestrial magnetism; while discoveries in
magnetism or its alter ego electricity must profoundly affect solar
inquiries.
The establishment of the new method of spectrum analysis drew[Pg 142]
far closer this alliance between celestial and terrestrial science.
Indeed, they have come to merge so intimately one into the other,
that it is no easier to trace their respective boundaries than it is
to draw a clear dividing-line between the animal and vegetable
kingdoms. Yet up to the middle of the last century, astronomy,
while maintaining her strict union with mathematics, looked with
indifference on the rest of the sciences; it was enough that she
possessed the telescope and the calculus. Now the materials for her
inductions are supplied by the chemist, the electrician, the inquirer
into the most recondite mysteries of light and the molecular constitution
of matter. She is concerned with what the geologist, the
meteorologist, even the biologist, has to say; she can afford to close
her ears to no new truth of the physical order. Her position of
lofty isolation has been exchanged for one of community and mutual
aid. The astronomer has become, in the highest sense of the term,
a physicist; while the physicist is bound to be something of an
astronomer.
This, then, is what is designed to be conveyed by the “foundation
of astronomical or cosmical physics.” It means the establishment
of a science of Nature whose conclusions are not only presumed by
analogy, but are ascertained by observation, to be valid wherever
light can travel and gravity is obeyed—a science by which the
nature of the stars can be studied upon the earth, and the nature
of the earth can be made better known by study of the stars—a
science, in a word, which is, or aims at being, one and universal,
even as Nature—the visible reflection of the invisible highest Unity—is
one and universal.
It is not too much to say that a new birth of knowledge has
ensued. The astronomy so signally promoted by Bessel[403]—the
astronomy placed by Comte[404] at the head of the hierarchy of the
physical sciences—was the science of the movements of the heavenly
bodies. And there were those who began to regard it as a science
which, from its very perfection, had ceased to be interesting—whose
tale of discoveries was told, and whose farther advance must be in
the line of minute technical improvements, not of novel and stirring
disclosures. But the science of the nature of the heavenly bodies is
one only in the beginning of its career. It is full of the audacities,
the inconsistencies, the imperfections, the possibilities of youth. It
promises everything; it has already performed much; it will doubtless
perform much more. The means at its disposal are vast and are
being daily augmented. What has so far been secured by them it
must now be our task to extricate from more doubtful surroundings
and place in due order before our readers.
FOOTNOTES:
[347] Wolf, Gesch. der Astr., p. 655.
[348] Manuel Johnson, Mem. R.A.S., vol. xxvi., p. 197.
[349] Astronomie Théorique et Pratique, t. iii., p. 20.
[350] Wolf, Gesch. der Astr., p. 654.
[351] Month. Not., vol. xvii., p. 241.
[352] Mem. R.A.S., vol. xxvi., p. 200.
[353] Astr. Nach., No. 495.
[354] Gehler’s Physikalisches Wörterbuch, art. Sonnenflecken, p. 851.
[355] Zweite Abth., p. 401.
[356] Annalen der Physik (Poggendorff’s), Bd. lxxxiv., p. 580.
[357] Phil. Trans., vol. cxlii., p. 103.
[358] Mittheilungen der Naturforschenden Gesellschaft, 1852, p. 183.
[359] Archives des Sciences, t. xxi., p. 194.
[360] Neue Untersuchungen, Mitth. Naturf. Ges., 1852, p. 249.
[361] Phil. Trans., vol. xci., p. 316.
[362] Evidence of an eleven-yearly fluctuation in the price of food-grains in India
was collected some years ago by Mr. Frederick Chambers. Nature, vol. xxxiv.,
p. 100.
[363] Bibl. Un. de Genève, t. li., p. 336.
[364] Neue Untersuchungen, p. 269.
[365] Die Sonne und ihre Flecken, p. 30. Arago was the first who attempted to
decide the question by keeping, through a series of years, a parallel register of
sun-spots and weather; but the data regarding the solar condition amassed at the
Paris Observatory from 1822 to 1830 were not sufficiently precise to support any
inference.
[366] Phil. Trans., vol. xxix., p. 421.
[367] Ibid., vols. cxliii., p. 558, cxlvi., p. 505.
[368] Observations on Light and Colours, p. 35.
[369] Phil. Trans., vol. lxxv., p. 190.
[370] Denkschriften (Munich. Ac. of Sc.), 1814, 1815, Bd. v., p. 197.
[371] Edinburgh Journal of Science, vol. v., p. 77. See also Phil. Mag., Feb.,
1834, vol. iv., p. 112.
[372] Ed. Phil. Trans., vol. xxi., p. 411.
[373] On the Absorption of Light by Coloured Media, Ed. Phil. Trans., vol. ix.,
p. 445 (1823).
[374] Phil. Mag., vol. xxvii, (ser. iii.), p. 81.
[375] Report Brit. Ass., 1835, p. 11 (pt. ii.). Electrodes are the terminals from
one to the other of which the electric spark passes, volatilising and rendering
incandescent in its transit some particles of their substance, the characteristic
light of which accordingly flashes out in the spectrum.
[376] Phil. Mag., vol. xx., p. 93.
[377] Annalen der Physik, Bd. cxiii., p. 357.
[378] Phil. Trans., vol. xcii., p. 378.
[379] Denkschriften, Bd. v., p. 202.
[380] Ibid., p. 220; Edin. Jour. of Science, vol. viii., p. 9.
[381] Denkschriften, Bd. v., p. 222.
[382] Arch. des Sciences, 1849, p. 43.
[383] Phil. Trans., vol. cl., p. 159, note.
[384] Ed. Phil. Trans., vol. xii., p. 528.
[385] Phil. Trans., vol. cxxvi., p. 453. “I conceive,” he says, “that this result
proves decisively that the sun’s atmosphere has nothing to do with the production
of this singular phenomenon” (p. 455). And Brewster’s well-founded opinion
that it had much to do with it was thereby, in fact, overthrown.
[386] Monatsberichte, Berlin, 1859, p. 664.
[387] Abhandlungen, Berlin, 1861, pp. 80, 81.
[388] Ibid., 1861, p. 77; Annalen der Physik, Bd. cxix., p. 275. A similar
conclusion, reached by Balfour Stewart in 1858, for heat-rays (Ed. Phil. Trans.,
vol. xxii., p. 13), was, in 1860, without previous knowledge of Kirchhoff’s work,
extended to light (Phil. Mag., vol. xx., p. 534); but his experiments wanted the
precision of those executed at Heidelburg.
[389] Miscellaneous Works, vol. i., p. 189.
[390] Ed. Phil. Trans., vol. ix., p. 458.
[391] Ibid., vol. xii., p. 519.
[392] Quart. Jour. Chem. Soc., vol. x. p. 79.
[393] A facsimile accompanied Sir H. Roscoe’s translation of Kirchhoff’s “Researches
on the Solar Spectrum” (London, 1862-63).
[394] Estimated by Kirchhoff’s at a trillion to one. Abhandl., 1861, p. 79.
[395] Phil. Mag., vol. xxvii. (3rd series), p. 90.
[396] L’Institut, Feb. 7, 1849, p. 45; Phil. Mag., vol. xix. (4th series), p. 193.
[397] Ann. d. Phys., vol. cxviii., p. 110.
[398] Phil. Mag., vol. ix. (4th series), p. 327.
[399] Spectra may be produced by diffraction as well as by refraction; but we are
here only concerned with the subject in its simplest aspect.
[400] Astrologia Gallica (1661), p. 189.
[401] Pos. Phil., vol. i., pp. 114, 115 (Martineau’s trans.).
[402] Proem Astronomiæ Pars Optica (1640), Op., t. ii.
[403] Pop. Vorl., pp. 14, 19, 408.
[404] Pos. Phil., p. 115.
CHAPTER II
SOLAR OBSERVATIONS AND THEORIES
The zeal with which solar studies have been pursued during the last
half century has already gone far to redeem the neglect of the two
preceding ones. Since Schwabe’s discovery was published in 1851,
observers have multiplied, new facts have been rapidly accumulated,
and the previous comparative quiescence of thought on the great
subject of the constitution of the sun, has been replaced by a bewildering
variety of speculations, conjectures, and more or less justifiable
inferences. It is satisfactory to find this novel impulse not only
shared, but to a large extent guided, by our countrymen.
William Rutter Dawes, one of many clergymen eminent in
astronomy, observed, in 1852, with the help of a solar eye-piece of
his own devising, some curious details of spot-structure.[405] The umbra—heretofore
taken for the darkest part of the spot—was seen to be
suffused with a mottled, nebulous illumination, in marked contrast
with the striated appearance of the penumbra; while through this
“cloudy stratum” a “black opening” permitted the eye to divine
farther unfathomable depths beyond. The hole thus disclosed—evidently
the true nucleus—was found to be present in all considerable,
as well as in many small maculæ.
Again, the whirling motions of some of these objects were noticed
by him. The remarkable form of one sketched at Wateringbury,
in Kent, January 17, 1852, gave him the means of detecting and
measuring a rotatory movement of the whole spot round the black
nucleus at the rate of 100 degrees in six days. “It appeared,” he
said, “as if some prodigious ascending force of a whirlwind character,
in bursting through the cloudy stratum and the two higher and
luminous strata, had given to the whole a movement resembling its
own.”[406] An interpretation founded, as is easily seen, on the
Herschelian theory, then still in full credit.
An instance of the same kind was observed by Mr. W. R. Birt[Pg 144]
in 1860,[407] and cyclonic movements are now a recognised feature
of sun-spots. They are, however, as Father Secchi[408] concluded
from his long experience, but temporary and casual. Scarcely
three per cent. of all spots visible exhibit the spiral structure
which should invariably result if a conflict of opposing, or the
friction of unequal, currents were essential, and not merely incidental
to their origin. A whirlpool phase not unfrequently
accompanies their formation, and may be renewed at periods of
recrudescence or dissolution; but it is both partial and inconstant,
sometimes affecting only one side of a spot, sometimes slackening
gradually its movement in one direction, to resume it, after a
brief pause, in the opposite. Persistent and uniform notions, such
as the analogy of terrestrial storms would absolutely require, are
not to be found. So that the “cyclonic theory” of sun-spots,
suggested by Herschel in 1847,[409] and urged, from a different point
of view, by Faye in 1872, may be said to have completely broken
down.
The drift of spots over the sun’s surface was first systematically
investigated by Carrington, a self-constituted astronomer,
gifted with the courage and the instinct of thoughtful labour.
Born at Chelsea in May, 1826, Richard Christopher Carrington
entered Trinity College, Cambridge, in 1844. He was intended
for the Church, but Professor Challis’s lectures diverted him to
astronomy, and he resolved, as soon as he had taken his degree,
to prepare, with all possible diligence, to follow his new vocation.
His father, who was a brewer on a large scale at Brentford, offered
no opposition; ample means were at his disposal; nevertheless, he
chose to serve an apprenticeship of three years as observer in the
University of Durham, as though his sole object had been to earn
a livelihood. He quitted the post only when he found that its
restricted opportunities offered no farther prospect of self-improvement.
He now built an observatory of his own at Redhill in Surrey,
with the design of completing Bessel’s and Argelander’s survey
of the northern heavens by adding to it the circumpolar stars
omitted from their view. This project, successfully carried out
between 1854 and 1857, had another and still larger one superposed
upon it before it had even begun to be executed. In 1852,
while the Redhill Observatory was in course of erection, the
discovery of the coincidence between the sun-spot and magnetic
periods was announced. Carrington was profoundly interested,
and devoted his enforced leisure to the examination of records,[Pg 145]
both written and depicted, of past solar observations. Struck
with their fragmentary and inconsistent character, he resolved
to “appropriate,” as he said, by “close and methodical research,”
the eleven-year period next ensuing.[410] He calculated rightly
that he should have the field pretty nearly to himself; for many
reasons conspire to make public observatories slow in taking up
new subjects, and amateurs with freedom to choose, and means
to treat them effectually, were scarcer then than they are now.
The execution of this laborious task was commenced November 9,
1853. It was intended to be merely a parergon—a “second
subject,” upon which daylight energies might be spent, while the
hours of night were reserved for cataloguing those stars that “are
bereft of the baths of ocean.” Its results, however, proved of the
highest interest, although the vicissitudes of life barred the completion,
in its full integrity, of the original design. By the death, in
1858, of the elder Carrington, the charge of the brewery devolved
upon his son; and eventually absorbed so much of his care that it
was found advisable to bring the solar observations to a premature
close, on March 24, 1861.
His scientific life may be said to have closed with them. Attacked
four years later with severe, and, in its results, permanent illness, he
disposed of the Brentford business, and withdrew to Churt, near
Farnham, in Surrey. There, in a lonely spot, on the top of a
detached conical hill known as the “Devil’s Jump,” he built a
second observatory, and erected an instrument which he was no
longer able to use with pristine effectiveness; and there,
November 27, 1875, he died of the rupture of a blood vessel on the
brain, before he had completed his fiftieth year.[411]
His observations of sun-spots were of a geometrical character.
They concerned positions and movements, leaving out of sight
physical peculiarities. Indeed, the prudence with which he limited
his task to what came strictly within the range of his powers to
accomplish, was one of Carrington’s most valuable qualities. The
method of his observations, moreover, was chosen with the same
practical sagacity as their objects. As early as 1847, Sir John
Herschel had recommended the daily self-registration of sun-spots,[412]
and he enforced the suggestion, with more immediate prospect of
success, in 1854.[413] The art of celestial photography, however, was
even then in a purely tentative stage, and Carrington wisely resolved
to waste no time on dubious experiments, but employ the
means of registration and measurement actually at his command.[Pg 146]
These were very simple, yet very effective. To the “helioscope”
employed by Father Scheiner[414] two centuries and a quarter earlier, a
species of micrometer was added. The image of the sun was projected
upon a screen by means of a firmly-clamped telescope, in the
focus of which were placed two cross-wires forming angles of 45°
with the meridian. The six instants were then carefully noted at
which these were met by the edges of the disc as it traversed the
screen, and by the nucleus of the spot to be measured.[415] A short
process of calculation then gave the exact position of the spot as
referred to the sun’s centre.
From a series of 5,290 observations made in this way, together
with a great number of accurate drawings, Carrington derived conclusions
of great importance on each of the three points which he
had proposed to himself to investigate. These were: the law of the
sun’s rotation, the existence and direction of systematic currents,
and the distribution of spots on the solar surface.
Grave discrepancies were early perceived to exist between determinations
of the sun’s rotation by different observers. Galileo,
with “comfortable generality,” estimated the period at “about a
lunar month”;[416] Scheiner, at twenty-seven days.[417] Cassini, in 1678,
made it 25·58; Delambre, in 1775, no more than twenty-five days.
Later inquiries brought these divergences within no more tolerable
limits. Laugier’s result of 25·34 days—obtained in 1841—enjoyed
the highest credit, yet it differed widely in one direction from that
of Böhm (1852), giving 25·52 days, and in the other from that of
Kysæus (1846), giving 25·09 days. Now the cause of these variations
was really obvious from the first, although for a long time
strangely overlooked. Scheiner pointed out in 1630 that different
spots gave different periods, adding the significant remark that one
at a distance from the solar equator revolved more slowly than those
nearer to it.[418] But the hint was wasted. For upwards of two
centuries ideas on the subject were either retrograde or stationary.
What were called the “proper motions” of spots were, however,
recognised by Schröter,[419] and utterly baffled Laugier,[420] who despaired
of obtaining any concordant result as to the sun’s rotation except by
taking the mean of a number of discordant ones. At last, in 1855,[Pg 147]
a valuable course of observations made at Capo di Monte, Naples, in
1845-6, enabled C. H. F. Peters[421] to set in the clearest light the
insecurity of determinations based on the assumption of fixity in
objects plainly affected by movements uncertain both in amount and
direction.
Such was the state of affairs when Carrington entered upon his
task. Everything was in confusion; the most that could be said
was that the confusion had come to be distinctly admitted and
referred to its true source. What he discovered was this: that the
sun, or at least the outer shell of the sun visible to us, has no single
period of rotation, but drifts round, carrying the spots with it, at a
rate continually accelerated from the poles to the equator. In other
words, the time of axial revolution is shortest at the equator and
lengthens with increase of latitude. Carrington devised a mathematical
formula by which the rate or “law” of this lengthening was
conveniently expressed; but it was a purely empirical one. It was
a concise statement, but implied no physical interpretation. It
summarised, but did not explain the facts. An assumed “mean
period” for the solar rotation of 25·38 days (twenty-five days nine
hours, very nearly), was thus found to be actually conformed to only
in two parallels of solar latitude (14° north and south), while the
equatorial period was slightly less than twenty-five, and that of
latitude 50° rose to twenty-seven days and a half.[422] These curious
results gave quite a new direction to ideas on solar physics.
The other two “elements” of the sun’s rotation were also ascertained
by Carrington with hitherto unattained precision. He fixed
the inclination of its axis to the ecliptic at 82° 45′; the longitude of
the ascending node at 73° 40′ (for the epoch 1850 A.D.). These
data—which have scarcely yet been improved upon—suffice to
determine the position in space of the sun’s equator. Its north pole
is directed towards a star in the coils of the Dragon, midway
between Vega and the Pole-star; its plane intersects that of the
earth’s orbit in such a way that our planet finds itself in the same
level on or about the 3rd of June and the 5th of December, when
any spots visible on the disc cross it in apparently straight lines.
At other times, the paths pursued by them seem curved—downward
(to an observer in the northern hemisphere) between June and
December, upward between December and June.
A singular peculiarity in the distribution of sun-spots emerged
from Carrington’s studies at the time of the minimum of 1856.
Two broad belts of the solar surface, as we have seen, are frequented
by them, of which the limits may be put at 6° and 35° of north and[Pg 148]
south latitude. Individual equatorial spots are not uncommon, but
nearer to the poles than 35° they are a rare exception. Carrington
observed—as an extreme instance—in July, 1858, one in south
latitude 44°; and Peters, in June, 1846, watched, during several
days, a spot in 50° 24′ north latitude. But beyond this no true
macula has ever been seen; for Lahire’s reported observation of one
in latitude 70° is now believed to have had its place on the solar
globe erroneously assigned; and the “veiled spots” described by
Trouvelot in 1875[423] as occurring within 10° of the pole can only be
regarded as, at the most, the same kind of disturbance in an undeveloped
form.
But the novelty of Carrington’s observations consisted in the
detection of certain changes in distribution concurrent with the
progress of the eleven-year period. As the minimum approached,
the spot-zones contracted towards the equator, and there finally
vanished; then, as if by a fresh impulse, spots suddenly reappeared
in high latitude, and spread downwards with the development of
the new phase of activity. Scarcely had this remark been made
public,[424] when Wolf[425] found a confirmation of its general truth in
Böhm’s observations during the years 1833-36; and a perfectly
similar behaviour was noted both by Spörer and Secchi at the
minimum epoch of 1867. The ensuing period gave corresponding
indications; and it may now be looked upon as established that the
spot-zones close in towards the equator with the advance of each
cycle, their activity culminating, as a rule, in a mean latitude of
about 16°, and expiring when it is reduced to 6°. Before this
happens, however, a completely new disturbance will have manifested
itself some 35° north and south of the equator, and will have begun
to travel over the same course as its predecessor. Each series of
sun-spots is thus, to some extent, overlapped by the succeeding one;
so that while the average interval from one maximum to the next
is eleven years, the period of each distinct wave of agitation is
twelve or fourteen.[426] Curious evidence of the retarded character of
the maximum of 1883-4 was to be found in the unusually low
latitude of the spot-zones when it occurred. Their movement downward
having gone on regularly while the crisis was postponed, its
final symptoms were hence displaced locally as well as in time. The
“law of zones” was duly obeyed at the minima of 1890[427] and 1901,
and Spörer found evidence of conformity to it so far back as 1619.[428]
His researches, however, also showed that it was in abeyance[Pg 149]
during some seventy years previously to 1716, during which period
sun-spots remained persistently scarce, and auroral displays were
feeble and infrequent even in high northern latitudes. An unaccountable
suspension of solar activity is, in fact, indicated.[429]
Gustav Spörer, born at Berlin in 1822, began to observe sun-spots
with the view of assigning the law of solar rotation in December,
1860. His assiduity and success with limited means attracted attention,
and a Government endowment was procured for his little solar
observatory at Anclam, in Pomerania, the Crown Prince (afterwards
Emperor Frederick) adding a five-inch refractor to its modest equipment.
Unaware of Carrington’s discovery (not made known until
January, 1859), he arrived at and published, in June, 1861,[430] a
similar conclusion as to the equatorial quickening of the sun’s movement
on its axis. Appointed observer in the new Astrophysical
establishment at Potsdam in 1874, he continued his sun-spot determinations
there for twenty years, and died July 7, 1895.
The time had now evidently come for a fundamental revision of
current notions respecting the nature of the sun. Herschel’s theory
of a cool, dark, habitable globe, surrounded by, and protected
against, the radiations of a luminous and heat-giving envelope, was
shattered by the first dicta of spectrum analysis. Traces of it may
be found for a few years subsequent to 1859,[431] but they are obviously
survivals from an earlier order of ideas, doomed to speedy extinction.
It needs only a moment’s consideration of the meaning at
last found for the Fraunhofer lines to see the incompatibility of the
new facts with the old conceptions. They implied not only the
presence near the sun, as glowing vapours, of bodies highly refractory
to heat, but that these glowing vapours formed the relatively
cool envelope of a still hotter internal mass. Kirchhoff, accordingly,
included in his great memoir “On the Solar Spectrum,” read before
the Berlin Academy of Sciences, July 11, 1861, an exposition of
the views on the subject to which his memorable investigations had
led him. They may be briefly summarised as follows:
Since the body of the sun gives a continuous spectrum, it must be
either solid or liquid,[432] while the interruptions in its light prove it
to be surrounded by a complex atmosphere of metallic vapours,
somewhat cooler than itself. Spots are simply clouds due to local
depressions of temperature, differing in no respect from terrestrial
clouds except as regards the kinds of matter composing them.[Pg 150]
These sun-clouds take their origin in the zones of encounter between
polar and equatorial currents in the solar atmosphere.
This explanation was liable to all the objections urged against the
“cumulus theory” on the one hand, and the “trade-wind theory”
on the other. Setting aside its propounder, it was consistently upheld
perhaps by no man eminent in science except Spörer; and his
advocacy of it proved ineffective to secure its general adoption.
M. Faye, of the Paris Academy of Sciences, was the first to
propose a coherent scheme of the solar constitution covering the
whole range of new discovery. The fundamental ideas on the
subject now in vogue here made their first connected appearance.
Much, indeed, remained to be modified and corrected; but the
transition was finally made from the old to the new order of
thought. The essence of the change may be conveyed in a single
sentence. The sun was thenceforth regarded, not as a mere heated
body, or—still more remotely from the truth—as a cool body unaccountably
spun round with a cocoon of fire, but as a vast heat-radiating
machine. The terrestrial analogy was abandoned in one
more particular besides that of temperature. The solar system of
circulation, instead of being adapted, like that of the earth, to the
distribution of heat received from without, was seen to be directed
towards the transportation towards the surface of the heat contained
within. Polar and equatorial currents, tending to a purely
superficial equalisation of temperature, were replaced by vertical
currents bringing up successive portions of the intensely heated
interior mass, to contribute their share in turn to the radiation
into space which might be called the proper function of a sun.
Faye’s views, which were communicated to the Academy of
Sciences, January 16, 1865,[433] were avowedly based on the anomalous
mode of solar rotation discovered by Carrington. This may be
regarded either as an acceleration increasing from the poles to the
equator, or as a retardation increasing from the equator to the poles,
according to the rate of revolution we choose to assume for the
unseen nucleus. Faye preferred to consider it a retardation
produced by ascending currents continually left behind as the
sphere widened in which the matter composing them was forced
to travel. He further supposed that the depth from which these
vertical currents rose, and consequently the amount of retardation
effected by their ascent to the surface, became progressively greater
as the poles were approached, owing to the considerable flattening
of the spheroidal surface from which they started;[434] but the adoption
of this expedient has been shown to involve inadmissible
consequences.
The extreme internal mobility betrayed by Carrington’s and
Spörer’s observations led to the inference that the matter composing
the sun was mainly or wholly gaseous. This had already been
suggested by Father Secchi[435] a year earlier, and by Sir John Herschel
in April, 1864;[436] but it first obtained general currency through
Faye’s more elaborate presentation. A physical basis was afforded
for the view by Cagniard de la Tour’s experiments in 1822,[437] proving
that, under conditions of great heat and pressure, the vaporous
state was compatible with a very considerable density. The position
was strengthened when Andrews showed, in 1869,[438] that above a
fixed limit of temperature, varying for different bodies, true liquefaction
is impossible, even though the pressure be so tremendous as
to retain the gas within the same space that enclosed the liquid.
The opinion that the mass of the sun is gaseous now commands a
very general assent; although the gaseity admitted is of such a
nature as to afford the consistence rather of honey or pitch than of
the aeriform fluids with which we are familiar.
On another important point the course of subsequent thought
was powerfully influenced by Faye’s conclusions in 1865. Arago
somewhat hastily inferred from experiments with the polariscope
the wholly gaseous nature of the visible disc of the sun. Kirchhoff,
on the contrary, believed (erroneously, as we now know) that the
brilliant continuous spectrum derived from it proved it to be a
white-hot solid or liquid. Herschel and Secchi[439] indicated a cloud-like
structure as that which would best harmonise the whole of the
evidence at command. The novelty introduced by Faye consisted
in regarding the photosphere no longer “as a defined surface, in the
mathematical sense, but as a limit to which, in the general fluid
mass, ascending currents carry the physical or chemical phenomena
of incandescence.”[440] Uprushing floods of mixed vapours with strong
affinities—say of calcium or sodium and oxygen—at last attain a
region cool enough to permit their combination; a fine dust of
solid or liquid compound particles (of lime or soda, for example)
there collects into the photospheric clouds, and descending by its
own weight in torrents of incandescent rain, is dissociated by the
fierce heat below, and replaced by ascending and combining currents
of similar constitution.
This first attempt to assign the part played in cosmical physics by
chemical affinities was marked by the importation into the theory[Pg 152]
of the sun of the now familiar phrase dissociation. It is indeed
tolerably certain that no such combinations as those contemplated
by Faye occur at the photospheric level, since the temperature there
must be enormously higher than would be needed to reduce all
metallic earths and oxides; but molecular changes of some kind,
dependent perhaps in part upon electrical conditions, in part upon
the effects of radiation into space, most likely replace them. The
conjecture was emitted by Dr. Johnstone Stoney in 1867[441] that the
photospheric clouds are composed of carbon-particles precipitated
from their mounting vapour just where the temperature is lowered
by expansion and radiation to the boiling-point of that substance.
But this view, though countenanced by Ångström,[442] and advocated
by Hastings of Baltimore,[443] and other authorities,[444] is open to grave
objections.[445]
In Faye’s theory, sun-spots were regarded as simply breaks in the
photospheric clouds, where the rising currents had strength to tear
them asunder. It followed that they were regions of increased heat—regions,
in fact, where the temperature was too high to permit
the occurrence of the precipitations to which the photosphere is due.
Their obscurity was attributed, as in Dr. Brester’s more recent
Théorie du Soleil, to deficiency of emissive power. Yet here the
verdict of the spectroscope is adverse and irreversible.
After every deduction, however, has been made, we still find that
several ideas of permanent value were embodied in this comprehensive
sketch of the solar constitution. The principal of these
were; first, that the sun is a mainly gaseous body; secondly, that
its stores of heat are rendered available at the surface by means of
vertical convection-currents—by the bodily transport, that is to say,
of intensely hot matter upward, and of comparatively cool matter
downward; thirdly, that the photosphere is a surface of condensation,
forming the limit set by the cold of space to this circulating
process, and that a similar formation must attend, at a certain stage,
the cooling of every cosmical body.
To Warren de la Rue belongs the honour of having obtained the
earliest results of substantial value in celestial photography. What
had been done previously was interesting in the way of promise, but
much could not be claimed for it as actual performance. Some
“pioneering experiments” were made by Dr. J. W. Draper of New
York in 1840, resulting in the production of a few “moon-pictures”[Pg 153]
one inch in diameter;[446] but slight encouragement was derived from
them, either to himself or others. Bond of Cambridge (U.S.), however,
secured in 1850 with the Harvard 15-inch refractor that
daguerreotype of the moon with which the career of extra-terrestrial
photography may be said to have formally opened. It was shown
in London at the Great Exhibition of 1851, and determined the
direction of De la Rue’s efforts. Yet it did little more than prove
the art to be a possible one.
Warren de la Rue was born in Guernsey in 1815, and died in
London April 19, 1889. Educated at the École Sainte-Barbe in
Paris, he made a large fortune as a paper manufacturer in England,
and thus amply and early provided the material supplies for his
scientific campaign. Towards the end of 1853 he took some
successful lunar photographs. They were remarkable as the first
examples of the application to astronomical light-painting of the
collodion process, invented by Archer in 1851; and also of the use
of reflectors (De la Rue’s was one of thirteen inches, constructed by
himself) for that kind of work. The absence of a driving apparatus
was, however, very sensibly felt; the difficulty of moving the
instrument by hand so as accurately to follow the moon’s apparent
motion being such as to cause the discontinuance of the experiments
until 1857, when the want was supplied. De la Rue’s new
observatory, built in that year at Cranford, was expressly dedicated
to celestial photography; and there he applied to the heavenly
bodies the stereoscopic method of obtaining relief, and turned his
attention to the delicate business of photographing the sun.
A solar daguerreotype was taken at Paris, April 2, 1845,[447] by
Foucault and Fizeau, acting on a suggestion from Arago. But the
attempt, though far from being unsuccessful, does not, at that time,
seem to have been repeated. Its great difficulty consisted in the
enormous light-power of the object to be represented, rendering an
inconceivably short period of exposure indispensable, under pain of
getting completely “burnt-up” plates. In 1857 De la Rue was
commissioned by the Royal Society to construct an instrument
specially adapted to the purpose for the Kew Observatory. The
resulting “photoheliograph” may be described as a small telescope
(of 3-1/2 inches aperture and 50 focus), with a plate-holder at the eye-end,
guarded in front by a spring-slide, the rapid movement of which
across the field of view secured for the sensitive plate a virtually
instantaneous exposure. By its means the first solar light-pictures
of real value were taken, and the autographic record of the solar[Pg 154]
condition recommended by Sir John Herschel was commenced and
continued at Kew during fourteen years—1858-72. The work
of photographing the sun is now carried on in every quarter of
the globe, from Mauritius to Massachusetts, and the days are few
indeed on which the self-betrayal of the camera can be evaded by
our chief luminary. In the year 1883 the incorporation of Indian
with Greenwich pictures afforded a record of the state of the solar
surface on 340 days; and 364 were similarly provided for in 1897
and 1899.
The conclusions arrived at by photographic means at Kew were
communicated to the Royal Society in a series of papers drawn up
jointly by De la Rue, Balfour Stewart, and Benjamin Loewy, in
1865 and subsequent years. They influenced materially the progress
of thought on the subject they were concerned with.
By its rotation the sun itself offers opportunities for bringing the
stereoscope to bear upon it. Two pictures, taken at an interval of
twenty-six minutes, show just the amount of difference needed to
give, by their combination, the maximum effect of solidity.[448] De la
Rue thus obtained, in 1861, a stereoscopic view of a sun-spot and
surrounding faculæ, representing the various parts in their true
mutual relations. “I have ascertained in this way,” he wrote,[449]
“that the faculæ occupy the highest portions of the sun’s photosphere,
the spots appearing like holes in the penumbræ, which
appeared lower than the regions surrounding them; in one case,
parts of the faculæ were discovered to be sailing over a spot apparently
at some considerable height above it.” Thus Wilson’s
inference as to the depressed nature of spots received, after the
lapse of not far from a century, proof of the most simple, direct, and
convincing kind. A careful application of Wilson’s own geometrical
test gave results only a trifle less decisive. Of 694 spots observed,
78 per cent. showed, as they traversed the disc, the expected effects
of perspective;[450] and their absence in the remaining 22 per cent.
might be explained by internal commotions producing irregularities
of structure. The absolute depth of spot-cavities—at least of their
sloping sides—was determined by Father Secchi through measurement
of the “parallax of profundity”[451]—that is, of apparent displacements
attendant on the sun’s rotation, due to depression below
the sun’s surface. He found that in every case it fell short of
4,000 miles, and averaged not more than 1,321, corresponding, on[Pg 155]
the terrestrial scale, to an excavation in the earth’s crust of 1-1/5 miles.
Of late, however, the reality of even this moderate amount of depression
has been denied. Mr. Howlett’s persevering observations,
extending over a third of a century, the results of which were
presented to the Royal Astronomical Society in December, 1894,[452]
availed to shatter the consensus of opinion which had so long been
maintained on the subject of spot-structure.[453] It has become
impossible any longer to hold that it is uniformly cavernous; and
what seem like actually protruding umbræ are occasionally vouched
for on unimpeachable authority.[454] We can only infer that the forms
of sun-spots are really more various than had been supposed; that
they are peculiarly subject to disturbance; and that the level of the
nuclei may rise and fall during the phases of commotion, like lavas
within volcanic craters.
The opinion of the Kew observers as to the nature of such
disturbances was strongly swayed by another curious result of
the “statistical method” of inquiry. They found that of 1,137
instances of spots accompanied by faculæ, 584 had those faculæ
chiefly or entirely on the left, 508 showed a nearly equal distribution,
while 45 only had faculous appendages mainly on the
right side.[455] Now the rotation of the sun, as we see it, is performed
from left to right; so that the marked tendency of the
faculæ was a lagging one. This was easily accounted for by
supposing the matter composing them to have been flung upwards
from a considerable depth, whence it would reach the surface
with the lesser absolute velocity belonging to a smaller circle of
revolution, and would consequently fall behind the cavities or
“spots” formed by its abstraction. An attempt, it is true, made
by M. Wilsing at Potsdam in 1888[456] to determine the solar rotation
from photographs of faculæ had an outcome inconsistent with this
view of their origin. They unexpectedly gave a uniform period.
No trace of the retardation poleward from the equator, shown by the
spots, could be detected in their movements. But the experiment
was obviously inconclusive;[457] and M. Stratonoff’s[458] repetition of it
with ampler materials gave a full assurance that faculæ rotate like
spots in periods lengthening as latitude augments.
The ideas of M. Faye were, on two fundamental points, contradicated[Pg 156]
by the Kew investigators. He held spots to be regions
of uprush and of heightened temperature; they believed their
obscurity to be due to a downrush of comparatively cool vapours.
Now M. Chacornac, observing, at Ville-Urbanne, March 6, 1865,
saw floods of photospheric matter visibly precipitating themselves
into the abyss opened by a great spot, and carrying with them
small neighbouring maculæ.[459] Similar instances were repeatedly
noted by Father Secchi, who considered the existence of a kind
of suction in spots to be quite beyond question.[460] The tendency in
their vicinity, to put it otherwise, is centripetal, not centrifugal;
and this alone seems to negative the supposition of a central
uprush.
A fresh witness was by this time at hand. The application of
the spectroscope to the direct examination of the sun’s surface
dates from March 4, 1866, when Sir Norman Lockyer (to give him
his present title) undertook an inquiry into the cause of the darkening
in spots.[461] It was made possible by the simple device of throwing
upon the slit of the spectroscope an image of the sun, any part
of which could be subjected to special scrutiny, instead of, as had
hitherto been done, admitting rays from every portion of his surface
indiscriminately. The answer to the inquiry was prompt and unmistakable,
and was again, in this case, adverse to the French
theorist’s view. The obscurations in question were found to be
produced by no deficiency of emissive power, but by an increase
of absorptive action. The background of variegated light remains
unchanged, but more of it is stopped by the interposition of a dense
mass of relatively cool vapours. The spectrum of a sun-spot is
crossed by the same set of multitudinous dark lines, with some
minor differences, visible in the ordinary solar spectrum. We must
then conclude that the same vapours (speaking generally) which are
dispersed over the unbroken solar surface are accumulated in the
umbral cavity, the compression incident to such accumulation being
betrayed by the thickening of certain lines of absorption. But
there is also a general absorption, extending almost continuously
from one end of the spot-spectrum to the other. Using, however,
a spectroscope of exceptionally high dispersive power, Professor
Young of Princeton, New Jersey, succeeded in 1883 in “resolving”
the supposed continuous obscurity of spot-spectra into a countless
multitude of fine dark lines set very close together.[462] Their structure
was seen still more perfectly, about five years later, by M.
Dunér,[463] Director of the Upsala Observatory, who traced besides some[Pg 157]
shadowy vestiges of the crowded doublets and triplets forming the
array, from the spots on to the general solar surface. They cease
to be separable in the blue part of the spectrum; and the ultra-violet
radiations of spots show nothing distinctive.[464]
As to the movements of the constipated vapours forming spots,
the spectroscope is also competent to supply information. The
principle of the method by which it is procured will be explained
farther on. Suffice it here to say that the transport, at any considerable
velocity, to or from the eye of the gaseous material giving
bright or dark lines, can be measured by the displacement of such
lines from their previously known normal positions. In this way
movements have been detected in or above spots of enormous
rapidity, ranging up to 320 miles per second. But the result, so far,
has been to negative the ascription to them of any systematic
direction. Uprushes and downrushes are doubtless, as Father
Cortie remarks,[465] “correlated phenomena in the production of a
sun-spot”; but neither seem to predominate as part of its regular
internal economy.
The same kind of spectroscopic evidence tells heavily against a
theory of sun-spots started by Faye in 1872. He had been foremost
in pointing out that the observations of Carrington and Spörer
absolutely forbade the supposition that any phenomenon at all
resembling our trade-winds exists in the sun. They showed, indeed,
that beyond the parallels of 20° there is a general tendency in
spots to a slow poleward displacement, while within that zone they
incline to approach the equator; but their “proper movements”
gave no evidence of uniformly flowing currents in latitude. The
systematic drift of the photosphere is strictly a drift in longitude;
its direction is everywhere parallel to the equator. This fact being
once clearly recognised, the “solar tornado” hypothesis at once fell
to pieces; but M. Faye[466] perceived another source of vorticose motion
in the unequal rotating velocities of contiguous portions of the
photosphere. The “pores” with which the whole surface of the
sun is studded he took to be the smaller eddies resulting from these
inequalities; the spots to be such eddies developed into whirlpools.
It only needs to thrust a stick into a stream to produce the kind of
effect designated. And it happens that the differences of angular
movement adverted to attain a maximum just about the latitudes
where spots are most frequent and conspicuous.
There are, however, grave difficulties in identifying the two kinds
of phenomena. One (already mentioned) is the total absence of the
regular swirling motion—in a direction contrary to that of the hands
of a watch north of the solar equator, in the opposite sense south of
it—which should impress itself upon every lineament of a sun-spot
if the cause assigned were a primary producing, and not merely (as
it possibly may be) a secondary determining one. The other,
pointed out by Young,[467] is that the cause is inadequate to the effect.
The difference of movement, or relative drift, supposed to occasion
such prodigious disturbances, amounts, at the utmost, for two
portions of the photosphere 123 miles apart, to about five yards a
minute. Thus the friction of contiguous sections must be quite
insignificant.
A view better justified by observation was urged by Secchi in and
after the year 1872, and was presented in an improved form by
Professor Young in his excellent little book on The Sun, published
in 1882.[468] Spots are manifestly associated with violent eruptive
action, giving rise to the faculæ and prominences which usually
garnish their borders. It is accordingly contended that upon the
withdrawal of matter from below by the flinging up of a prominence
must ensue a sinking-in of the surface, into which the partially
cooled erupted vapours rush and settle, producing just the kind
of darkening by increased absorption told of by the spectroscope.
Round the edges of the cavity the rupture of the photospheric shell
will form lines of weakness provocative of further eruptions, which
will, in their turn, deepen and enlarge the cavity. The phenomenon
thus tends to perpetuate itself, until equilibrium is at last restored
by internal processes. A sun-spot might then be described as an
inverted terrestrial volcano, in which the outbursts of heated
matter take place on the borders instead of at the centre of the
crater, while the cooled products gather in the centre instead of at
the borders.
But on the earth, the solid crust forcibly represses the steam
gathering beneath until it has accumulated strength for an
explosion, while there is no such restraining power that we know
of in the sun. Zöllner, indeed, adapted his theory of the solar
constitution to the special purpose of procuring it; yet with very
partial success, since almost every new fact has proved adverse to
his assumptions. Volcanic action is essentially spasmodic. It
implies habitual constraint varied by temporary outbreaks, inconceivable
in a gaseous globe, such as we believe the sun to be.
If the “volcanic hypothesis” represented the truth, no spot
could possibly appear without a precedent eruption. The real
order of the phenomenon, however, is exceedingly difficult to
ascertain; nor is it perhaps invariable. Although, in most cases,
the “opening” shows first, that may be simply because it is more
easily seen. According to Father Sidgreaves,[469] the disturbance has
then already passed the incipient stage. He considers it indeed
“highly probable that the preparatory sign of a new spot is always
a small, bright patch of facula.”
This sequence, if established, would be fatal to Lockyer’s theory
of sun-spots, communicated to the Royal Society, May 6, 1886,[470]
and further developed some months later in his work on The
Chemistry of the Sun. Spots are represented in it as incidental to a
vast system of solar atmospheric circulation, starting with the polar
out- and up-flows indicated by observations during some total
eclipses, and eventuating in the plunge downward from great heights
upon the photosphere of prodigious masses of condensed materials.
From these falls result, primarily, spots; secondarily, through the
answering uprushes in which chemical and mechanical forces co-operate,
their girdles of flame-prominences. The evidence is, however,
slight that such a circulatory flow as would be needed to maintain
this supposed cycle of occurrences really prevails in the sun’s atmosphere;
and a similar objection applies to an “anticyclonic theory”
(so to designate it) elaborated by Egon von Oppolzer in 1893.[471]
August Schmidt’s optical rationale of solar phenomena[472] was, on the
other hand, a complete novelty, both in principle and development.
Attractive to speculators from its recondite nature and far-reaching
scope, it by no means commended itself to practical observers,
intolerant of finding the all but palpable realities of their daily
experience dealt with as illusory products of “circular refraction.”
A singular circumstance has now to be recounted. On the
1st of September, 1859, while Carrington was engaged in his daily
work of measuring the positions of sun-spots, he was startled by the
sudden appearance of two patches of peculiarly intense light within
the area of the largest group visible. His first idea was that a ray
of unmitigated sunshine had penetrated the screen employed to
reduce the brilliancy of the image; but, having quickly convinced
himself to the contrary, he ran to summon an additional witness of
an unmistakably remarkable occurrence. On his return he was disappointed
to find the strange luminous outburst already on the[Pg 160]
wane; shortly afterwards the last trace vanished. Its entire duration
was five minutes—from 11.18 to 11.23 A.M., Greenwich time;
and during those five minutes it had traversed a space estimated at
35,000 miles! No perceptible change took place in the details of
the group of spots visited by this transitory conflagration, which, it
was accordingly inferred, took place at a considerable height
above it.[473]
Carrington’s account was precisely confirmed by an observation
made at Highgate. Mr. R. Hodgson described the appearance seen
by him as that “of a very brilliant star of light, much brighter than
the sun’s surface, most dazzling to the protected eye, illuminating
the upper edges of the adjacent spots and streaks, not unlike in
effect the edging of the clouds at sunset.”[474]
This unique phenomenon seemed as if specially designed to
accentuate the inference of a sympathetic relation between the earth
and the sun. From the 28th of August to the 4th of September,
1859, a magnetic storm of unparalleled intensity, extent, and duration,
was in progress over the entire globe. Telegraphic communication
was everywhere interrupted—except, indeed, that it was, in
some cases, found practicable to work the lines without batteries, by
the agency of the earth-currents alone:[475] sparks issued from the
wires; gorgeous auroræ draped the skies in solemn crimson over
both hemispheres, and even within the tropics; the magnetic needle
lost all trace of continuity in its movements, and darted to and fro
as if stricken with inexplicable panic. The coincidence was drawn
even closer. At the very instant[476] of the solar outburst witnessed by
Carrington and Hodgson, the photographic apparatus at Kew
registered a marked disturbance of all the three magnetic elements;
while, shortly after the ensuing midnight, the electric agitation
culminated, thrilling the earth with subtle vibrations, and lighting
up the atmosphere from pole to pole with the coruscating splendours
which, perhaps, dimly recall the times when our ancient planet itself
shone as a star.
Here then, at least, the sun was—in Professor Balfour Stewart’s
phrase—”taken in the act”[477] of stirring up terrestrial commotions.
Nor have instances since been wanting of an indubitable connection
between outbreaks of individual spots and magnetic disturbances.
Four such were registered in 1882; and symptoms of the same kind,
including the beautiful “Rose Aurora,” marked the progress across[Pg 161]
the sun of the enormous spot-group of February, 1892—the largest
ever recorded at Greenwich. This extraordinary formation, which
covered about 1/300 of the sun’s disc, survived through five complete
rotations.[478] It was remarkable for a persistent drift in latitude, its place
altering progressively from 17° to 30° south of the solar equator.
Again, the central passage of an enormous spot on September 9,
1898, synchronised with a sharp magnetic disturbance and brilliant
aurora;[479] and the coincidence was substantially repeated in March,
1899,[480] when it was emphasised by the prevalent cosmic calm. The
theory of the connection is indeed far from clear. Lord Kelvin, in
1892,[481] pronounced against the possibility of any direct magnetic
action by the sun upon the earth, on the ground of its involving an
extravagant output of energy; but the fact is unquestionable that—in
Professor Bigelow’s words—”abnormal agitations affect the sun
and the earth as a whole and at the same time.”[482]
The nearer approach to the event of September 1, 1859, was
photographically observed by Professor George E. Hale at Chicago,
July 15, 1892.[483] An active spot in the southern hemisphere was the
scene of this curiously sudden manifestation. During an interval of
12m. between two successive exposures, a bridge of dazzling light
was found to have spanned the boundary-line dividing the twin-nuclei
of the spot; and these, after another 27m., were themselves
almost obliterated by an overflow of far-spreading brilliancy. Yet
two hours later, no trace of the outburst remained, the spot and its
attendant faculæ remaining just as they had been previously to its
occurrence. Unlike that seen by Carrington, it was accompanied
by no exceptional magnetic phenomena, although a “storm” set in next
day.[484] Possibly a terrestrial analogue to the former might be discovered
in the “auroral beam” which traversed the heavens during
a vivid display of polar lights, November 17, 1882, and shared,
there is every reason to believe, their electrical origin and character.[485]
Meantime M. Rudolf Wolf, transferred to the direction of the
Zürich Observatory, where he died, December 6, 1893, had relaxed
none of his zeal in the investigation of sun-spot periodicity. A
laborious revision of the entire subject with the aid of fresh[Pg 162]
materials led him, in 1859,[486] to the conclusion that while the mean
period differed little from that arrived at in 1852 of 11.11 years,
very considerable fluctuations on either side of that mean were
rather the rule than the exception. Indeed, the phrase “sun-spot
period” must be understood as fitting very loosely the great fact it
is taken to represent; so loosely, that the interval between two
maxima may rise to sixteen and a half or sink below seven and a
half years.[487] In 1861[488] Wolf showed, and the remark was fully confirmed
at Kew, that the shortest periods brought the most acute
crises, and vice versâ; as if for each wave of disturbance a strictly equal
amount of energy were available, which might spend itself lavishly
and rapidly, or slowly and parsimoniously, but could in no case be
exceeded. The further inclusion of recurring solar commotions
within a cycle of fifty-five and a half years was simultaneously pointed
out; and Hermann Fritz showed soon afterwards that the aurora
borealis is subject to an identical double periodicity.[489] The same
inquirer has more recently detected both for auroræ and sun-spots
a “secular period” of 222 years,[490] and the Kew observations indicate
for the latter, oscillations accomplished within twenty-six and
twenty-four days,[491] depending, most likely, upon the rotation of the
sun. This is certainly reflected in magnetic, and perhaps in auroral
periodicity. The more closely, in fact, spot-fluctuations are looked
into, the more complex they prove. Maxima of one order are superposed
upon, or in part neutralised by, maxima of another order;[492]
originating causes are masked by modifying causes; the larger waves
of the commotion are indented with minor undulations, and these
again crisped with tiny ripples, while the whole rises and falls with
the swell of the great secular wave, scarcely perceptible in its
progress because so vast in scale.
The idea that solar maculation depends in some way upon the[Pg 163]
position of the planets occurred to Galileo in 1612.[493] It has been
industriously sifted by a whole bevy of modern solar physicists.
Wolf in 1859[494] found reason to believe that the eleven-year curve is
determined by the action of Jupiter, modified by that of Saturn, and
diversified by influences proceeding from the earth and Venus. Its
tempting approach to agreement with Jupiter’s period of revolution
round the sun, indeed, irresistibly suggested a causal connection;
yet it does not seem that the most skilful “coaxing” of figures can
bring about a fundamental harmony. Carrington pointed out in
1863, that while, during eight successive periods, from 1770 downwards,
there were approximate coincidences between Jupiter’s
aphelion passages and sun-spot maxima, the relation had been
almost exactly reversed in the two periods preceding that date;[495] and
Wolf himself finally concluded that the Jovian origin must be
abandoned.[496] M. Duponchel’s[497] prediction, nevertheless, of an abnormal
retardation of the maximum due in 1881 through certain peculiarities
in the positions of Uranus and Neptune about the time it fell
due, was partially verified by the event, since, after an abortive
phase of agitation in April, 1882, the final outburst was postponed
to January, 1894. The interval was thus 13.5 instead of 11.1 years;
and it is noticeable that the delay affected chiefly the southern
hemisphere. Alternations of activity in the solar hemispheres were
indeed a marked feature of the maximum of 1884, which, in
M. Faye’s view,[498] derived thence its indecisive character, while
sharp, strong crises arise with the simultaneous advance of agitation
north and south of the solar equator. The curve of magnetic
disturbance followed with its usual strict fidelity the anomalous
fluctuations of the sun-spot curve. The ensuing minimum occurred
early in 1889, and was succeeded in 1894 by a maximum slightly
less feeble than its predecessor.[499]
It cannot be said that much progress has been made towards
the disclosure of the cause, or causes, of the sun-spot cycle. No
external influence adequate to the effect has, at any rate, yet been
pointed out. Most thinkers on this difficult subject provide a
quasi-explanation of the periodicity in question through certain
assumed vicissitudes affecting internal processes;[500] Sir Norman
Lockyer and E. von Oppolzer reach the same end by establishing
self-compensatory fluctuations in the solar atmospheric circulation;[Pg 164]
Dr. Schuster resorts to changes in the electrical conductivity of
space near the sun.[501] In all these theories, however, the course of
transition is arbitrarily arranged to suit a period, which imposes
itself as a fact peremptorily claiming admittance, while obstinately
defying explanation.
The question so much discussed, as to the influence of sun-spots
on weather, does not admit of a satisfactory answer. The facts
of meteorology are too complex for easy or certain classification.
Effects owning dependence on one cause often wear the livery of
another; the meaning of observed particulars may be inverted by
situation; and yet it is only by the collection and collocation of
particulars that we can hope to reach any general law. There is,
however, a good deal of evidence to support the opinion—the
grounds for which were primarily derived from the labours of
Dr. Meldrum at Mauritius—that increased rainfall and atmospheric
agitation attend spot-maxima; while Herschel’s conjecture of a more
copious emission of light and heat about the same epochs has
recently obtained some countenance from Savélieff’s measures
showing a gain in the strength of the sun’s radiation pari passu
with increase in the number of spots visible on his surface.[502]
The examination of what we may call the texture of the sun’s
surface derived new interest from a remarkable announcement made
by Mr. James Nasmyth in 1862.[503] He had made (as he supposed) the
discovery that the entire luminous stratum of the sun is composed
of a multitude of elongated shining objects on a darker background,
shaped much like willow-leaves, of vast size, crossing each other in
all possible directions, and possessed of unceasing relative motions.
A lively controversy ensued. In England and abroad the most
powerful telescopes were directed to a scrutiny encompassed with
varied difficulties. Mr. Dawes was especially emphatic in declaring
that Nasmyth’s “willow-leaves” were nothing more than the
“nodules” of Sir William Herschel seen under a misleading aspect
of uniformity; and there is little doubt that he was right. It is,
nevertheless, admitted that something of the kind may be seen in
the penumbræ and “bridges” of spots, presenting an appearance
compared by Dawes himself in 1852 to that of a piece of coarse
straw-thatching left untrimmed at the edges.[504]
The term “granulated,” suggested by Dawes in 1864,[505] best
describes the mottled aspect of the solar disc as shown by modern
telescopes and cameras. The grains, or rather the “floccules,”[Pg 165]
with which it is thickly strewn, have been resolved by Langley,
under exceptionally favourable conditions, into “granules” not
above 100 miles in diameter; and from these relatively minute
elements, composing, jointly, about one-fifth of the visible photosphere,[506]
he estimates that three-quarters of the entire light of the
sun are derived.[507] Janssen agrees, so far as to say that if the whole
surface were as bright as its brightest parts, its luminous emission
would be ten to twenty times greater than it actually is.[508]
The rapid changes in the forms of these solar cloud-summits are
beautifully shown in the marvellous photographs taken by Janssen
at Meudon, with exposures reduced at times to 1/100000 of a second!
By their means, also, the curious phenomenon known as the réseau
photosphérique has been made evident.[509] This consists in the diffusion
over the entire disc of fleeting blurred patches, separated by a
reticulation of sharply-outlined and regularly-arranged granules.
The imperfect definition in the smudged areas may be due to
agitations in the solar or terrestrial atmosphere, unless it be—as
Dr. Schemer thinks possible[510]—merely a photographic effect.
M. Janssen considers that the photospheric cloudlets change their
shape and character with the progress of the sun-spot period;[511] but
this is as yet uncertain.
The “grains,” or more brilliant parts of the photosphere, are
now generally held to represent the upper termination of ascending
and condensing currents, while the darker interstices (Herschel’s
“pores”) mark the positions of descending cooler ones. In the
penumbræ of spots, the glowing streams rushing up from the
tremendous sub-solar furnace are bent sideways by the powerful
indraught, so as to change their vertical for a nearly horizontal
motion, and are thus taken, as it were, in flank by the eye, instead
of being seen end-on in mamelon-form. This gives a plausible
explanation of the channelled structure of penumbræ which suggested
the comparison to a rude thatch. Accepting this theory as
in the main correct, we perceive that the very same circulatory
process which, in its spasms of activity, gives rise to spots, produces
in its regular course the singular “marbled” appearance, for the
recording of which we are no longer at the mercy of the fugitive
or delusive impressions of the human retina. And precisely this
circulatory process it is which gives to our great luminary its permance
as a sun, or warming and illuminating body.
FOOTNOTES:
[405] Mem. R. A. S., vol. xxi., p. 157.
[406] Ibid., p. 160.
[407] Month. Not., vol. xxi., p. 144.
[408] Le Soleil, t. i., pp. 87-90 (2nd ed., 1871).
[409] See ante, p. 58.
[410] Observations at Redhill (1863), Introduction.
[411] Month. Not., vol. xxxvi., p. 142.
[412] Cape Observations, p. 435, note.
[413] Month. Not., vol. x., p. 158.
[414] Rosa Ursina, lib. iii., p. 348.
[415] Observations at Redhill, p. 8.
[416] Op., t. iii., p. 402.
[417] Rosa Ursina, lib. iv., p. 601. Both Galileo and Scheiner spoke of the
apparent or “synodical” period, which is about one and a third days longer than
the true or “sidereal” one. The difference is caused by the revolution of the
earth in its orbit in the same direction with the sun’s rotation on its axis.
[418] Rosa Ursina, lib. iii., p. 260.
[419] Faye, Comptes Rendus, t. lx., p. 818.
[420] Ibid., t. xii., p. 648.
[421] Proc. Am. Ass. Adv. of Science, 1885, p. 85.
[422] Observations at Redhill, p. 221.
[423] Am. Jour. of Science, vol. xi., p. 169.
[424] Month. Not., vol. xix., p. 1.
[425] Vierteljahrsschrift der Naturfors. Gesellschaft (Zürich), 1859, p. 252.
[426] Lockyer, Chemistry of the Sun, p. 428.
[427] Maunder, Knowledge, vol. xv., p. 130.
[428] Month. Mon., vol. l., p. 251.
[429] Maunder, Knowledge, vol. xvii., p. 173.
[430] Astr. Nach., No. 1,315.
[431] As late as 1866 an elaborate treatise in its support was written by F. Coyteux,
entitled Qu’est-ce que le Soleil? Peut-il être habité? and answering the
question in the affirmative.
[432] The subsequent researches of Plücker, Frankland, Wüllner, and others,
showed that gases strongly compressed give an absolutely unbroken spectrum.
[433] Comptes Rendus, t. lx., pp. 89, 138.
[434] Ibid., t. c., p. 595.
[435] Bull. Meteor. dell Osservatorio dell Coll. Rom., Jan. 1, 1864, p. 4.
[436] Quart. Jour. of Science, vol. i., p. 222.
[437] Ann. de Chim. et de Phys., t. xxii., p. 127.
[438] Phil. Trans., vol. clix., p. 575.
[439] Les Mondes, Dec. 22, 1864, p. 707.
[440] Comptes Rendus, t. lx., p. 147.
[441] Proc. Roy. Society, vol. xvi., p. 29.
[442] Recherches sur le Spectre Solaire, p. 38.
[443] Am. Jour. of Science, 1881, vol. xxi., p. 41. Hastings stipulated only for
some member of the triad, carbon, silicon, and boron.
[444] Ranyard, Knowledge, vol. xvi., p. 190.
[445] Young, The Sun, p. 337, ed. 1897.
[446] H. Draper, Quart. Journ. of Sc., vol. i., p. 381; also Phil. Mag., vol. xvii.,
1840, p. 222.
[447] Reproduced in Arago’s Popular Astronomy, plate xii., vol. 1.
[448] Report Brit. Ass., 1859, p. 148.
[449] Phil. Trans., vol. clii., p. 407.
[450] Researches in Solar Physics, part i., p. 20.
[451] Both the phrase and the method were suggested by Faye, who estimated the
average depth of the luminous sheath of spots at 2,160 miles. Comptes Rendus,
t. lxi., p. 1082; t. xcvi., p. 356.
[452] Month. Not., vol. lv., p. 74.
[453] Sidgreaves, Ibid., p. 282; Cortie, Ibid., vol. lviii., p. 91.
[454] Explained by East as refraction-effects. Jour. Brit. Astr. Ass., vol. viii.,
p. 187.
[455] Proc. Roy. Soc., vol. xiv., p. 39.
[456] Potsdam Publicationen, No. 18; Astr. Nach., Nos. 3,000, 3,287.
[457] Faye, Comptes Rendus, t. cxi., p. 77; Bélopolsky, Astr. Nach., No. 2,991.
[458] Ibid., Nos. 3,275, 3,344.
[459] Lockyer, Contributions to Solar Physics, p. 70.
[460] Le Soleil, p. 87.
[461] Proc. Roy. Soc., vol. xv., p. 256.
[462] Phil. Mag., vol. xvi., p. 460.
[463] Recherches sur la Rotation du Soleil, p. 12.
[464] Hale, Astr. and Astrophysics, vol. xi., p. 814.
[465] Jour. Brit. Astr. Ass., vol. i., p. 177.
[466] Comptes Rendus, t. lxxv., p. 1664; Revue Scientifique, t. v., p. 359 (1883).
Mr. Herbert Spencer had already (in The Reader, Feb. 25, 1865) put forward an
opinion that spots were of the nature of “cyclonic clouds.”
[467] The Sun, p. 174. For Faye’s answer to the objection, see Comptes Rendus,
t. xcv., p. 1310.
[468] A revised edition appeared in 1897.
[469] Astr. and Astrophysics, vol. xii., p. 832.
[470] Proc. Roy. Soc., No. 244.
[471] Astr. Nach., No. 3,146; Astr. and Astrophysics, vol. xii., pp. 419, 736.
[472] Sirius, Sept., 1893; ibid., vol. xxiii., p. 97; Astrophy. Jour., vol. i., p. 112
(Wilczynski), p. 178 (Keeler); vol. ii., p. 73 (Hale).
[473] Month. Not., vol. xx., p. 13.
[474] Ibid., p. 15.
[475] Am. Jour., vol. xxix. (2nd series), pp. 94, 95.
[476] The magnetic disturbance took place at 11.15 A.M., three minutes before the
solar blaze compelled the attention of Carrington.
[477] Phil. Trans., vol. cli., p. 428.
[478] Maunder, Journal Brit. Astr. Ass., vol. ii., p. 386; Miss E. Brown, Ibid.,
p. 210; Month. Not., vol. lii., p. 354.
[479] Observatory, vol. xxi., p. 387; Maunder, Knowledge, vol. xxi., p. 228; Fényi,
Astroph. Jour., vol. x., p. 333.
[480] Ibid., p. 336; W. Anderson, Observatory, vol. xxii., p. 196.
[481] Proc. Roy. Society, vol. lii., p. 307; Rev. W. Sidgreaves, Mem. R. A. S.,
vol. liv., p. 85.
[482] Report on Solar and Terrestrial Magnetism, Washington, 1898, p. 27.
[483] Astr. and Astrophysics, vol. xi., p. 611.
[484] Ibid., p. 819 (Sidgreaves).
[485] See J. Rand Capron, Phil. Mag., vol. xv., p. 318.
[486] Mittheilungen über die Sonnenflecken, No. ix., Vierteljahrsschrift der Naturforschenden
Gesellschaft in Zürich, Jahrgang 4.
[487] Mitth., No. lii., p. 58 (1881).
[488] Ibid., No. xii., p. 192. Baxendell, of Manchester, reached independently a
similar conclusion. See Month. Not., vol. xxi., p. 141.
[489] Wolf, Mitth., No. xv., p. 107, etc. Olmsted, following Hansteen, had already,
in 1856, sought to establish an auroral period of sixty-five years. Smithsonian
Contributions, vol. viii., p. 37.
[490] Hahn, Ueber die Reziehungen der Sonnenfleckenperiode zu meteorologischen
Erscheinungen, p. 99 (1877).
[491] Report Brit. Ass., 1881, p. 518; 1883, p. 418.
[492] The Rev. A. Cortie (Month. Not., vol. lx., p. 538) detects the influence of a
short subsidiary cycle, Dr. W. J. S. Lockyer that of a thirty-five year period
(Nature, June 20, 1901). Professor Newcomb (Astroph. Jour., vol. xiii., p. 11)
considers that solar activity oscillates uniformly in 11.13 years, with superposed
periodic variations.
[493] Opere, t. iii., p. 412.
[494] Mitth., Nos. vii. and xviii.
[495] Observations at Redhill, p. 248.
[496] Comptes Rendus, t. xcv., p. 1249.
[497] Ibid., t. xciii., p. 827; t. xcvi., p. 1418.
[498] Ibid., t. c, p. 593.
[499] Ellis, Proc. Roy. Society, vol. lxiii., p. 70.
[500] Schultz, Astr. Nach., Nos. 2,817-18, 2,847-8; Wilsing, Ibid., No. 3,039;
Bélopolsky, Ibid., No. 2,722.
[501] Report Brit. Ass., 1892, p. 635.
[502] A. W. Augur, Astroph. Jour., vol. xiii., p. 346.
[503] Report Brit. Ass., 1862, p. 16 (pt. ii.).
[504] Mem. R. A. S., vol. xxi., p. 161.
[505] Month. Not., vol. xxiv., p. 162.
[506] Am. Jour. of Science, vol. vii., 1874, p. 92.
[507] Young, The Sun, p. 103.
[508] Ann. Bur. Long., 1879, p. 679.
[509] Ibid., 1878, p. 689.
[510] Himmelsphotographie, p. 273.
[511] Ranyard, Knowledge, vols. xiv., p. 14, xvi., p. 189; see also the accompanying
photographs.
CHAPTER III
RECENT SOLAR ECLIPSES
By observations made during a series of five remarkable eclipses,
comprised within a period of eleven years, knowledge of the solar
surroundings was advanced nearly to its present stage. Each of
these events brought with it a fresh disclosure of a definite and
unmistakable character. We will now briefly review this orderly
sequence of discovery.
Photography was first systematically applied to solve the problems
presented by the eclipsed sun, July 18, 1860. It is true that a
daguerreotype,[512] taken by Berkowski with the Königsberg heliometer
during the eclipse of 1851, is still valuable as a record of the corona
of that year; and some subsequent attempts were made to register
partial phases of solar occultation, notably by Professor Bartlett at
West Point in 1854;[513] but the ground remained practically unbroken
until 1860.
In that year the track of totality crossed Spain, and thither,
accordingly, Warren de la Rue transported his photo-heliograph,
and Father Secchi his six-inch Cauchoix refractor. The question
then primarily at issue was that relating to the nature of the red
protuberances. Although, as already stated, the evidence collected
in 1851 gave a reasonable certainty of their connection with the
sun, objectors were not silenced; and when the side of incredulity
was supported by so considerable an authority as M. Faye, it was
impossible to treat it with contempt. Two crucial tests were
available. If it could be shown that the fantastic shapes suspended
above the edge of the dark moon were seen under an identical
aspect from two distant stations, that fact alone would annihilate
the theory of optical illusion or “mirage”; while the certainty that
they were progressively concealed by the advancing moon on one
side, and uncovered on the other, would effectually detach them[Pg 167]
from dependence on our satellite, and establish them as solar
appendages.
Now both these tests were eminently capable of being applied by
photography. But the difficulty arose that nothing was known as
to the chemical power of the rosy prominence-light, while everything
depended on its right estimation. A shot had to be fired, as
it were, in the dark. It was a matter of some surprise, and of no
small congratulation, that, in both cases, the shot took effect.
De la Rue occupied a station at Rivabellosa, in the Upper Ebro
valley; Secchi set up his instrument at Desierto de las Palmas,
about 250 miles to the south-east, overlooking the Mediterranean.
From the totally eclipsed sun, with its strange garland of flames,
each observer derived several perfectly successful impressions, which
were found, on comparison, to agree in the most minute details.
This at once settled the fundamental question as to the substantial
reality of these objects; while their solar character was demonstrated
by the passage of the moon in front of them, indisputably
attested by pictures taken at successive stages of the eclipse. That
forms seeming to defy all laws of equilibrium were, nevertheless,
not wholly evanescent, appeared from their identity at an interval
of seven minutes, during which the lunar shadow was in transit
from one station to the other; and the singular energy of their
actinic rays was shown by the record on the sensitive plates of
some prominences invisible in the telescope. Moreover, photographic
evidence strongly confirmed the inference—previously
drawn by Grant and others, and now with fuller assurance by
Secchi—that an uninterrupted stratum of prominence-matter encompasses
the sun on all sides, forming a reservoir from which gigantic
jets issue, and into which they subside.
Thus, first-fruits of accurate knowledge regarding the solar surroundings
were gathered, while the value of the brief moments of
eclipse gained indefinite increase, by supplementing transient visual
impressions with the faithful and lasting records of the camera.
In the year 1868 the history of eclipse spectroscopy virtually
began, as that of eclipse photography in 1860; that is to say, the
respective methods then first gave definite results. On the 18th of
August, 1868, the Indian and Malayan peninsulas were traversed by
a lunar shadow producing total obscuration during five minutes and
thirty-eight seconds. Two English and two French expeditions
were despatched to the distant regions favoured by an event so
propitious to the advance of knowledge, chiefly to obtain the
verdict of the prism as to the composition of prominences. Nor
were they despatched in vain. An identical discovery was made
by nearly all the observers. At Jamkandi, in the Western Ghauts,[Pg 168]
where Lieutenant (now Colonel) Herschel was posted, unremitting
bad weather threatened to baffle his eager expectations; but during
the lapse of the critical five and a half minutes the clouds broke, and
across the driving wrack a “long, finger-like projection” jutted
out over the margin of the dark lunar globe. In another moment
the spectroscope was pointed towards it; three bright lines—red,
orange, and blue—flashed out, and the problem was solved.[514] The
problem was solved in this general sense, that the composition out
of glowing vapours of the objects infelicitously termed “protuberances”
or “prominences” was no longer doubtful; although further
inquiry was needed for the determination of the particular species
to which those vapours belonged.
Similar, but more complete observations were made, with less
atmospheric hindrance, by Tennant and Janssen at Guntoor, by
Pogson at Masulipatam, and by Rayet at Wha-Tonne, on the
coast of the Malay peninsula, the last observer counting as many as
nine bright lines.[515] Among them it was not difficult to recognise
the characteristic light of hydrogen; and it was generally, though
over-hastily, assumed that the orange ray matched the luminous
emissions of sodium. But fuller opportunities were at hand.
The eclipse of 1868 is chiefly memorable for having taught
astronomers to do without eclipses, so far, at least, as one particular
branch of solar inquiry is concerned. Inspired by the beauty and
brilliancy of the variously tinted prominence-lines revealed to him
by the spectroscope, Janssen exclaimed to those about him, “Je
verrai ces lignes-là en dehors des éclipses!” On the following
morning he carried into execution the plan which formed itself in
his brain while the phenomenon which suggested it was still before
his eyes. It rests upon an easily intelligible principle.
The glare of our own atmosphere alone hides the appendages of
the sun from our daily view. To a spectator on an airless planet,
the central globe would appear attended by all its splendid retinue
of crimson prominences, silvery corona, and far-spreading zodiacal
light projected on the star-spangled black background of an
absolutely unilluminated sky. Now the spectroscope offers the
means of indefinitely weakening atmospheric glare by diffusing a
constant amount of it over an area widened ad libitum. But monochromatic
or “bright-line” light is, by its nature, incapable of
being so diffused. It can, of course, be deviated by refraction to
any extent desired; but it always remains equally concentrated, in
whatever direction it may be thrown. Hence, when it is mixed up
with continuous light—as in the case of the solar flames shining
through our atmosphere—it derives a relative gain in intensity from[Pg 169]
every addition to the dispersive power of the spectroscope with
which the heterogeneous mass of beams is analysed. Employ
prisms enough, and eventually the undiminished rays of persistent
colour will stand out from the continually fading rainbow-tinted
band, by which they were at first effectually veiled.
This Janssen saw by a flash of intuition while the eclipse was
in progress; and this he realised at 10 A.M. next morning,
August 19, 1868—the date of the beginning of spectroscopic work
at the margin of the unobscured sun. During the whole of that
day and many subsequent ones, he enjoyed, as he said, the advantage
of a prolonged eclipse. The intense interest with which he
surveyed the region suddenly laid bare to his scrutiny was
heightened by evidences of rapid and violent change. On the
18th of August, during the eclipse, a vast spiral structure, at least
89,000 miles high, was perceived, planted in surprising splendour
on the rim of the interposed moon. If was formed as General
Tennant judged from its appearance in his photographs, by the encounter
of two mounting torrents of flame, and was distinguished
as the “Great Horn.” Next day it was in ruins; hardly a trace
remained to show where it had been.[516] Janssen’s spectroscope
furnished him besides with the strongest confirmation of what had
already been reported by the telescope and the camera as to the
continuous nature of the scarlet “sierra” lying at the base of the
prominences. Everywhere at the sun’s edge the same bright lines
appeared.
It was not until the 19th of September that Janssen thought fit
to send news of his discovery to Europe. It seemed little likely to
be anticipated; yet a few minutes before his despatch was handed
to the Secretary of the Paris Academy of Sciences, a communication
similar in purport had been received from Sir Norman Lockyer.
There is no need to discuss the narrow and wearisome question of
priority; each of the competitors deserves, and has obtained, full
credit for his invention. With noteworthy and confident prescience,
Lockyer, in 1866, before anything was yet known regarding the
constitution of the “red flames,” ordered a strongly dispersive
spectroscope for the express purpose of viewing, apart from eclipses,
the bright-line spectrum which he expected them to give. Various
delays, however, supervened, and the instrument was not in his hands
until October 16, 1868. On the 20th he picked up the vivid rays,
of which the presence and (approximately) the positions had in the
interim become known. But there is little doubt that, even without
that previous knowledge, they would have been found; and that the
eclipse of August 18 only accelerated a discovery already assured.
Sir William Huggins, meanwhile, had been tending towards the
same goal during two and a half years in his observatory at Tulse
Hill. The principle of the spectroscopic visibility of prominence-lines
at the edge of an uneclipsed sun was quite explicitly stated
by him in February, 1868,[517] and he devised various apparatus for
bringing them into actual view; but not until he knew where to
look did he succeed in seeing them.
Astronomers, thus liberated, by the acquisition of power to
survey them at any time, from the necessity of studying prominences
during eclipses, were able to concentrate the whole of their attention
on the corona. The first thing to be done was to ascertain the
character of its spectrum. This was seen in 1868 only as a faintly
continuous one; for Rayet, who seems to have perceived its distinctive
bright line far above the summits of the flames, connected
it, nevertheless, with those objects. On the other hand, Lieutenant
Campbell ascertained on the same occasion the polarisation of the
coronal light in planes passing through the sun’s centre,[518] thereby
showing that light to be, in whole or in part, reflected sunshine.
But if reflected sunshine, it was objected, the chief at least of the
dark Fraunhofer lines should be visible in it, as they are visible in
moonbeams, sky illumination, and all other sun-derived light. The
objection was well founded, but was prematurely urged, as we
shall see.
On the 7th of August, 1869, a track of total eclipse crossed
the continent of North America diagonally, entering at Behring’s
Straits, and issuing on the coast of North Carolina. It was beset
with observers; but the most effective work was done in Iowa. At
Des Moines, Professor Harkness of the Naval Observatory, Washington,
obtained from the corona an “absolutely continuous spectrum,”
slightly less bright than that of the full moon, but traversed by a
single green ray.[519] The same green ray was seen at Burlington and
its position measured by Professor Young of Dartmouth College.[520]
It appeared to coincide with that of a dark line of iron in the solar
spectrum, numbered 1,474 on Kirchhoff’s scale. But in 1876 Young
was able, by the use of greatly increased dispersion, to resolve the
Fraunhofer line “1474′ into a pair, the more refrangible member
of which he considered to be the reversal of the green coronal ray.[521]
Scarcely called in question for over twenty years, the identification
nevertheless broke down through the testimony of the eclipse-photographs
of 1898. Sir Norman Lockyer derived from them a[Pg 171]
position for the line in question notably higher up in the spectrum
than that previously assigned to it. Instead of 5,317, its true
wave-length proved to be 5,303 ten millionths of a millimetre;[522] nor
does it make any show by absorption in dispersed sunlight. The
originating substance, designated “coronium,” of which nothing is
known to terrestrial chemistry, continues luminous[523] at least 300,000
miles above the sun’s surface, and is hence presumably much lighter
even than hydrogen.
A further trophy was carried off by American skill[524] sixteen
months after the determination due to it of the distinctive spectrum
of the corona. The eclipse of December 22, 1870, though lasting
only two minutes and ten seconds, drew observers from the New, as
well as from the Old World to the shores of the Mediterranean.
Janssen issued from beleaguered Paris in a balloon, carrying with
him the vital parts of a reflector specially constructed to collect
evidence about the corona. But he reached Oran only to find himself
shut behind a cloud-curtain more impervious than the Prussian
lines. Everywhere the sky was more or less overcast. Lockyer’s
journey from England to Sicily, and shipwreck in the Psyche, were
recompensed with a glimpse of the solar aureola during one second
and a half! Three parties stationed at various heights on Mount
Etna saw absolutely nothing. Nevertheless important information
was snatched in despite of the elements.
The prominent event was Young’s discovery of the “reversing
layer.” As the surviving solar crescent narrowed before the
encroaching moon, “the dark lines of the spectrum,” he tells us,
“and the spectrum itself, gradually faded away, until all at once, as
suddenly as a bursting rocket shoots out its stars, the whole field of
view was filled with bright lines more numerous than one could
count. The phenomenon was so sudden, so unexpected, and so
wonderfully beautiful, as to force an involuntary exclamation.”[525] Its
duration was about two seconds, and the impression produced was
that of a complete reversal of the Fraunhofer spectrum—that is, the
substitution of a bright for every dark line.
Now something of the kind was theoretically necessary to account
for the dusky rays in sunlight which have taught us so much, and
have yet much more to teach us; so that, although surprising from
its transitory splendour, the appearance could not strictly be called
“unexpected.” Moreover, its premonitory symptom in the fading
out of these rays had been actually described by Secchi in 1868,[526][Pg 172]
and looked for by Young as the moon covered the sun in August
1869. But with the slit of his spectroscope placed normally to the
sun’s limb, the bright lines gave a flash too thin to catch the eye.
In 1870 the position of the slit was tangential—it ran along the
shallow bed of incandescent vapours, instead of cutting across it:
hence his success.
The same observation was made at Xerez de la Frontera by
Mr. Pye, a member of Young’s party; and, although an exceedingly
delicate one, has since frequently been repeated. The whole Fraunhofer
series appeared bright (omitting other instances) to Maclear,
Herschel, and Fyers in 1871, at the beginning or end of totality;
to Pogson, at the break-up of an annual eclipse, June 6, 1872; to
Stone at Klipfontein, April 16, 1874, when he saw “the field full of
bright lines.”[527] But between the picture presented by the “véritable
pluie de lignes brilliantes,”[528] which descended into M. Trépied’s
spectroscope for three seconds after the disappearance of the sun,
May 17, 1882, and the familiar one of the dark-line solar spectrum,
certain differences were perceiving, showing their relation to be not
simply that of a positive to a negative impression.
A “reversing layer,” or stratum of mixed vapours, glowing, but
at a lower temperature than that of the actual solar surface, was an
integral part of Kirchhoff’s theory of the production of the Fraunhofer
lines. Here it was assumed that the missing rays were
stopped, and here also it was assumed that the missing rays would
be seen bright, could they be isolated from the overpowering
splendour of their background. This isolation is effected by
eclipses, with the result—beautifully confirmatory of theory—of
reversing, or turning from dark to bright, the Fraunhofer spectrum.
The completeness and precision of the reversal, however, could not
be visually attested; and a quarter of a century elapsed before a
successful “snap-shot” provided photographic evidence on the
subject. It was taken at Novaya Zemlya by Mr. Shackleton, a
member of the late Sir George Baden-Powell’s expedition to observe
the eclipse of August 9, 1896;[529] and similar records in abundance
were secured during the Indian eclipse of January 22, 1898,[530] and
the Spanish-American eclipse of May 28, 1900.[531] The result of
their leisurely examination has been to verify the existence of a
“reversing-layer,” in the literal sense of the term. It is true that
no single “flash” photograph is an inverted transcript of the[Pg 173]
Fraunhofer spectrum. The lines are, indeed, in each case—speaking
broadly—the same; but their relative intensities are widely different.
Yet this need occasion no surprise when we remember that the
Fraunhofer spectrum integrates the absorption of multitudinous
strata, various in density and composition, while only the upper
section of the formation comes within view of the sensitive plates
exposed at totalities, the low-lying vaporous beds being necessarily
covered by the moon. The total depth of this glowing envelope
may be estimated at 500 to 600 miles, and its normal state seems to
be one of profound tranquillity, judging from the imperturbable
aspect of the array of dark lines due to its sifting action upon light.
The last of the five eclipses which we have grouped together for
separate consideration was visible in Southern India and Australia,
December 12, 1871. Some splendid photographs were secured by
the English parties on the Malabar coast, showing, for the first time,
the remarkable branching forms of the coronal emanations; but the
most conspicuous result was Janssen’s detection of some of the dark
Fraunhofer lines, long vainly sought in the continuous spectrum
of the corona. Chief among these was the D-line of sodium, the
original index, it might be said, to solar chemistry. No proof could
be afforded more decisive that this faint echoing back of the
distinctive notes of the Fraunhofer spectrum, that the polariscope
had spoken the truth in asserting a large part of the coronal
radiance to be reflected sunlight. But it is usually so drenched in
original luminosity, that its special features are almost obliterated.
Janssen’s success in seizing them was due in part to the extreme
purity of the air at Sholoor, in the Neilgherries, where he was
stationed; in part to the use of an instrument adapted by its large
aperture and short focus to give an image of the utmost brilliancy.
His observation, repeated during the Caroline Island eclipse of
1883, was photographically verified ten years later by M. de la
Baume Pluvinel in Senegal.[532]
An instrument of great value for particular purposes was introduced
into eclipse-work in 1871. The “slitless spectroscope”
consists simply of a prism placed outside the object-glass of
a telescope or the lens of a camera, whereby the radiance encompassing
the eclipsed sun is separated into as many differently
tinted rings as it contains different kinds of light. These tinted
rings were simultaneously viewed by Respighi at Poodacottah, and
by Lockyer at Baikul. Their photographic registration by the
latter in 1875 initiated the transformation of the slitless spectroscope
into the prismatic camera.[533] Meanwhile, the use of an ordinary[Pg 174]
spectroscope by Herschel and Tennant at Dodabetta showed the
green ray of coronium to be just as bright in a rift as in the adjacent
streamer. The visible structure of the corona was thus seen to be
independent of the distribution of the gases which enter into its
composition.
By means, then, of the five great eclipses of 1860-71 it was
ascertained: first, that the prominences, and at least the lower
part of the corona, are genuine solar appurtenances; secondly,
that the prominences are composed of hydrogen and other
gases in a state of incandescence, and rise, as irregular outliers,
from a continuous envelope of the same materials, some
thousands of miles in thickness; thirdly, that the corona is of a
highly complex constitution, being made up in part of glowing
vapours, in part of matter capable of reflecting sunlight. We may
now proceed to consider the results of subsequent eclipses.
These have raised, and have helped to solve, some very curious
questions. Indeed, every carefully watched total eclipse of the
sun stimulates as well as appeases curiosity, and leaves a legacy
of outstanding doubt, continually, as time and inquiry go on,
removed, but continually replaced. It cannot be denied that the
corona is a perplexing phenomenon, and that it does not become
less perplexing as we know more about it. It presented itself
under quite a new and strange aspect on the occasion of the
eclipse which visited the Western States of North America, July 29,
1878. The conditions of observation were peculiarly favourable.
The weather was superb; above the Rocky Mountains the sky
was of such purity as to permit the detection of Jupiter’s satellites
with the naked eye on several successive nights. The opportunity
for advancing knowledge was made the most of. Nearly
a hundred astronomers, including several Englishmen, occupied
twelve separate posts, and prepared for an attack in force.
The question had often suggested itself, and was a natural one
to ask, whether the corona sympathises with the general condition
of the sun? whether, either in shape or brilliancy, it varies
with the progress of the sun-spot period? A more propitious
moment for getting this question answered could hardly have
been chosen than that at which the eclipse occurred. Solar disturbance
was just then at its lowest ebb. The development of
spots for the month of July, 1878, was represented on Wolf’s
system of “relative numbers” by the fraction 0·1, as against
135·4 for December, 1870, an epoch of maximum activity. The
“chromosphere”[534] was, for the most part, shallow and quiescent;[Pg 175]
its depth, above the spot zones, had sunk from about 6,000 to 2,000
miles;[535] prominences were few and faint. Obviously, if a type of
corona corresponding to a minimum of sun-spots existed, it should
be seen then or never. It was seen; but while, in some respects, it
agreed with anticipation, in others it completely set it at naught.
The corona of 1878, as compared with those of 1869, 1870, and
1871, was generally admitted to be shrunken in its main outlines
and much reduced in brilliancy. Lockyer pronounced it ten times
fainter than in 1871; Harkness estimated its light at less than
one-seventh that derived from the mist-blotted aureola of 1870.[536]
In shape, too, it was markedly different. When sun-spots are
numerous, the corona appears to be most fully developed above the
spot-zones, thus offering to our eyes a rudely quadrilateral contour.
The four great luminous sheaves forming the corners of the square
are made up of rays curving together from each side into
“synclinal” or ogival groups, each of which may be compared to
the petal of a flower. To Janssen, in 1871, the eclipsing moon
seemed like the dark heart of a gigantic dahlia, painted in light on
the sky; and the similitude to the ornament on a compass-card,
used by Airy in 1851, well conveys the decorative effect of the
beamy, radiated kind of aureola, never, it would appear, absent
when solar activity is at a tolerably high pitch. In his splendid
volume on eclipses,[537] with which the systematic study of coronal
structure may be said to have begun, Mr. Ranyard first generalised
the synclinal peculiarity by a comparison of records; but the
symmetry of the arrangement, though frequently striking, is liable
to be confused by secondary formations. He further pointed out,
with the help of careful drawings from the photographs of 1871
made by Mr. Wesley, the curved and branching shapes assumed by
the component filaments of massive bundles of rays. Nothing of
all this, however, was visible in 1878. Instead, there was seen, as
the groundwork of the corona, a ring of pearly light, nebulous to
the eye, but shown by telescopes and in photographs to have a
fibrous texture, as if made up of tufts of fine hairs. North and
south, a series of short, vivid, electrical-looking flame-brushes
diverged with conspicuous regularity from each of the solar poles.
Their direction was not towards the centre of the sun, but towards
each summit of his axis, so that the farther rays on either side
started almost tangentially to the surface.
But the leading, and a truly amazing, characteristic of the[Pg 176]
phenomenon was formed by two vast, faintly-luminous wings of
light, expanded on either side of the sun in the direction of the
ecliptic. These were missed by very few careful onlookers; but the
extent assigned to them varied with skill in, and facilities for seeing.
By far the most striking observations were made by Newcomb at
Separation (Wyoming), by Cleveland Abbe from the shoulder of
Pike’s Peak, and by Langley at its summit, an elevation of 14,100
feet above the sea. Never before had an eclipse been viewed
from anything approaching that altitude, or under so translucent
a sky. A proof of the great reduction in atmospheric glare
was afforded by the perceptibility of the corona four minutes after
totality was over. For the 165 seconds of its duration, the remarkable
streamers above alluded to continued “persistently visible,”
stretching away right and left of the sun to a distance of at least
ten million miles! One branch was traced over an apparent extent
of fully twelve lunar diameters, without sign of a definite termination
having been reached; and there were no grounds for supposing
the other more restricted.
The resemblance to the zodiacal light was striking; and a community
of origin between that enigmatical member of our system
and the corona was irresistibly suggested. We should, indeed,
expect to see, under such exceptionally favourable atmospheric
conditions as Professor Langley enjoyed on Pike’s Peak, the roots of
the zodiacal light presenting near the sun just such an appearance
as he witnessed; but we can imagine no reason why their visibility
should be associated with a low state of solar activity. Nevertheless
this seems to be the case with the streamers which astonished
astronomers in 1878. For in August, 1867, when similar equatorial
emanations, accompanied by similar symptoms of polar excitement,
were described and depicted by Grosch[538] of the Santiago Observatory,
sun-spots were at a minimum; while the corona of 1715, which
appears from the record of it by Roger Cotes[539] to have been of the
same type, preceded by three years the ensuing maximum. The
eclipsed sun was seen by him at Cambridge, May 2, 1715, encompassed
with a ring of light about one-sixth of the moon’s diameter
in breadth, upon which was superposed a luminous cross formed of
long bright branches lying very nearly in the plane of the ecliptic,
and shorter polar arms so faint as to be only intermittently visible.
The resemblance between his sketch and Cleveland Abbe’s drawing
of the corona of 1878 is extremely striking. It should, nevertheless,
be noted that some conspicuous spots were visible on the sun’s disc[Pg 177]
at the time of Cotes’s eclipse, and that the preceding minimum
(according to Wolf) occurred in 1712. Thus, the coincidence of
epochs is imperfect.
Professor Cleveland Abbe was fully persuaded that the long rays
carefully observed by him from Pike’s Peak were nothing else than
streams of meteorites rushing towards or from perihelion; and it is
quite certain that the solar neighbourhood must be crowded with
such bodies. But the peculiar structure at the base of the streamers
displayed in the photographs, the curved rays meeting in pointed
arches like Gothic windows, the visible upspringing tendency, the
filamentous texture,[540] speak unmistakably of the action of forces
proceeding from the sun, not of extraneous matter circling round
him.
A further proof of sympathetic change in the corona is afforded
by the analysis of its light. In 1878 the bright line so conspicuous
in the coronal spectrum in 1870 and 1871 had faded to the very
limit of visibility. Several skilled observers failed to see it at all;
but Young and Eastman succeeded in tracing the green “coronium”
ray all round the sun, to a height estimated at 340,000 miles. The
substance emitting it was thus present, though in a low state of
incandescence. The continuous spectrum was relatively strong;
faint traces of the Fraunhofer lines attested for it an origin, in part
by reflection; and polarisation was undoubted, increasing towards
the limb, whereas in 1870 it reached a maximum at a considerable
distance from it. Experiments with Edison’s tasimeter seemed to
show that the corona radiates a sensible amount of heat.
The next promising eclipse occurred May 17, 1882. The concourse
of astronomers which has become usual on such occasions
assembled this time at Sohag, in Upper Egypt. Rarely have
seventy-four seconds been turned to such account. To each
observer a special task was assigned, and the advantages of a strict
division of labour were visible in the variety and amount of the information
gained.
The year 1882 was one of numerous sun-spots. On the eve
of the eclipse twenty-three separate maculæ were counted. If there
were any truth in the theory which connected coronal forms with
fluctuations in solar activity, it might be anticipated that the vast
equatorial expansions and polar “brushes” of 1878 would be found
replaced by the star-like structure of 1871. This expectation was
literally fulfilled. No lateral streamers were to be seen. The
universal failure to perceive them, after express search in a sky of
the most transparent purity, justifies the emphatic assertion that
they were not there. Instead, the type of corona observed in India[Pg 178]
eleven years earlier, was reproduced with its shining aigrettes, complex
texture and brilliant radiated aspect.
Concordant testimony was given by the spectroscope. The
reflected light derived from the corona was weaker than in 1878,
while its original emissions were proportionately intensified. Nevertheless,
most of the bright lines recorded as coronal[541] were really
due, there can be no doubt, to diffused chromospheric light. On
this occasion, the first successful attempt was made to photograph
the coronal spectrum procured in the ordinary way with a slit and
prisms, while the prismatic camera was also profitably employed.
It served to bring out at least one important fact—that of the uncommon
strength in chromospheric regions of the twin violet beams
of calcium, designated “H” and “K”; and prominence-photography
signalised its improvement by the registration, in the spectrum of
one such object, of twenty-nine rays, including many of the ultra-violet
hydrogen series discovered by Sir William Huggins in the
emission of white stars.[542]
Dr. Schuster’s photographs of the corona itself were the most
extensive, as well as the most detailed, of any yet secured. One
rift imprinted itself on the plates to a distance of nearly a diameter
and a half from the limb; and the transparency of the streamers
was shown by the delineation through them of the delicate tracery
beyond. The singular and picturesque feature was added of a
bright comet, self-depicted in all the exquisite grace of swift movement
betrayed by the fine curve of its tail, hurrying away from one
of its rare visits to our sun, and rendered momentarily visible by the
withdrawal of the splendour in which it had been, and was again
quickly veiled.
From a careful study of these valuable records Sir William
Huggins derived the idea of a possible mode of photographing the
corona without an eclipse.[543] As already stated, its ordinary invisibility
is entirely due to the “glare” or reflected light diffused through
our atmosphere. But Huggins found, on examining Schuster’s
negatives, that a large proportion of the light in the coronal
spectrum, both continuous and interrupted, is collected in the violet
region between the Fraunhofer lines G and H. There, then, he
hoped that, all other rays being excluded, it might prove strong
enough to vanquish inimical glare, and stamp on prepared plates,
through local superiority in illuminative power, the forms of the
appendage by which it is emitted.
His experiments were begun towards the end of May, 1882, and
by September 28 he had obtained a fair earnest of success. The
exclusion of all other qualities of light save that with which he
desired to operate, was accomplished by using chloride of silver as
his sensitive material, that substance being chemically inert to all
other but those precise rays in which the corona has the advantage.[544]
Plates thus sensitised received impressions which it was hardly
possible to regard as spurious. “Not only the general features,”
Captain Abney affirmed,[545] “are the same, but details, such as rifts
and streamers, have the same position and form.” It was found,
moreover, that the corona photographed during the total eclipse
of May 6, 1883, was intermediate in shape between the coronas
photographed by Sir William Huggins before and after that event,
each picture taking its proper place in a series of progressive
modifications highly interesting in themselves, and full of promise
for the value of the method employed to record them.[546] But experiments
on the subject were singularly interrupted. The volcanic
explosion in the Straits of Sunda in August, 1883, brought to
astronomers a peculiarly unwelcome addition to their difficulties.
The magnificent sunglows due to the diffractive effects on light
of the vapours and fine dust flung in vast volumes into the air,
and rapidly diffused all round the globe, betokened an atmospheric
condition of all others the most prejudicial to delicate
researches in the solar vicinity. The filmy coronal forms, accordingly,
which had been hopefully traced on the Tulse Hill plates ceased
to appear there; nor were any substantially better results obtained
by Mr. C. Ray Woods, in the purer air either of the Riffel or the Cape
of Good Hope, during the three ensuing years. Moreover, attempts
to obtain coronal photographs during the partial phases of the eclipse
of August 29, 1886, completely failed. No part of the lunar globe
became visible in relief against circumfluous solar radiance on any of
the plates exposed at Grenada; and what vestiges of “structure”
there were, came out almost better upon the moon than beside her,
thus stamping themselves at once as of atmospheric origin.
That the effect sought is a perfectly possible one is proved by the
distinct appearance of the moon projected on the corona, in photographs
of the partially eclipsed sun in 1858, 1889, and 1890, and
very notably in 1898 and 1900.[547]
In the spring of 1893, Professor Hale[548] attacked the problem of[Pg 180]
coronal daylight photography, employing the “double-slit” method
so eminently serviceable for the delineation of prominences.[549] But
neither at Kenwood nor at the summit of Pike’s Peak, whither,
in the course of the summer, he removed his apparatus, was any
action of the desired kind secured. Similar ill success attended his
and Professor Riccò’s employment, on Mount Etna in July, 1894,
of a specially designed coronagraph. Yet discouragement did not
induce despair. The end in view is indeed too important to be
readily abandoned; but it can be reached only when a more particular
acquaintance with the nature of coronal light than we now
possess indicates the appropriate device for giving it a preferential
advantage in self-portraiture. Moreover, the effectiveness of this
device may not improbably be enhanced, through changes in the
coronal spectrum at epochs of sun-spot maximum.
The prosperous result of the Sohag observations stimulated the
desire to repeat them on the first favourable opportunity. This
offered itself one year later, May 6, 1883, yet not without the
drawbacks incident to terrestrial conditions. The eclipse promised
was of rare length, giving no less than five minutes and twenty-three
seconds of total obscurity, but its path was almost exclusively
a “water-track.” It touched land only on the outskirts of the
Marquesas group in the Southern Pacific, and presented, as the one
available foothold for observers, a coral reef named Caroline Island,
seven and a half miles long by one and a half wide, unknown
previously to 1874, and visited only for the sake of its stores of
guano. Seldom has a more striking proof been given of the vividness
of human curiosity as to the condition of the worlds outside
our own, than in the assemblage of a group of distinguished men
from the chief centres of civilisation, on a barren ridge, isolated in
a vast and tempestuous ocean, at a distance, in many cases, of
11,000 miles and upwards from the ordinary scene of their labours.
And all these sacrifices—the cost and care of preparation, the
transport and readjustment of delicate instruments, the contrivance
of new and more subtle means of investigating phenomena—on
the precarious chance of a clear sky during one particular five
minutes! The event, though fortunate, emphasised the hazard
of the venture. The observation of the eclipse was made possible
only by the happy accident of a serene interval between two
storms.
The American expedition was led by Professor Edward S. Holden,
and to it were courteously permitted to be attached Messrs.
Lawrance and Woods, photographers, sent out by the Royal Society
of London. M. Janssen was chief of the French Academy mission;[Pg 181]
he was accompanied from Meudon by Trouvelot, and joined from
Vienna by Palisa, and from Rome by Tacchini. A large share of
the work done was directed to assuring or negativing previous
results. The circumstances of an eclipse favour illusion. A single
observation by a single observer, made under unfamiliar conditions,
and at a moment of peculiar excitement, can scarcely be regarded as
offering more than a suggestion for future inquiry. But incredulity
may be carried too far. Janssen, for instance, felt compelled, by the
survival of unwise doubts, to devote some of the precious minutes
of obscurity at Caroline Island to confirming what, in his own persuasion,
needed no confirmation—that is, the presence of reflected
Fraunhofer lines in the spectrum of the corona. Trouvelot and
Palisa, on the other hand, instituted an exhaustive, but fruitless
search for the spurious “intramercurian” planets announced by Swift
and Watson in 1878.
New information, however, was not deficient. The corona proved
identical in type with that of 1882,[550] agreeably to what was expected
at an epoch of protracted solar activity. The characteristic aigrettes
were of even greater brilliancy than in the preceding year, and the
chemical effects of the coronal light proved unusually intense.
Janssen’s photographs, owing to the considerable apertures (six and
eight inches) of his object-glasses, and the long exposures permitted
by the duration of totality, were singularly perfect; they gave a
greater extension to the coronal than could be traced with the
telescope,[551] and showed its forms as absolutely fixed and of remarkable
complexity.
The English pictures, taken with exposures up to sixty seconds,
were likewise of great value. They exhibited details of structure
from the limb to the tips of the streamers, which terminated
definitely, and as it seemed actually, where the impressions on the
plates ceased. The coronal spectrum was also successfully photographed,
and although the reversing layer in its entirety evaded
record, a print was caught of some of its more prominent rays just
before and after totality. The use of the prismatic camera was
baffled by the anomalous scarcity of prominences.
Using an ingenious apparatus for viewing simultaneously the
spectrum from both sides of the sun, Professor Hastings noticed at
Caroline Island alternations, with the advance of the moon, in the
respective heights above the right and left solar limbs of the coronal
green line, which were thought to imply that the corona, with its
rifts and sheaves and “tangled hanks” of rays, is, after all, merely
an illusive appearance produced by the diffraction of sunlight at the[Pg 182]
moon’s edge.[552] But the observation was assuredly misleading or
misinterpreted. Atmospheric diffusion may indeed, under favouring
circumstances, be effective in deceptively enlarging solar appendages;
but always to a very limited extent.
The controversy is an old one as to the part played by our air
in producing the radiance visible round the eclipsed sun. In its
original form, it is true, it came to an end when Professor Harkness,
in 1869,[553] pointed out that the shadow of the moon falls equally over
the air and on the earth, and that if the sun had no luminous
appendages, a circular space of almost absolute darkness would consequently
surround the apparent places of the superposed sun and
moon. Mr. Proctor,[554] with his usual ability, impressed this mathematically
certain truth upon public attention; and Sir John Herschel
calculated that the diameter of the “negative halo” thus produced
would be, in general, no less than 23°.
But about the same time a noteworthy circumstance relating
to the state of things in the solar vicinity was brought into view.
On February 11, 1869, Messrs. Frankland and Lockyer communicated
to the Royal Society a series of experiments on gaseous
spectra under varying conditions of heat and density, leading them
to the conclusion that the higher solar prominences exist in a
medium of excessive tenuity, and that even at the base of the
chromosphere the pressure is far below that at the earth’s surface.[555]
This inference was fully borne out by the researches of Wüllner;
and Janssen expressed the opinion that the chromospheric gases are
rarefied almost to the degree of an air-pump vacuum.[556] Hence was
derived a general and fully justified conviction that there could be
outside, and incumbent upon the chromosphere, no such vast atmosphere
as the corona appeared to represent. Upon the strength of
which conviction the “glare” theory entered, chiefly under the
auspices of Sir Norman Lockyer, upon the second stage of its
existence.
The genuineness of the “inner corona” to the height of 5′ or 6′
from the limb was admitted; but it was supposed that by the detailed
reflection of its light in our air the far more extensive “outer
corona” was optically created, the irregularities of the moon’s edge
being called in to account for the rays and rifts by which its
structure was varied. This view received some countenance from
Admiral Maclear’s observation, during the eclipse of 1870, of bright
lines “everywhere”—even at the centre of the lunar disc. Here,[Pg 183]
indeed, was an undoubted case of atmospheric diffusion; but here,
also, was a safe index to the extent of its occurrence. Light scatters
equally in all directions; so that when the moon’s face at the time
of an eclipse shows (as is the common case) a blank in the spectroscope,
it is quite certain that the corona is not noticeably enlarged
by atmospheric causes. A sky drifted over with thin cirrus clouds
and air changed with aqueous vapour amply accounted for the
abnormal amount of scattering in 1870.
But even in 1870 positive evidence was obtained of the substantial
reality of the radiated outer corona, in the appearance on
the photographic plates exposed by Willard in Spain and by
Brothers in Sicily of identical dark rifts. The truth is, that far
from being developed by misty air, it is peculiarly liable to be
effaced by it. The purer the sky, the more extensive, brilliant, and
intricate in the details of its structure the corona appears. Take
as an example General Myer’s description of the eclipse of 1869, as
seen from the summit of White Top Mountain, Virginia, at an elevation
above the sea of 5,523 feet, in an atmosphere of peculiar
clearness.
“To the unaided eye,” he wrote,[557] “the eclipse presented, during
the total obscuration, a vision magnificent beyond description. As
a centre stood the full and intensely black disc of the moon,
surrounded by the aureola of a soft bright light, through which shot
out, as if from the circumference of the moon, straight, massive,
silvery rays, seeming distinct and separate from each other, to a
distance of two or three diameters of the solar disc; the whole
spectacle showing as on a background of diffused rose-coloured
light.”
On the same day, at Des Moines, Newcomb could perceive,
through somewhat hazy air, no long rays, and the four-pointed outline
of the corona reached at its farthest only a single semidiameter of
the moon from the limb. The plain fact, that our atmosphere acts
rather as a veil to hide the coronal radiance than as the medium
through which it is visually formed, emerges from further innumerable
records.
No observations of importance were made during the eclipse of
September 9, 1885. The path of total obscurity touched land
only on the shores of New Zealand, and two minutes was the
outside limit of available time. Hence local observers had the
phenomenon to themselves; nor were they even favoured by the
weather in their efforts to make the most of it. One striking
appearance was, however, disclosed. It was that of two “white”
prominences of unusual brilliancy, shining like a pair of electric[Pg 184]
lamps hung one at each end of a solar diameter, right above the
places of two large spots.[558] This coincidence of diametrically
opposite disturbances is of too frequent occurrence to be accidental.
M. Trouvelot observed at Meudon, June 26, 1885, two active and
evanescent prominences thus situated, each rising to the enormous
height of 300,000 miles; and on August 16, one scarcely less remarkable,
balanced by an antipodal spot-group.[559] It towered upward, as
if by a process of unrolling, to a quarter of a million of miles; after
which, in two minutes, the light died out of it; it had become completely
extinct. The development, again from the ends of a
diameter, of a pair of similar objects was watched, September 19 and
20, 1893, by Father Fényi, Director of the Kalocsa Observatory;
and the phenomenon has been too often repeated to be accidental.
The eclipse of August 29, 1886, was total during about four
minutes over tropical Atlantic regions; and an English expedition,
led by Sir Norman Lockyer, was accordingly despatched to Grenada
in the West Indies, for the purpose of using the opportunity it
offered. But the rainy season was just then at its height: clouds
and squalls were the order of the day; and the elaborately planned
programme of observation could only in part be carried through.
Some good work, none the less, was done. Professor Tacchini, who
had been invited to accompany the party, ascertained besides some
significant facts about prominences. From a comparison of their
forms and sizes during and after the eclipse, it appeared that only
the growing vaporous cores of these objects are shown by the
spectroscope under ordinary circumstances; their upper sections,
giving a faint continuous spectrum, and composed of presumably
cooler materials, can only be seen when the veil of scattered light
usually drawn over them is removed by an eclipse. Thus all
modestly tall prominences have silvery summits; but all do not
appear to possess the “red heart of flame,” by which alone they
can be rendered perceptible to daylight observation. Some
prove to be ordinarily invisible, because silvery throughout—”sheeted
ghosts,” as it were, met only in the dark.
Specimens of the class had been noted as far back as 1842, but
Tacchini first drew particular attention to them. The one observed
by him in 1886 rose in a branching form to a height of 150,000
miles, and gave a brilliantly continuous spectrum, with bright lines
at H and K, but no hydrogen-lines.[560] Hence the total invisibility of
the object before and after the eclipse. During the eclipse, it was
seen framed, as it were, in a pointed arch of coronal light, the[Pg 185]
symmetrical arrangement of which with regard to it was obviously
significant. Both its unspringing shape, and the violet rays of
calcium strongly emitted by it, contradicted the supposition that
“white prominences” represent a downrush of refrigerated
materials.
The corona of 1886, as photographed by Dr. Schuster and
Mr. Maunder, showed neither the petals and plumes of 1871, nor
the streamers of 1878. It might be called of a transition type.[561]
Wide polar rifts were filled in with tufted radiations, and bounded
on either side by irregularly disposed, compound luminous masses.
In the south-western quadrant, a triangular ray, conspicuous to the
naked eye, represented, Mr. W. H. Pickering thought, the projection
of a huge, hollow cone.[562] Branched and recurving jets were
curiously associated with it. The intrinsic photographic brightness
of the corona proved, from Pickering’s measures, to be about 1/54 that
of the average surface of the full moon.
The Russian eclipse of August 19, 1887, can only be remembered
as a disastrous failure. Much was expected of it. The shadow-path
ran overland from Leipsic to the Japanese sea, so that the solar
appurtenances would, it was hoped, be disclosed to observers
echeloned along a line of 6,000 miles. But the incalculable element
of weather rendered all forecasts nugatory. The clouds never
parted, during the critical three minutes, over Central Russia, where
many parties were stationed, and Professor D. P. Todd was
equally unfortunate in Japan. Some good photographs were,
nevertheless, secured by Professor Arai, Director of the Tokio
Observatory, as well as by MM. Bélopolsky and Glasenapp at
Petrovsk and Jurjevitch respectively. They showed a corona of
simpler form than that of the year before, but not yet of the pronounced
type first associated by Mr. Ranyard with the lowest stage
of solar activity.
The genuineness of the association was ratified by the duplicate
spectacle of the next-ensuing minimum year. Two total eclipses
of the sun distinguished 1889. The first took place on New Year’s
Day, when a narrow shadow-path crossed California, allowing
less than two minutes for the numerous experiments prompted
by the varied nature of modern methods of research. American
astronomers availed themselves of the occasion to the full. The
heavens were propitious. Photographic records were obtained
in unprecedented abundance, and of unusual excellence. Their
comparison and study placed it beyond reasonable doubt that
the radiated corona belonging to periods of maximum sun-spots[Pg 186]
gives place, at periods of minimum, to the “winged” type of 1878.
Professor Holden perceived further that the equatorial extensions
characterising the latter tend to assume a “trumpet-shape.”[563] Their
extremities diverge, as if mutually repellent, instead of flowing
together along a medial plane. The maximum actinic brilliancy of
the corona of January 1, 1889, was determined at Lick to be twenty-one
times less than that of the full moon.[564] Its colour was described
as “of an intense luminous silver, with a bluish tinge, similar to the
light of an electric arc.”[565] Its spectrum was comparatively simple.
Very few bright lines besides those of hydrogen and coronium,
and apparently no dark ones, stood out from the prismatic background.
“The marked structural features of the corona, as presented by
the negatives” taken by Professors Nipher and Charroppin, were
the filaments and the streamers. The filaments issued from polar
calottes of 20° radius.
“The impression conveyed to the eye,” Professor Pritchett wrote,[566]
“is that the equatorial stream of denser coronal matter extends across
and through the filaments, simply obscuring them by its greater
brightness. The effect is just as if the equatorial belt were superposed
upon, or passed through, the filamentary structure. There is
nothing in the photographs to prove that the filaments do not exist
all round the sun.[567] The testimony from negatives of different
lengths of exposure goes to show that the equatorial streamers are
made up of numerous interlacing parts inclined at varying angles to
the sun’s equator.”
The coronal extensions, perceptible with the naked eye to a
distance of more than 3° from the sun, appeared barely one-third
of that length on the best negatives. Little more could be
seen of them either in Barnard’s exquisite miniature pictures, or
in the photographs obtained by W. H. Pickering with a thirteen-inch
refractor—the largest instrument so far used in eclipse-photography.
The total eclipse of December 22, 1889, held out a prospect,
unfortunately not realized, of removing some of the doubts and
difficulties that impeded the progress of coronal photography.[568]
Messrs. Burnham and Schaeberle secured at Cayenne some excellent
impressions, showing enough of the corona to prove its identical[Pg 187]
character with that depicted in the beginning of the year, but not
enough to convey additional information about its terminal forms
or innermost structure. Any better result was indeed impossible,
the moisture-laden air having cut down the actinic power of the
coronal light to one-fourth its previous value.
Two English expeditions organized by the Royal Astronomical
Society fared still worse. Mr. Taylor was stationed on the West
Coast of Africa, one hundred miles south of Loanda; Father Perry
chose as the scene of his operations the Salut Islands, off French
Guiana. Each was supplied with a reflector constructed by Dr.
Common, endowed, by its extremely short focal length of forty-five,
combined with an aperture of twenty inches, with a light-concentrating
force capable, it was hoped, of compelling the very filmiest
coronal branches to self-registration. Had things gone well two
sets of coronal pictures, absolutely comparable in every respect, and
taken at an interval of two hours and a half, would have been at
the disposal of astronomers. But things went very far from well.
Clouds altogether obscured the sun in Africa; they only separated
to allow of his shining through a saturated atmosphere in South
America. Father Perry’s observations were the last heroic effort of
a dying man. Stricken with malaria, he crawled to the hospital as
soon as the eclipse was over, and expired five days later, at sea, on
board the Comus. He was buried at Barbados. And the sacrifice
of his life had, after all, purchased no decisive success. Most of the
plates exposed by him suffered deterioration from the climate, or from
an inevitably delayed development. A drawing from the best of
them by Miss Violet Common[569] represented a corona differing from
its predecessor of January 1, chiefly through the oppositely
unsymmetrical relations of its parts. Then the western wing had
been broader at its base than the eastern; now the inequality was
conspicuously the other way.[570]
The next opportunity for retrieving the mischances of the past
was offered April 16, 1893. The line of totality charted for that day
ran from Chili to Senegambia. American parties appropriated the
Andes; both shores of the Atlantic were in English occupation;
French expeditions, led by Deslandres and Bigourdan, took up posts
south of Cape Verde. A long totality of more than four minutes
was favoured by serene skies; hence an ample store of photographic
data was obtained. Professor Schaeberle, of the Lick Observatory,
took, almost without assistance, at Mina Bronces, a mining station
6,600 feet above the Pacific, fifty-two negatives, eight of them with
a forty-foot telescope, on a scale of four and a half inches to the[Pg 188]
solar diameter. Not only the inner corona, but the array of
prominences then conspicuous, appeared in them to be composed of
fibrous jets and arches, held to be sections of elliptic orbits described
by luminous particles about the sun’s centre.[571] One plate
received the impression of a curious object,[572] entangled amidst
coronal streamers, and the belief in its cometary nature was ratified
by the bestowal of a comet-medal in recognition of the discovery.
Similiar paraboloidal forms had, nevertheless, occasionally been seen
to make an integral part of earlier coronas; and it remains extremely
doubtful whether Schaeberle’s “eclipse-comet” was justly entitled to
the character claimed for it.
The eclipse of 1893 disclosed a radiated corona such as a year of
spot-maximum was sure to bring. An unexpected fact about it
was, however, ascertained. The coronal has been believed to have
much in common with the chromospheric spectrum; it proved, on
investigation with a large prismatic camera, employed under Sir
Norman Lockyer’s directions by Mr. Fowler at Fundium, to be
absolutely distinct from it. The fundamental green ray had, on
the West African plates, seven more refrangible associates;[573] but all
alike are of unknown origin. They may be due to many substances,
or to one; future research will perhaps decide; we can at present
only say that the gaseous emission of the corona include none
from hydrogen, helium, calcium, or any other recognisable terrestrial
element. Deslandres’ attempt to determine the rotation of the
corona through opposite displacements, east and west of the interposed
moon, of the violet calcium-lines supposed to make part of
the coronal spectrum, was thus rendered nugatory. Yet it gave an
earnest of success, by definitely introducing the subject into the
constantly lengthened programme of eclipse-work. There is, however,
little prospect of its being treated effectively until the green
line is vivified by a fresh access of solar activity.
The flight of the moon’s shadow was, on August 9, 1896, dogged
by atrocious weather. It traversed, besides, some of the most
inhospitable regions on the earth’s surface, and afforded, at the
best, but a brief interval of obscurity. At Novaya Zemlya, however,
of all places, the conditions were tolerably favourable, and, as
we have seen, the trophy of a “flash-spectrograph” was carried off.
Some coronal photographs, moreover, taken by the late Sir George
Baden-Powell[574] and by M. Hansky, a member of a Russian party,
were marked by features of considerable interest. They made[Pg 189]
apparent a close connection between coronal outflows and chromospheric
jets, cone-shaped beams serving as the sheaths, or envelopes,
of prominences. M. Hansky,[575] indeed, thought that every streamer
had a chromospheric eruption at its base. Further, dark veinings
of singular shapes unmistakably interrupted the coronal light, and
bordered brilliant prominences,[576] reminding us of certain “black
lines” traced by Swift across the “anvil protuberance” August 7,
1869.[577] In type the corona of 1896 reproduced that of 1886, as
befitted its intermediate position in the solar cycle.
The eclipse-track on January 22, 1898, crossed the Indian peninsula
from Viziadrug, on the Malabar coast, to Mount Everest in the
Himalayas. Not a cloud obstructed the view anywhere, and an
unprecedented harvest of photographic records was garnered. The
flash-spectrum, in its successive phases, appeared on plates taken by
Sir Norman Lockyer, Mr. Evershed, Professor Campbell,[578] and others;
Professor Turner[579] set on foot a novel mode of research by picturing
the corona in the polarised ingredient of its light; Mrs. Maunder[580]
practically solved the problem of photographing the faint coronal
extensions, one ray on her plates running out to nearly six diameters
from the moon’s limb. Yet she used a Dallmeyer lens of only one
and a half inches aperture. Her success accorded perfectly with
Professor Wadsworth’s conclusion that effectiveness in delineation
by slight contrasts of luminosity varies inversely with aperture.
Triple-coated plates, and a comparatively long exposure of twenty
seconds, contributed to a result unlikely, for some time, to be surpassed.
The corona of 1898 presented a mixed aspect. The polar
plumes due at minimum were combined in it with the quadrilateral
ogives belonging to spot-maxima. A slow course of transformation,
in fact, seemed in progress; and it was found to be completed in
1900, when the eclipse of May 28 revealed the typical halo of a
quiescent sun.
The obscurity on this occasion was short—less than 100 seconds—but
was well observed east and west of the Atlantic. No striking
gain in knowledge, however, resulted. Important experiments were
indeed made on the heat of the corona with Langley’s bolometer,
but their upshot can scarcely be admitted as decisive. They indicated
a marked deficiency of thermal radiations, implying for
coronal light, in Professor Langley’s opinion,[581] an origin analogous
to that of the electric glow-discharge, which, at low pressures, was[Pg 190]
found by K. Ångström in 1893 to have no invisible heat-spectrum.[582] The
corona was photographed by Professor Barnard, at Wadesborough,
North Carolina, with a 61-1/2-foot horizontal “coelostat.” In this instrument,
of a type now much employed in eclipse operations and first
recommended by Professor Turner, a six-inch photographic objective
preserved an invariable position, while a silvered plane mirror, revolving
by clockwork once in forty-eight hours (since the angle of movement
is doubled by reflection), supplied the light it brought to a focus.
A temporary wooden tube connected the lens with the photographic
house where the plates were exposed. Pictures thus obtained with
exposures of from one to fourteen seconds, were described as “remarkably
sharp and perfectly defined, showing the prominences and inner
corona very beautifully. The polar fans came out magnificently.”[583]
The great Sumatra eclipse left behind it manifold memories of
foiled expectations. A totality of above six minutes drew observers
to the Far East from several continents, each cherishing a plan of
inquiry which few were destined to execute. All along the line of
shadow, which, on May 18, 1901, crossed Réunion and Mauritius,
and again met land at Sumatra and Borneo, the meteorological forecast
was dubious, and the meteorological actuality in the main
deplorable. Nevertheless, the corona was seen, and fairly well
photographed through drifting clouds, and proved to resemble in
essentials the appendage viewed a year previously. Negatives taken
by members of the Lick Observatory expedition led by Mr. Perrine[584]
disclosed the unique phenomenon of a violent coronal disturbance,
with a small compact prominence as its apparent focus. Tumbling
masses and irregular streamers radiating from a point subsequently
shown by the Greenwich photographs to be the seat of a conspicuous
spot, suggested the recent occurrence of an explosion, the far-reaching
effects of which might be traced in the confused floccular
luminosity of a vast surrounding region. Again, photographs in
polarised light attested the radiance of the outer corona to be in
large measure reflected, while that of the inner ring was original;
and the inference was confirmed by spectrographs, recording many
Fraunhofer lines when the slit lay far from the sun’s limb, but none
in its immediate vicinity. On plates exposed by Mr. Dyson and
Dr. Humphrys with special apparatus, the coronal spectrum,
continuous and linear, impressed itself more extensively in the ultra-violet
than on any previous occasion; and Dr. Mitchell succeeded in
photographing the reversing layer by means of a grating spectroscope.
Finally, Mrs. Maunder, at Mauritius, despite mischievous[Pg 191]
atmospheric tremors, obtained with the Newbegin telescope an
excellent series of coronal pictures.[585]
The principles of explanation applied to the corona may be
briefly described as eruptive and electrical. The first was adopted
by Professor Schaeberle in his “Mechanical Theory,” advanced in
1890.[586] According to this view, the eclipse-halo consists of streams
of matter shot out with great velocity from the spot-zones by
forces acting perpendicularly to the sun’s surface. The component
particles return to the sun after describing sections of extremely
elongated ellipses, unless their initial speed happen to equal or
exceed the critical rate of 383 miles a second, in which case they
are finally driven off into space. The perspective overlapping and
interlacing of these incandescent outflows was supposed to occasion
the intricacies of texture visible in the corona; and it should be
recorded that a virtually identical conclusion was reached by
Mr. Perrine in 1901,[587] by a different train of reasoning, based upon
a distinct set of facts. A theory on very much the same lines was,
moreover, worked out by M. Bélopolsky in 1897.[588] Schaeberle,
however, had the merit of making the first adequate effort to deduce
the real shape of the corona, as it exists in three dimensions, from
its projection upon the surface of the sphere. He failed, indeed, to
account for the variation in coronal types by the changes in our
situation with regard to the sun’s equator. It is only necessary to
remark that, if this were so, they should be subject to an annual
periodicity, of which no trace can be discerned.
Electro-magnetic theories have the charm, and the drawback, of
dealing largely with the unknown. But they are gradually losing
the vague and intangible character which long clung to them; and
the improved definition of their outlines has not, so far, brought
them into disaccord with truth. The most promising hypothesis of
the kind is due to Professor Bigelow of Washington. His able
discussion of the eclipse photographs of January 1, 1889,[589] showed
a striking agreement between the observed coronal forms and the
calculated effects of a repulsive influence obeying the laws of electric
potential, also postulated by Huggins in 1885.[590] Finely subdivided
matter, expelled from the sun along lines of force emanating from the
neighbourhood of his poles, thus tends to accumulate at “equipotential[Pg 192]
surfaces.” In deference, however, to a doubt more strongly felt then
than now, whether the presence of free electricity is compatible with
the solar temperature, he avoided any express assertion that the
coronal structure is an electrical phenomenon, merely pointing out
that, if it were, its details would be just what they are.
Later, in 1892, Pupin in America,[591] and Ebert in Germany,[592] imitated
the coronal streamers by means of electrical discharges in low vacua
between small conducting bodies and strips of tinfoil placed on the
outside of the containing glass receptacles. Finally, a critical experiment
made by Ebert in 1895 served, as Bigelow justly said, “to
clear up the entire subject, and put the theory on a working basis.”
Having obtained coronoidal effects in the manner described, he proceeded
to subject them to the action of a strong magnetic field,
with the result of marshalling the scattered rays into a methodical
and highly suggestive array. They followed the direction of the
magnetic lines of force, and, forsaking the polar collar of the
magnetised sphere, surrounded it like a ruffle. The obvious analogy
with the aurora polaris and the solar corona was insisted upon by
Ebert himself, and has been further developed by Bigelow.[593] According
to a recent modification of his hypothesis, the latter appendage
is controlled by two opposing systems of forces; the magnetic
causing the rays to diverge from the poles towards the equator, and
the electrostatic urging their spread, through the mutual repulsion
of the particles accumulated in the “wings,” from the equator
towards either pole. The cyclical change in the corona, he adds, is
probably due to a variation in the balance of power thus established,
the magnetic polar influence dominating at minima, the electrostatic
at maxima. And he may well feel encouraged by the fortunate
combination of many experimental details into one explanatory
whole, no less than by the hopeful prospect of further developments,
both practical and theoretical, along the same lines.
What we really know about the corona can be summed up in a
few words. It is certainly not a solar atmosphere. It does not
gravitate upon the sun’s surface and share his rotation, as our air
gravitates upon and shares the rotation of the earth; and this for
the simple reason that there is no visible growth of pressure
downwards (of which the spectroscope would infallibly give notice)
in its gaseous constituents; whereas under the sole influence of
the sun’s attractive power, their density should be multiplied many
million times in the descent through a mere fraction of their actual
depth.[594]
They are apparently in a perpetual state of efflux from, and
influx to our great luminary, under the stress of opposing forces.
It is not unlikely that some part, at least, of the coronal materials
are provided by eruptions from the body of the sun;[595] it is almost
certain that they are organized and arranged round it through
electro-magnetic action. This, however, would seem to be influential
only upon their white-hot or reflective ingredients, out of
which the streamers and aigrettes are composed; since the coronal
gases appear, from observations during eclipses, to form a shapeless
envelope, with condensations above the spot-zones, or at the bases
of equatorial extensions. The corona is undoubtedly affected both
in shape and constitution by the periodic ebb and flow of solar
activity, its low-tide form being winged, its high-tide form stellate;
while the rays emitted by the gases contained in it fade, and the
continuous spectrum brightens, at times of minimum sun-spots.
The appendage, as a whole, must be of inconceivable tenuity, since
comets cut their way through it without experiencing sensible retardation.
Not even Sir William Crookes’s vacua can give an idea of
the rarefaction which this fact implies. Yet the observed luminous
effects may not in reality bear witness contradictory of it. One
solitary molecule in each cubic inch of space might, in Professor
Young’s opinion, produce them; while in the same volume of
ordinary air at the sea-level, the molecules number (according to
Dr. Johnstone Stoney) 20,000 trillions!
The most important lesson, however, derived from eclipses is that
of partial independence of them. Some of its fruits in the daily
study of prominences the next chapter will collect; and the harvest
has been rendered more abundant, as well as more valuable, since it
has been found possible to enlist, in this department too, the versatile
aid of the camera.
FOOTNOTES:
[512] Vierteljahrsschrift Astr. Ges., Jahrg. xxvi., p. 274.
[513] Astr. Jour., vol. iv., p. 33.
[514] Proc. Roy. Soc., vol. xvii., p. 116.
[515] Comptes Rendus, t. lxvii., p. 757.
[516] Comptes Rendus, t. lxvii., p. 839.
[517] Month. Not., vol. xxvii., p. 88.
[518] Proc. Roy. Soc., vol. xvii., p. 123.
[519] Washington Observations, 1867, App. ii., Harkness’s Report, p. 60.
[520] Am. Jour., vol. xlviii. (2nd series), p. 377.
[521] Am. Jour., vol. xi. (3rd series), p. 429.
[522] Campbell, Astroph. Jour., vol. x., p. 186.
[523] Keeler, Reports on Eclipse of January 1, 1889, p. 47.
[524] Everything in such observations depends upon the proper manipulation of
the slit of the spectroscope.
[525] Mem. R. A. S., vol. xli., p. 435.
[526] Comptes Rendus, t. lxvii., p. 1019.
[527] Mem. R. A. S., vol. xli., p. 43.
[528] Comptes Rendus, t. xciv., p. 1640.
[529] Young, Pop. Astr., Oct., 1897, p. 333.
[530] J. Evershed, Indian Eclipse, 1898, p. 65; Month. Not., vol. lviii., p. 298;
Proc. Roy. Soc., Jan. 17, 1901.
[531] Frost, Astroph. Jour., vol. xii., p. 85; Lord, Ibid., vol. xiii., p. 149.
[532] Comptes Rendus, t. cxvii., No. 1; Jour. Brit. Astr. Ass., vol. iii., p. 532.
[533] Lockyer, Phil. Trans., vol. clvii., p. 551.
[534] The rosy envelope of prominence-matter was so named by Lockyer in 1868
(Phil. Trans., vol. clix., p. 430).
[535] According to Trouvelot (Wash. Obs., 1876, App. iii., p. 80), the subtracted
matter was, at least to some extent, accumulated in the polar regions.
[536] Bull. Phil. Soc. Washington, vol. iii., p. 118.
[537] Mem. R. A. S., vol. xli., 1879.
[538] Astr. Nach., No. 1,737.
[539] Correspondence with Newton, pp. 181-184; Ranyard, Mem. Astr. Soc.,
vol. xli., p. 501.
[540] S. P. Langley, Wash. Obs., 1876, App. iii., p. 209; Nature, vol. lxi., p. 443.
[541] Schuster (Proc. Roy. Soc., vol. xxxv., p. 154) measured and photographed
about thirty.
[542] Abney, Phil. Trans., vol. clxxv., p. 267.
[543] Proc. Roy. Soc., vol. xxxiv., p. 409. Experiments directed to the same end
had been made by Dr. O. Lohse at Potsdam, 1878-80. Astr. Nach., No. 2,486.
[544] The sensitiveness of chloride of silver extends from h to H; that is, over the
upper or more refrangible half of the space in which the main part of the coronal
light is concentrated.
[545] Proc. Roy. Soc., vol. xxxiv., p. 414.
[546] Report Brit. Assoc., 1883, p. 351.
[547] Maunder, Indian Eclipse, p. 125; Eclipse of 1900, p. 143.
[548] Astr. and Astrophysics, vol. xiii., p. 662.
[549] See infra, p. 197.
[550] Abney, Phil. Trans., vol. clxxx., p. 119.
[551] Comptes Rendus, t. xcvii., p. 592.
[552] Memoirs National Ac. of Sciences, vol. ii., p. 102.
[553] Wash. Obs., 1867, App. ii., p. 64.
[554] The Sun, p. 357.
[555] Proc. Roy. Soc., vol. xvii., p. 289.
[556] Comptes Rendus, t. lxxiii., p. 434.
[557] Wash. Obs., 1867, App. ii., p. 195.
[558] Stokes, Anniversary Address, Nature, vol. xxxv., p. 114.
[559] Comptes Rendus, t. ci., p. 50.
[560] Harvard Annals, vol. xviii., p. 99.
[561] Wesley, Phil. Trans., vol. clxxx., p. 350.
[562] Harvard Annals, vol. xviii, p. 108.
[563] Lick Report, p. 20.
[564] Ibid., p. 14.
[565] Ibid., p. 155.
[566] Pub. Astr. Soc. of the Pacific, vol. iii., p. 158.
[567] Professor Holden concluded, with less qualification, “that so-called ‘polar’
rays exist at all latitudes on the sun’s surface.” Lick Report, p. 19.
[568] Holden, Report on Eclipse of December, 1889, p. 18; Charroppin, Pub. Astr.
Soc. of the Pacific, vol. iii., p. 26.
[569] Published as the Frontispiece to the Observatory, No. 160.
[570] Wesley, Ibid., p. 107.
[571] Lick Observatory Contributions, No. 4, p. 108.
[572] Astr. and Astrophysics, vol. xiii. p. 307.
[573] Lockyer, Phil. Trans., vol. clxxxvii., p. 592.
[574] He died in London, November 20, 1898.
[575] Bull. Acad. St. Pétersbourg, t. vi., p. 253.
[576] W. H. Wesley, Phil. Trans., vol. cxc, p. 204.
[577] Lick Reports on Eclipse of January 1, 1889, p. 204.
[578] Astroph. Jour., vol. xi., p. 226.
[579] Observatory, vol. xxi., p. 157.
[580] The Indian Eclipse, 1898, p. 114.
[581] Science, June 22, 1900; Astroph. Jour., vol. xii., p. 370.
[582] Ann. der Physik, Bd. xlviii., p. 528. See also Wood, Physical Review,
vol. iv., p. 191, 1896.
[583] Science, August 3, 1900.
[584] Lick Observatory Bulletin, No. 9.
[585] Observatory, vol. xxiv., pp. 321, 375.
[586] Lick Report on Eclipse of December 22, 1889, p. 47; Month. Not., vol. l.,
p. 372.
[587] Lick Obs. Bull., No. 9.
[588] Bull. de l’Acad. St. Pétersbourg, t. iv., p. 289.
[589] The Solar Corona discussed by Spherical Harmonics, Smithsonian Institution,
1889.
[590] Bakerian Lecture, Proc. Roy. Soc., vol. xxxix.
[591] Astr. and Astrophysics, vol. xi., p. 483.
[592] Ibid., vol. xii., p. 804.
[593] Am. Journ. of Science, vol. xi., p. 253, 1901.
[594] See Huggins, Proc. Roy. Soc., vol. xxxix., p. 108; Young, North Am.
Review, February, 1885, p. 179.
[595] Professor W. A. Norton, of Yale College, appears to have been the earliest
formal advocate of the Expulsion Theory of the solar surroundings, in the second
(1845) and later editions of his Treatise on Astronomy.
CHAPTER IV
SOLAR SPECTROSCOPY
The new way struck out by Janssen and Lockyer was at once and
eagerly followed. In every part of Europe, as well as in North
America, observers devoted themselves to the daily study of the
chromosphere and prominences. Foremost among these were
Lockyer in England, Zöllner at Leipzig, Spörer at Anclam, Young
at Hanover, New Hampshire, Secchi and Respighi at Rome. There
were many others, but these names stood out conspicuously.
The first point to be cleared up was that of chemical composition.
Leisurely measurements verified the presence above the sun’s surface
of hydrogen in prodigious volumes, but showed that sodium had
nothing to do with the orange-yellow ray identified with it in the
haste of the eclipse. From its vicinity to the D-pair (than which
it is slightly more refrangible), the prominence-line was, however,
designated D_3, and the unknown substance emitting it was named
by Lockyer “helium.” Its terrestrial discovery ensued after twenty-six
years. In March, 1895, Professor Ramsay obtained from the
rare mineral clevite a volatile gas, the spectrum of which was
found to include the yellow prominence-ray. Helium was actually
at hand, and available for examination. The identification cleared
up many obscurities in chromospheric chemistry. Several bright
lines, persistently seen at the edge of the sun, and early suspected
by Young[596] to emanate from the same source as D_3, were now
derived from helium in the laboratory; and all the complex
emissions of that exotic substance ranged themselves into six sets
or series, the members of which are mutually connected by numerical
relations of a definite and simple kind. Helium is of rather more than
twice the density of hydrogen, and has no chemical affinities.
In almost evanescent quantities it lurks in the earth’s crust, and
is diffused through the earth’s atmosphere.
The importance of the part played in the prominence-spectrum by[Pg 195]
the violet line of calcium was noticed by Professor Young in 1872,
but since H and K lie near the limit of the visible spectrum, photography
was needed for a thorough investigation of their appearances.
Aided by its resources, Professor George E. Hale, then at
the beginning of his career, detected in 1889 their unfailing and
conspicuous presence.[597] The substance emitting them not only constitutes
a fundamental ingredient of the chromosphere, but rises, in
the fantastic jets thence issuing, to greater heights than hydrogen
itself. The isolation of H and K in solar prominences from any
other of the lines usually distinctive of calcium was experimentally
proved by Sir William and Lady Huggins in 1897 to be due to the
extreme tenuity of the emitting vapour.[598]
Hydrogen, helium, and calcium form, then, the chief and unvarying
materials of the solar sierra and its peaks; but a number of
metallic elements make their appearance spasmodically under the
influence of disturbances in the layers beneath. In September,
1871, Young[599] drew up at Dartmouth College a list of 103 lines
significant of injections into the chromosphere of iron, titanium,
chromium, magnesium, and many other substances. During two
months’ observation in the pure air of Mount Sherman (8,335 feet
high) in the summer of 1872, these tell-tale lines mounted up to
273;[600] and he believes their number might still be doubled by steady
watching. Indeed, both Young and Lockyer have more than once
seen the whole field of the spectroscope momentarily inundated
with bright rays, as if the “reversing layer” had been suddenly
thrust upwards into the chromosphere, and as quickly allowed to
drop back again. The opinion would thus appear to be well-grounded
that the two form one continuous region, of which the
lower parts are habitually occupied by the heaviest vapours, but
where orderly arrangement is continually overturned by violent
eruptive disturbances.
The study of the forms of prominences practically began with
Huggins’s observation of one through an “open slit” February 13,
1869.[601] At first it had been thought possible to examine them
only in sections—that is, by admitting mere narrow strips or
“lines” of their various kinds of light; while the actual shape
of the objects emitting those lines had been arrived at by such
imperfect devices as that of giving to the slit of the spectroscope a
vibratory moment rapid enough to enable the eye to retain the[Pg 196]
impression of one part while others were successively presented to
it. It was an immense gain to find that their rays had strength to
bear so much of dilution with ordinary light as was involved in
opening the spectroscopic shutter wide enough to exhibit the tree-like,
or horn-like, or flame-shaped bodies rising over the sun’s rim in
their undivided proportions. Several diversely-coloured images of
them are formed in the spectroscope; each may be seen under a
crimson, a yellow, a green, and a deep blue aspect. The crimson,
however (built up out of the C-line of hydrogen), is the most
intense, and is commonly used for purposes of observation and
illustration.
Friedrich Zöllner was, by a few days, beforehand with Huggins
in describing the open-slit method, but was somewhat less prompt
in applying it. His first survey of a complete prominence, pictured
in, and not simply intersected by, the slit of his spectroscope, was
obtained July 1, 1869.[602] Shortly afterwards the plan was successfully
adopted by the whole band of investigators.
A difference in kind was very soon perceived to separate these
objects into two well-marked classes. Its natural and obvious
character was shown by its having struck several observers independently.
The distinction of “cloud-prominences” from
“flame-prominences” was announced by Lockyer, April 27; by
Zöllner, June 2; and by Respighi, December 4, 1870.
The first description are tranquil and relatively permanent, sometimes
enduring without striking change for many days. Certain of
the included species mimic terrestrial cloud-scenery—now appearing
like fleecy cirrus transpenetrated with the red glow of sunset—now
like prodigious masses of cumulo-stratus hanging heavily above the
horizon. The solar clouds, however, have the peculiarity of possessing
stems. Slender columns can ordinarily be seen to connect the
surface of the chromosphere with its outlying portions. Hence the
fantastic likeness to forest scenery presented by the long ranges of
fiery trunks and foliage occasionally seeming to fringe the sun’s limb.
But while this mode of structure suggests an actual outpouring of incandescent
material, certain facts require a different interpretation.
At a distance, and quite apart from the chromosphere, prominences
have been perceived, both by Secchi and Young, to form, just as
clouds form in a clear sky, condensation being replaced by ignition.
Filaments were then thrown out downward towards the chromosphere,
and finally the usual appearance of a “stemmed prominence”
was assumed. Still more remarkable was an observation
made by Trouvelot at Harvard College Observatory, June 26, 1874.[603]
A gigantic comma-shaped prominence, 82,000 miles high, vanished[Pg 197]
from before his eyes by a withdrawal of light as sudden as the
passage of a flash of lightning. The same observer has frequently
witnessed a gradual illumination or gradual extinction of such
objects, testifying to changes in the thermal or electrical condition
of matter already in situ.
The first photograph of a prominence, as shown by the spectroscope
in daylight, was taken by Professor Young in 1870.[604] But
neither his method, nor that described by Dr. Braun in 1872,[605] had
any practical success. This was reserved to reward the efforts
towards the same end of Professor Hale. Begun at Harvard College
in 1889,[606] they were prosecuted soon afterwards at the Kenwood
Observatory, Chicago. The great difficulty was to extricate the
coloured image of the gaseous structure, spectroscopically visible at
the sun’s limb, from the encompassing glare, a very little of which
goes a long way in fogging sensitive plates. To counteract its
mischievous effects, a second slit,[607] besides the usual narrow one in
front of the collimator, was placed on guard, as it were, behind the
dispersing apparatus, so as to shut out from the sensitised surface
all light save that of the required quality. The sun’s image being
then allowed to drift across the outer slit, while the plate holder
was kept moving at the same rate, the successive sectional impressions
thus rapidly obtained finally “built up” a complete picture of
the prominence. Another expedient was soon afterwards contrived.[608]
The H and K rays of calcium are always, as we have seen, bright in
the spectrum of prominences. They are besides fine and sharp,
while the corresponding absorption-lines in the ordinary solar
spectrum are wide and diffuse. Hence, prominences formed by the
spectroscope out of these particular qualities of violet light, can be
photographed entire and at once, for the simple reason that they are
projected upon a naturally darkened background. Atmospheric
glare is abolished by local absorption. This beautiful method was
first realised by Professor Hale in June, 1891.
A “spectroheliograph,” consisting of a spectroscopic and a photographic
apparatus of special type, attached to the eye-end of an
equatoreal twelve inches in aperture, was erected at Kenwood in
March, 1891; and with its aid, Professor Hale entered upon
original researches of high promise for the advancement of solar
physics. Noteworthy above all is his achievement of photographing
both prominences and faculæ on the very face of the sun. The latter[Pg 198]
had, until then, been very imperfectly observed. They were only
visible, in fact, when relieved by their brilliancy against the dusky
edge of the solar disc. Their convenient emission of calcium light,
however, makes it possible to photograph them in all positions, and
emphasises their close relationship to prominences. The simultaneous
picturing, moreover, of the entire chromospheric ring, with
whatever trees or fountains of fire chance to be at the moment
issuing from it, has been accomplished by a very simple device.
The disc of the sun itself having been screened with a circular
metallic diaphragm, it is only necessary to cause the slit to traverse
the virtually eclipsed luminary, in order to get an impression of the
whole round of its fringing appendages. And the record can be
extended to the disc by removing the screen, and carrying the slit
back at a quicker rate, when an “image of the sun’s surface, with
the faculæ and spots, is formed on the plate exactly within the image
of the chromosphere formed during the first exposure. The whole
operation,” Professor Hale continues, “is completed in less than a
minute, and the resulting photographs give the first true pictures of
the sun, showing all of the various phenomena at its surface.”[609] Most
of these novel researches were, by a remarkable coincidence, pursued
independently and contemporaneously by M. Deslandres, of the
Paris Observatory.[610]
The ultra-violet prominence spectrum was photographed for the
first time from an uneclipsed sun, in June, 1891, at Chicago. Besides
H and K, four members of the Huggins-series of hydrogen-lines
imprinted themselves on the plate.[611] Meanwhile M. Deslandres
was enabled, by fitting quartz lenses to his spectroscope,
and substituting a reflecting for a refracting telescope, to
get rid of the obstructive action of glass upon the shorter
light-waves, and thus to widen the scope of his inquiry into
the peculiarities of those derived from prominences.[612] As the
result, not only all the nine white-star lines were photographed
from a brilliant sun-flame, but five additional ones were found
to continue the series upward. The wave-lengths of these last
had, moreover, been calculated beforehand with singular exactness,
from a simple formula known as “Balmer’s Law.”[613] The new
lines, accordingly, filled places in a manner already prepared for
them, and were thus unmistakably associated with the hydrogen-spectrum.
This is now known to be represented in prominences by
twenty-seven lines,[614] forming a kind of harmonic progression, only
Photographs of the Solar Chromosphere and Prominences.
Taken with the Spectroheliograph of the Kenwood Observatory, Chicago,
by Professor George E. Hale.
[Pg 199]
four of which are visibly darkened in the Fraunhofer spectrum of
the sun.
The chemistry of “cloud-prominences” is simple. Hydrogen,
helium, and calcium are their chief constituents. “Flame-prominences,”
on the other hand, show, in addition, the characteristic rays
of a number of metals, among which iron, titanium, barium,
strontium, sodium, and magnesium are conspicuous. They are
intensely brilliant; sharply defined in their varying forms of jets,
spikes, fountains, waterspouts; of rapid formation and speedy
dissolution, seldom attaining to the vast dimensions of the more
tranquil kind. Eruptive or explosive by origin, they occur in close
connection with spots; whether causally, the materials ejected as
“flames” cooling and settling down as dark, depressed patches of
increased absorption;[615] or consequentially, as a reactive effect of
falls of solidified substances from great heights in the solar atmosphere.[616]
The two classes of phenomena, at any rate, stand in a
most intimate relation; they obey the same law of periodicity, and
are confined to the same portions of the sun’s surface, while quiescent
prominences may be found right up to the poles and close to the
equator.
The general distribution of prominences, including both genera,
follows that of faculæ much more closely than that of spots. From
Father Secchi’s and Professor Respighi’s observations, 1869-71, were
derived the first clear ideas on the subject, which have been supplemented
and modified by the later researches of Professors Tacchini
and Riccò at Rome and Palermo. The results are somewhat complicated,
but may be stated broadly as follows. The district of
greatest prominence-frequency covers and overlaps by several
degrees that of the greatest spot-frequency. That is to say, it extends
to about 40° north and south of the equator.[617] There is a visible
tendency to a second pair of maxima nearer the poles. The poles
themselves, as well as the equator, are regions of minimum occurrence.
Distribution in time is governed by the spot-cycle, but the
maximum lasts longer for prominences than for spots.
The structure of the chromosphere was investigated in 1869 and
subsequent years by Professor Respighi, director of the Capitoline
Observatory, as well as by Spörer, and Brédikhine of the Moscow
Observatory. They found this supposed solar envelope to be of
the same eruptive nature as the vast protrusions from it, and to be
made up of a congeries of minute flames[618] set close together like[Pg 200]
blades of grass. “The appearance,” Professor Young writes,[619]
“which probably indicates a fact, is as if countless jets of heated
gas were issuing through vents and spiracles over the whole surface,
thus clothing it with flame which heaves and tosses like the blaze
of a conflagration.”
The summits of these filaments of fire are commonly inclined, as
if by a wind sweeping over them, when the sun’s activity is near its
height, but erect during his phase of tranquillity. Spörer, in 1871,
inferred the influence of permanent polar currents,[620] but Tacchini
showed in 1876 that the deflections upon which this inference was
based ceased to be visible as the spot-minimum drew near.[621]
Another peculiarity of the chromosphere, denoting the remoteness
of its character from that of a true atmosphere,[622] is the irregularity
of its distribution over the sun’s surface. There are no signs
of its bulging out at the equator, as the laws of fluid equilibrium in
a rotating mass would require; but there are some that the fluctuations
in its depth are connected with the phases of solar agitation.
At times of minimum it seems to accumulate and concentrate its
activity at the poles; while maxima probably bring a more equable
general distribution, with local depressions at the base of great
prominences and above spots.
A low-lying stratum of carbon-vapour was, in 1897, detected in
the chromosphere by Professor Hale with a grating-spectroscope
attached to the 40-inch Yerkes refractor.[623] The eclipse-photographs
of 1893 disclosed to Hartley’s examination the presence there of
gallium;[624] and those taken by Evershed in 1898 were found by
Jewell[625] to be crowded with ultra-violet lines of the equally rare
metal scandium. The general rule had been laid down by Sir
Norman Lockyer that the metallic radiations from the chromosphere
are those “enhanced” in the electric spark.[626] Hence, the comparative
study of conditions prevalent in the arc and the spark has
acquired great importance in solar physics.
The reality of the appearance of violent disturbance presented by
the “flaming” kind of prominence can be tested in a very remarkable
manner. Christian Doppler,[627] professor of mathematics at
Prague, enounced in 1842 the theorm that the colour of a luminous
body, like the pitch of a sonorous body, must be changed by movements[Pg 201]
of approach or recession. The reason is this. Both colour
and pitch are physiological effects, depending, not upon absolute
wave-length, but upon the number of waves entering the eye or ear
in a given interval of time. And this number, it is easy to see,
must be increased if the source of light or sound is diminishing its
distance, and diminished if it is decreasing it. In the one case, the
vibrating body pursues and crowds together the waves emanating
from it; in the other, it retreats from them, and so lengthens out
the space covered by an identical number. The principle may be
thus illustrated. Suppose shots to be fired at a target at fixed
intervals of time. If the marksman advances, say twenty paces
between each discharge of his rifle, it is evident that the shots will
fall faster on the target than if he stood still; if, on the contrary,
he retires by the same amount, they will strike at correspondingly
longer intervals. The result will of course be the same whether
the target or the marksman be in movement.
So far Doppler was altogether right. As regards sound, anyone
can convince himself that the effect he predicted is a real one, by
listening to the alternate shrilling and sinking of the steam-whistle
when an express train rushes through a station. But in applying
this principle to the colours of stars he went widely astray; for he
omitted from consideration the double range of invisible vibrations
which partake of, and to the eye exactly compensate, changes of
refrangibility in the visible rays. There is, then, no possibility of
finding a criterion of velocity in the hue of bodies shining, like the
sun and stars, with continuous light. The entire spectrum is
slightly shifted up or down in the scale of refrangibility; certain
rays normally visible become exalted or degraded (as the case may
be) into invisibility, and certain other rays at the opposite end
undergo the converse process; but the sum total of impressions on
the retina continues the same.
We are not, however, without the means of measuring this sub-sensible
transportation of the light-gamut. Once more the wonderful
Fraunhofer lines came to the rescue. They were called by the
earlier physicists “fixed lines;” but it is just because they are not
fixed that, in this instance, we find them useful. They share, and
in sharing betray, the general shift of the spectrum. This aspect of
Doppler’s principle was adverted to by Fizeau in 1848,[628] and
the first tangible results in the estimation of movements of
approach and recession between the earth and the stars, were communicated
by Sir William Huggins to the Royal Society, April 23,[Pg 202]
1868. Eighteen months later, Zöllner devised his “reversion-spectroscope”[629]
for doubling the measurable effects of line-displacements;
aided by which ingenious instrument, and following a
suggestion of its inventor, Professor H. C. Vogel succeeded at
Bothkamp, June 9, 1871,[630] in detecting effects of that nature due to
the solar rotation. This application constitutes at once the test and
the triumph of the method.[631]
The eastern edge of the sun is continually moving towards us
with an equatorial speed of about a mile and a quarter per second,
the western edge retreating at the same rate. The displacements—towards
the violet on the east, towards the red on the west—corresponding
to this velocity are very small; so small that it seems
hardly credible that they should have been laid bare to perception.
They amount to but 1/150th part of the interval between the two
constituents of the D-line of sodium; and the D-line of sodium
itself can be separated into a pair only by a powerful spectroscope.
Nevertheless, Professor Young[632] was able to show quite satisfactorily,
in 1876, not only deviations in the solar lines from their proper
places indicating a velocity of rotation (1·42 miles per second)
slightly in excess of that given by observations of spots, but the
exemption of terrestrial lines (those produced by absorption in the
earth’s atmosphere) from the general push upwards or downwards.
Shortly afterwards, Professor Langley, then director of the Allegheny
Observatory, having devised a means of comparing with great
accuracy light from different portions of the sun’s disc, found that
while the obscure rays in two juxtaposed spectra derived from the
solar poles were absolutely continuous, no sooner was the instrument
rotated through 90°, so as to bring its luminous supplies
from opposite extremities of the equator, than the same rays became
perceptibly “notched.” The telluric lines, meanwhile, remained
unaffected, so as to be “virtually mapped” by the process.[633] This
rapid and unfailing mode of distinction was used by Cornu with
perfect ease during his investigation of atmospheric absorption near
Loiret in August and September, 1883.[634]
A beautiful experiment of the same kind was performed by
M. Thollon, of M. Bischoffsheim’s observatory at Nice, in the
summer of 1880.[635] He confined his attention to one delicately
defined group of four lines in the orange, of which the inner
pair are solar (iron) and the outer terrestrial. At the centre of[Pg 203]
the sun the intervals separating them were sensibly equal; but
when the light was taken alternately from the right and left
limbs, a relative shift in alternate directions of the solar, towards
and from the stationary telluric rays became apparent. A
parallel observation was made at Dunecht, December 14, 1883,
when it was noticed that a strong iron-line in the yellow part of
the solar spectrum is permanently double on the sun’s eastern,
but single on his western limb;[636] opposite motion-displacements
bringing about this curious effect of coincidence with, and separation
from, an adjacent stationary line of our own atmosphere’s production,
according as the spectrum is derived from the retreating or
advancing margin of the solar globe. Statements of fact so precise
and authoritative amount to a demonstration that results of this
kind are worthy of confidence; and they already occupy an important
place among astronomical data.
The subtle method of which they served to assure the validity
was employed in 1887-9 by M. Dunér to test and extend
Carrington’s and Spörer’s conclusions as to the anomalous nature of
the sun’s axial movement.[637] His observations for the purpose, made
with a fine diffraction-spectroscope, just then mounted at the
observatory of Upsala, were published in 1891.[638] Their upshot was
to confirm and widen the law of retardation with increasing latitude
derived from the progressive motions of spots. Determinations
made within 15° of the pole, consequently far beyond the
region of spots, gave a rotation-period of 38-1/2, that of the
equatorial belt being of 25-1/2 days. Spots near the equator indeed
complete their rounds in a period shorter by at least half a day;
and proportionate differences were found to exist elsewhere in
corresponding latitudes; but Dunér’s observations, it must be
remembered, apply to a distinct part of the complex solar machine
from the disturbed photospheric surface. It is amply possible that
the absorptive strata producing the Fraunhofer lines, significant, by
their varying displacements at either limb, of the inferred varying
rates of rotation, may gyrate more slowly than the spot-generating
level. Moreover, faculæ appear to move at a quicker pace than
either;[639] so that we have, for three solar formations, three different
periods of average rotation, the shortest of which belongs to the
faculæ, one of intermediate length to the spots, and the most
protracted to the reversing layer. All, however, agree in lengthening
progressively from the equator towards the poles. Professor
Holden aptly compared the sun to “a vast whirlpool where the[Pg 204]
velocities of rotation depend not only on the situation of the
rotating masses as to latitude, but also as to depth beneath the
exterior surface.”[640]
Sir Norman Lockyer[641] promptly perceived the applicability of
the surprising discovery of line-shiftings through end-on motion to
the study of prominences, the discontinuous light of which affords
precisely the same means of detecting movement without seeming
change of place, as do lines of absorption in a continuous spectrum.
Indeed, his observations at the sun’s edge almost compelled recourse
to an explanation made available just when the need of it
began to be felt. He saw bright lines, not merely pushed aside
from their normal places by a barely perceptible amount, but
bent, torn, broken, as if by the stress of some tremendous violence.
These remarkable appearances were quite simply interpreted as
the effects of movements varying in amount and direction in the
different parts of the extensive mass of incandescent vapours falling
within a single field of view. Very commonly they are of a
cyclonic character. The opposite distortions of the same coloured
rays betray the fury of “counter-gales” rushing along at the rate
of 120 miles a second; while their undisturbed sections prove the
persistence of a “heart of peace” in the midst of that unimaginable
fiery whirlwind. Velocities up to 250 miles a second, or 15,000 times
that of an express train at the top of its speed, were thus observed
by Young during his trip to Mount Sherman, August 2, 1872; and
these were actually doubled in an extraordinary outburst observed
by Father Jules Fényi, on June 17, 1891, at the Haynald Observatory
in Hungary, as well as by M. Trouvelot at Meudon.[642]
Motions ascertainable in this way near the limb are, of course,
horizontal as regards the sun’s surface; the analogies they present
might, accordingly, be styled meteorological rather than volcanic.
But vertical displacements on a scale no less stupendous can also be
shown to exist. Observations of the spectra of spots centrally
situated (where motions in the line of sight are vertical) disclose the
progress of violent uprushes and downrushes of ignited gases, for
the most part in the penumbral or outlying districts. They appear
to be occasioned by fitful and irregular disturbances, and have none
of the systematic quality which would be required for the elucidation
of sun-spot theories. Indeed, they almost certainly take
place at a great height above the actual openings in the photosphere.
As to vertical motions above the limb, on the other hand, we have
direct visual evidence of a truly amazing kind. The projected[Pg 205]
glowing matter has, by the aid of the spectroscope, been watched in
its ascent. On September 7, 1871, Young examined at noon a vast
hydrogen cloud 100,000 miles long, as it showed to the eye, and
54,000 high. It floated tranquilly above the chromosphere at an
elevation of some 15,000 miles, and was connected with it by
three or four upright columns, presenting the not uncommon
aspect compared by Lockyer to that of a grove of banyans. Called
away for a few minutes at 12.30, on returning at 12.55 the observer
found—
“That in the meantime the whole thing had been literally blown
to shreds by some inconceivable uprush from beneath. In place of
the quiet cloud I had left, the air, if I may use the expression, was
filled with flying débris—a mass of detached, vertical, fusiform
filaments, each from 10′ to 30′ long by 2′ or 3′ wide,[643] brighter and
closer together where the pillars had formerly stood, and rapidly
ascending. They rose, with a velocity estimated at 166 miles a
second, to fully 200,000 miles above the sun’s surface, then gradually
faded away like a dissolving cloud, and at 1.15 only a few filmy
wisps, with some brighter streamers low down near the photosphere,
remained to mark the place.”[644]
A velocity of projection of at least 500 miles per second was, by
Proctor’s[645] calculation, required to account for this extraordinary
display, to which the earth immediately responded by a magnetic
disturbance, and a fine aurora. It has proved by no means an
isolated occurrence. Young saw its main features repeated,
October 7, 1881,[646] on a still vaster scale; for the exploded prominence
attained, this time, an altitude of 350,000 miles—the
highest yet chronicled. Lockyer, moreover, has seen a prominence
40,000 miles high shattered in ten minutes; while uprushes
have been witnessed by Respighi, of which the initial velocities were
judged by him to be 400 or 500 miles a second. When it is
remembered that a body starting from the sun’s surface at the rate
of 383 miles a second would, if it encountered no resistance, escape
for ever from his control, it is obvious that we have, in the enormous
forces of eruption or repulsion manifested in the outbursts just
described, the means of accounting for the vast diffusion of matter
in the solar neighbourhood. Nor is it possible to explain them
away, as Cornu,[647] Faye,[648] and others have sought to do, by substituting
for the rush of matter in motion, progressive illumination[Pg 206]
through electric discharges, chemical processes,[649] or even through
the mere reheating of gases cooled by expansion.[650] All the appearances
are against such evasions of the difficulty presented by
velocities stigmatised as “fabulous” and “improbable,” but which,
there is the strongest reason to believe, really exist.
On the 12th of December, 1878, Sir Norman Lockyer formally
expounded before the Royal Society his hypothesis of the compound
nature of the “chemical elements.”[651] An hypothesis, it is
true, over and over again propounded from the simply terrestrial
point of view. What was novel was the supra-terrestrial evidence
adduced in its support; and even this had been, in a general and
speculative way, anticipated by Professor F. W. Clarke of Washington.[652]
Lockyer had been led to his conclusion along several
converging lines of research. In a letter to M. Dumas, dated
December 3, 1873, he had sketched out the successive stages of
“celestial dissociation” which he conceived to be represented in
the sun and stars. The absence from the solar spectrum of
metalloidal absorption he explained by the separation, in the fierce
solar furnace, of such substances as oxygen, nitrogen, sulphur, and
chlorine, into simpler constituents possessing unknown spectra;
while metals were at that time still admitted to be capable of existing
there in a state of integrity. Three years later he shifted his
position onward. He announced, as the result of a comparative
study of the Fraunhofer and electric-arc spectra of calcium, that
the “molecular grouping” of that metal, which at low temperatures
gives a spectrum with its chief line in the blue, is nearly broken up
in the sun into another or others with lines in the violet.[653] This
came to be regarded by him as “a truly typical case.”[654]
During four years (1875-78 inclusive) this diligent observer was
engaged in mapping a section of the more refrangible part of the
solar spectrum (wave-lengths 3,800-4,000) on a scale of magnitude
such that, if completed down to the infra-red, its length would have
been about half a furlong. The attendant laborious investigation,
by the aid of photography, of metallic spectra, seemed to indicate
the existence of what he called “basic lines.” These held their
ground persistently in the spectra of two or more metals after all[Pg 207]
possible “impurities” had been eliminated, and were therefore held
to attest the presence of a common substratum of matter in a
simpler state of aggregation than any with which we are ordinarily
acquainted.
Later inquiries have shown, however, that between the spectral
lines of different substances there are probably no absolute coincidences.
“Basic” lines are really formed of doublets or triplets
merged together by insufficient dispersion. Of Thalèn’s original
list of seventy rays common to several spectra,[655] very few resisted
Thollon’s and Young’s powerful spectroscopes; and the process of
resolution was completed by Rowland. Thus the argument from
community of lines to community of substance has virtually
collapsed. It was replaced by one founded on certain periodical
changes on the spectra of sun-spots. They emerged from a series
of observations begun at South Kensington under Sir Norman
Lockyer’s direction in 1879, and continued for fifteen years.[656]
The principle of the method employed is this. The whole range
of Fraunhofer lines is visible when the light from a spot is examined
with the spectroscope; but relatively few are widened. Now these
widened lines alone constitute (presumably) the true spot-spectrum;
they, and they alone, tell what kinds of vapour are thrust down
into the strange dusky pit of the nucleus, the unaffected lines taking
their accustomed origin from the over-lying strata of the normal
solar atmosphere. Here then we have the criterion that was wanted—the
means of distinguishing, spectroscopically and chemically,
between the cavity and the absorbing layers piled up above it. By
its persistent employment some marked peculiarities have been
brought out, such as the unfamiliar character of numerous lines in
spot-spectra, especially at epochs of disturbance; and the strange
individuality in the behaviour of every one of these darkened and
distended rays. Each seems to act on its own account; it comports
itself as if it were the sole representative of the substance emitting
it; its appearance is unconditioned by that of any of its terrestrial
companions in the same spectrum.
The most curious fact, however, elicited by these inquiries was
that of the attendance of chemical vicissitudes upon the advance of
the sun-spot period. As the maximum approached, unknown replaced
known components of the spot-spectra in a most pronounced
and unmistakable way.[657] It seemed as if the vapours emitting lines of
iron, titanium, nickel, etc., had ceased to exist as such, and their[Pg 208]
room been taken by others, total strangers in terrestrial laboratories.
These were held by Lockyer to be simply the finer constituents of
their predecessors, dissociation having been effected by the higher
temperature ensuing upon increased solar activity. But Father
Cortie’s supplementary investigations at Stonyhurst[658] modified, while
they in the main substantiated, the South Kensington results. They
showed that the substitution of unknown for known lines characterizes
disturbed spots, at all stages of the solar cycle, so that no
systematic course of chemical change can be said to affect the sun
as a whole. They showed further[659]—from evidence independent of
that obtained by Young in 1892[660]—the remarkable conspicuousness
in spot-spectra of vanadium lines excessively faint in the Fraunhofer
spectrum. Lockyer’s “unknown lines” may probably thus be
accounted for. They represent absorption, not by new, but by
scarce elements, especially, Father Cortie thinks, those with atomic
weights of about 50. The circumstance of their development in solar
commotions, largely to the exclusion of iron, is none the less curious;
but it cannot be explained by any process of dissociation.
The theory has, however, to be considered under still another
aspect. It frequently happens that the contortions or displacements
due to motion are seen to affect a single line belonging
to a particular substance, while the other lines of that same substance
remain imperturbable. Now, how is this most singular fact,
which seems at first sight to imply that a body may be at rest
and in motion at one and the same instant, to be accounted for?
It is accounted for, on the present hypothesis, easily enough, by
supposing that the rays thus discrepant in their testimony, do not
belong to one kind of matter, but to several, combined at ordinary
temperatures to form a body in appearance “elementary.” Of
these different vapours, one or more may of course be rushing
rapidly towards or from the observer, while the others remain still;
and since the line of sight across the average prominence-region
penetrates, at the sun’s edge, a depth of about 300,000 miles,[661] all
the incandescent materials separately occurring along which line are
projected into a single “flame” or “cloud,” it will be perceived that
there is ample room for diversities of behaviour.
The alternative mode of escape from the perplexity consists in
assuming that the vapour in motion is rendered luminous under[Pg 209]
conditions which reduce its spectrum to a few rays, the unaffected
lines being derived from a totally distinct mass of the same substance
shining with its ordinary emissions.[662] Thus, calcium can be
rendered virtually monochromatic by attenuation, and analogous
cases are not rare.
Sir Norman Lockyer only asks us to believe that effects which
follow certain causes on the earth are carried a stage further in the
sun, where the same causes must be vastly intensified. We find that
the bodies we call “compound” split asunder at fixed degrees of
heat within the range of our resources. Why should we hesitate to
admit that the bodies we call “simple” do likewise at degrees of
heat without the range of our resources? The term “element” simply
expresses terrestrial incapability of reduction. That, in celestial
laboratories, the means and their effect here absent should be
present, would be an inference challenging, in itself, no expression
of incredulity.
There are indeed theoretical objections to it which, though
probably not insuperable, are unquestionably grave. Our seventy
chemical “elements,” for instance, are placed by the law of specific
heats on a separate footing from their known compounds. We are
not, it is true, compelled by it to believe their atoms to be really
and absolutely such—to contain, that is, the “irreducible minimum”
of material substance; but we do certainly gather from it that they
are composed on a different principle from the salts and oxides made
and unmade at pleasure by chemists. Then the multiplication of
the species of matter with which Lockyer’s results menace us, is
at first sight startling. They may lead, we are told, to eventual
unification, but the prospect appears remote. Their only obvious
outcome is the disruption into several constituents of each terrestrial
“element.” The components of iron alone should be counted
by the dozen. And there are other metals, such as cerium,
which, giving a still more complex spectrum, would doubtless be
still more numerously resolved. Sir Norman Lockyer interprets
the observed phenomena as indicating the successive combinations,
in varying proportions, of a very few original ingredients;[663] but no
definite sign of their existence is perceptible; “protyle” seems
likely long to evade recognition; and the only intelligible underlying
principle for the reasonings employed—that of “one line, one
element”—implies a throng beyond counting of formative material
units.
Thus, added complexity is substituted for that fundamental unity
of matter which has long formed the dream of speculators. And it[Pg 210]
is extremely remarkable that Sir William Crookes, working along
totally different lines, has been led to analogous conclusions. To
take only one example. As the outcome of extremely delicate
operations of sifting and testing carried on for years, he finds that the
metal yttrium splits up into five, if not eight constituents.[664] Evidently,
old notions are doomed, nor are any preconceived ones likely
to take their place. It would seem, on the contrary, as if their complete
reconstruction were at hand. Subversive facts are steadily
accumulating; the revolutionary ideas springing from them tend,
if we interpret them aright, towards the substitution of electrical
for chemical theories of matter. Dissociation by the brute force of
heat is already nearly superseded, in the thoughts of physicists, by
the more delicate process of “ionisation.” Precisely what this implies
and involves we do not know; but the symptoms of its occurrence
are probably altogether different from those gathered by Sir Norman
Lockyer from the collation of celestial spectra.
A. J. Ångström of Upsala takes rank after Kirchhoff as a subordinate
founder, so to speak, of solar spectroscopy. His great map
of the “normal” solar spectrum[665] was published in 1868, two years
before he died. Robert Thalèn was his coadjutor in its execution,
and the immense labour which it cost was amply repaid by its
eminent and lasting usefulness. For more than a score of years it
held its ground as the universal standard of reference in all spectroscopic
inquiries within the range of the visible emanations. Those
that are invisible by reason of the quickness of their vibrations were
mapped by Dr. Henry Draper, of New York, in 1873, and with
superior accuracy by M. Cornu in 1881. The infra-red part of the
spectrum, investigated by Langley, Abney, and Knut Ångström,
reaches perhaps no definite end. The radiations oscillating too
slowly to affect the eye as light may pass by insensible gradations
into the long Hertzian waves of electricity.[666]
Professor Rowland’s photographic map of the solar spectrum,
published in 1886, and in a second enlarged edition in 1889, opened
fresh possibilities for its study, from far down in the red to high up
in the ultra-violet, and the accompanying scale of absolute wave-lengths[667]
has been, with trifling modifications, universally adopted.[Pg 211]
His new table of standard solar lines was published in 1893.[668]
Through his work, indeed, knowledge of the solar spectrum so far
outstripped knowledge of terrestrial spectra, that the recognition of
their common constituents was hampered by intolerable uncertainties.
Thousands of the solar lines charted with minute precision remained
unidentified for want of a corresponding precision in the
registration of metallic lines. Rowland himself, however, undertook
to provide a remedy. Aided by Lewis E. Jewell, he redetermined,
at the Johns Hopkins University, the wave-lengths of about 16,000
solar lines,[669] photographing for comparison with them the spectra of
all the known chemical elements except gallium, of which he could
procure no specimen. The labour of collation was well advanced
when he died at the age of fifty-two, April 16, 1901. Investigations
of metallic arc-spectra have also been carried out with signal
success by Hasselberg,[670] Kayser and Runge, O. Lohse,[671] and others.
Another condition sine quâ non of progress in this department is
the separation of true solar lines from those produced by absorption
in our own atmosphere. And here little remains to be done.
Thollon’s great Atlas[672] was designed for this purpose of discrimination.
Each of its thirty-three maps exhibits in quadruplicate a
subdivision of the solar spectrum under varied conditions of weather
and zenith-distance. Telluric effects are thus made easily legible,
and they account wholly for 866, partly for 246, out of a total of
3,200 lines. But the death of the artist, April 8, 1887, unfortunately
interrupted the half-finished task of the last seven years of his life.
A most satisfactory record, meanwhile, of selective atmospheric
action has been supplied by the experiments and determinations of
Janssen, Cornu and Egoroff, by Dr. Becker’s drawings,[673] and Mr.
McClean’s photographs of the analysed light of the sun at high,
low, and medium altitudes; and the autographic pictures obtained by
Mr. George Higgs, of Liverpool, of certain rhythmical groups in the
red, emerging with surprising strength near sunset, excite general
and well-deserved admiration.[674] The main interest, however, of all
these documents resides in the information afforded by them regarding
the chemistry of the sun.
The discovery that hydrogen exists in the atmosphere of the
sun was made by Ångström in 1862. His list of solar elements[Pg 212]
published in that year,[675] the result of an investigation separate
from, though conducted on the same principle as Kirchhoff’s,
included the substance which we now know to be predominant
among them. Dr. Plücker of Bonn had identified in 1859 the
Fraunhofer line F with the green ray of hydrogen, but drew no
inference from his observation. The agreement was verified by
Ångström; two further coincidences were established; and in
1866 a fourth hydrogen line in the extreme violet (named h) was
detected in the solar spectrum. With Thalèn, he besides added
manganese, titanium, and cobalt to the constituents of the sun
enumerated by Kirchhoff, and raised the number of identical
rays in the solar and terrestrial spectra of iron to no less
than 460.[676]
Thus, when Sir Norman Lockyer entered on that branch of
inquiry in 1872, fourteen substances were recognised as common
to the earth and sun. Early in 1878 he was able to increase the
list provisionally to thirty-three,[677] all except hydrogen metals. This
rapid success was due to his adoption of the test of length in lieu of
that of strength in the comparison of lines. He measured their
relative significance, in other words, rather by their persistence
through a wide range of temperature, than by their brilliancy at
any one temperature. The distinction was easily drawn. Photographs
of the electric arc, in which any given metal had been volatilised,
showed some of the rays emitted by it stretching across the
axis of the light to a considerable distance on either side, while many
others clung more or less closely to its central hottest core. The
former “long lines,” regarded as certainly representative, were
those primarily sought in the solar spectrum; while the attendant
“short lines,” often, in point of fact, due to foreign admixtures, were
set aside as likely to be misleading.[678] The criterion is a valuable
one, and its employment has greatly helped to quicken the progress
of solar chemistry.
Carbon was the first non-metallic element discovered in the
sun. Messrs. Trowbridge and Hutchins of Harvard College
concluded in 1887,[679] on the ground of certain spectral coincidences,
that this protean substance is vaporised in the solar atmosphere
at a temperature approximately that of the voltaic arc. Partial
evidence to the same effect had earlier been alleged by Lockyer,
as well as by Liveing and Dewar; and the case was rendered[Pg 213]
tolerably complete by photographs taken by Kayser and Runge
in 1889.[680] It was by Professor Rowland shown to be irresistible.
Two hundred carbon-lines were, through his comparisons, sifted
out from sunlight, and it contains others significant of the presence
of silicon—a related substance, and one as important to rock-building
on the earth, as carbon is to the maintenance of life.
The general result of Rowland’s labours was the establishment
among solar materials, not only of these two out of the fourteen
metalloids, or non-metallic substances, but of thirty-three metals,
including silver and tin. Gold, mercury, bismuth, antimony, and
arsenic were discarded from the catalogue; platinum and uranium,
with six other metals, remained doubtful; while iron was recorded
as crowding the spectrum with over two thousand obscure rays.[681]
Gallium-absorption was detected in it by Hartley and Ramage in
1889.[682]
Dr. Henry Draper[683] announced, in 1877, his imagined discovery,
in the solar spectrum, of eighteen especially brilliant spaces corresponding
to oxygen-emissions. But the agreement proved, when
put to the test of very high dispersion, to be wholly illusory.[684] Nor
has it yet been found possible to identify, in analysed sunlight, any
significant bright beams.[685]
The book of solar chemistry must be read in characters exclusively
of absorption. Nevertheless, the whole truth is unlikely to be
written there. That a substance displays none of its distinctive
beams in the spectrum of the sun or of a star, affords scarcely a presumption
against its presence. For it may be situated below the
level where absorption occurs, or under a pressure such as to efface
lines by widening and weakening them; it may be at a temperature
so high that it gives out more light than it takes up, and yet its
incandescence may be masked by the absorption of other bodies;
finally, it may just balance absorption by emission, with the result
of complete spectral neutrality. An instructive example is that
of the chromospheric element helium. Father Secchi remarked
in 1868[686] that there is no dark line in the solar spectrum matching
its light; and his observation has been fully confirmed.[687] Helium-absorption[Pg 214]
is, however, occasionally noticed in the penumbræ
of spots.[688]
Our terrestrial vital element might then easily subsist unrecognisably
in the sun. The inner organisation of the oxygen molecule is
a considerably plastic one. It is readily modified by heat, and these
modifications are reflected in its varying modes of radiating light.
Dr. Schuster enumerated in 1879[689] four distinct oxygen spectra,
corresponding to various stages of temperature, or phases of
electrical excitement; and a fifth has been added by M. Egoroff’s
discovery in 1883[690] that certain well-known groups of dark lines in
the red end of the solar spectrum (Fraunhofer’s A and B) are due to
absorption by the cool oxygen of our air. These persist down to
the lowest temperatures, and even survive a change of state. They
are produced essentially the same by liquid, as by aërial oxygen.[691]
It seemed, however, possible to M. Janssen that these bands
owned a joint solar and terrestrial origin. Oxygen in a fit condition
to produce them might, he considered, exist in the outer atmosphere
of the sun; and he resolved to decide the point. No one could
bring more skill and experience to bear upon it than he.[692] By
observations on the summit of the Faulhorn, as well as by direct
experiment, he demonstrated, nearly thirty years ago, the leading
part played by water-vapour in generating the atmospheric spectrum;
and he had recourse to similar means for appraising the share in it
assignable to oxygen. An electric beam, transmitted from the
Eiffel Tower to Meudon in the summer of 1888, having passed
through a weight of oxygen about equal to that piled above the
surface of the earth, showed the groups A and B just as they appear
in the high-sun spectrum.[693] Atmospheric action is then adequate to
produce them. But M. Janssen desired to prove, in addition, that
they diminish proportionately to its amount. His ascent of Mont
Blanc[694] in 1890 was undertaken with this object. It was perfectly
successful. In the solar spectrum, examined from that eminence,
oxygen-absorption was so much enfeebled as to leave no possible
doubt of its purely telluric origin. Under another form, nevertheless,
it has been detected as indubitably solar. A triplet of dark
lines low down in the red, photographed from the sun by Higgs and[Pg 215]
McClean, was clearly identified by Runge and Paschen in 1896[695] with
the fundamental group of an oxygen series, first seen by Piazzi
Smyth in the spectrum of a vacuum-tube in 1883.[696] The pabulum
vitæ of our earth is then to some slight extent effective in arresting
transmitted sunlight, and oxygen must be classed as a solar element.
The rays of the sun, besides being stopped selectively in our
atmosphere, suffer also a marked general absorption. This tells
chiefly upon the shortest wave-lengths; the ultra-violet spectrum is
in fact closed, as if by the interposition of an opaque screen. Nor
does the screen appear very sensibly less opaque from an elevation
of 10,000 feet. Dr. Simony’s spectral photographs, taken on the
Peak of Teneriffe,[697] extended but slightly further up than M. Cornu’s,
taken in the valley of the Loire. Could the veil be withdrawn,
some indications as to the originating temperature of the solar
spectrum might be gathered from its range, since the proportion
of quick vibrations given out by a glowing body grows with the
intensity of its incandescence. And this brings us to the subject of
our next Chapter.
FOOTNOTES:
[596] Phil. Mag., vol. xlii., p. 380, 1871.
[597] Astr. Nach., No. 3,053, Amer. Jour., vol. xlii., p. 162; Deslandres, Comptes
Rendus, t. cxiii., p. 307.
[598] Proc. Roy. Society, vol. lxi., p. 433.
[599] Phil. Mag., vol. xlii., p. 377.
[600] Frost-Scheiner, Astr. Spectroscopy, pp. 184, 423.
[601] Proc. Roy. Soc., vol. xvii., p. 302.
[602] Astr. Nach., No. 1,769.
[603] Am. Jour. of Science, vol. xv., p. 85.
[604] Journ. Franklin Institute, vol. xl., p. 232a.
[605] Pogg. Annalen, Bd. cxlvi., p. 475; Astr. Nach., No. 3,014.
[606] Astr. Nach., Nos. 3,006, 3,037.
[607] This device was suggested by Janssen in 1869.
[608] Astr. and Astrophysics, vol. xi., pp. 70, 407.
[609] Astr. and Astrophysics, vol. xi., p. 604.
[610] Comptes Rendus, t. cxiii., p. 307.
[611] Astr. and Astrophysics, vol. xi., p. 50.
[612] Ibid., pp. 60, 314.
[613] Wiedemann’s Annalen der Physik, Bd. xxv., p. 80.
[614] Evershed, Knowledge, vol. xxi., p. 133.
[615] Secchi, Le Soleil, t. ii., p. 294.
[616] Lockyer, Chemistry of the Sun, p. 418.
[617] L’Astronomie, August, 1884, p. 292 (Riccò); see also Evershed, Jour. British
Astr. Ass., vol. ii., p. 174.
[618] Averaging about 100 miles across and 300 high. Le Soleil, t. ii., p. 35.
[619] The Sun, p. 192.
[620] Astr. Nach., No. 1,854.
[621] Mem. degli Spettroscopisti Italiani, t. v., p. 4; Secchi, ibid., t. vi.,
p. 56.
[622] Its non-atmospheric character was early defined by Proctor, Month. Not.,
vol. xxxi., p. 196.
[623] Astroph. Jour., vol. vi., p. 412.
[624] Ibid., vol. xi., p. 165.
[625] Ibid., p. 243.
[626] Sun’s Place in Nature, pp. 111, 288.
[627] Abh. d. Kön. Böhm Ges. d. Wiss., Bd. ii., 1841-42, p. 467.
[628] In a paper read before the Société Philomathique de Paris, December 23, 1848,
and first published in extenso in Ann. de Chim. et de Phys., t. xix., p. 211 (1870).
Hippolyte Fizeau died in September, 1896.
[629] Astr. Nach., No. 1,772.
[630] Ibid., No. 1,864.
[631] A. Cornu, Sur la Méthode Doppler-Fizeau, p. D. 23.
[632] Am. Jour. of Sc., vol. xii., p. 321.
[633] Ibid., vol. xiv., p. 140.
[634] Bull. Astronom., February, 1884, p. 77.
[635] Comptes Rendus, t. xci., p. 368.
[636] Month. Not., vol. xliv., p. 170.
[637] See ante, p. 147.
[638] Recherches sur la Rotation du Soleil, Upsal, 1891.
[639] Harzer, Astr. Nach., No. 3,026; Stratonoff, Ibid., No. 3,344.
[640] Publ. Astr. Pacific Soc., vol. ii., p. 193.
[641] Proc. Roy. Society, vols. xvii., p. 415; xviii., p. 120.
[642] Comptes Rendus, t. cxii., p. 1421; t. cxiii., p. 310.
[643] At the sun’s distance, one second of arc represents about 450 miles.
[644] Amer. Jour. of Sc., vol. ii., p. 468, 1871.
[645] Month. Not., vol. xxxii., p. 51.
[646] Nature, vol. xxiii., p. 281.
[647] Comptes Rendus, t. lxxxvii., p. 532.
[648] Ibid., t. xcvi., p. 359.
[649] A. Brester, Théorie du Soleil, p. 66.
[650] Such prominences as have been seen to grow by the spread of incandescence
are of the quiescent kind, and present no deceptive appearance of violent motion.
[651] Proc. Roy. Soc., vol. xxviii., p. 157.
[652] “Evolution and the Spectroscope,” Pop. Science Monthly, January, 1873.
[653] Proc. Roy. Soc., vol. xxiv., p. 353. These are the H and K of prominences.
H. W. Vogel discovered in 1879 a hydrogen-line nearly coincident with H
(Monatsb. Preuss. Ak., February, 1879, p. 118).
[654] Proc. Roy. Soc., vol. xxviii., p. 444.
[655] Many of these were referred by Lockyer himself, who first sifted the
matter, to traces of the metals concerned.
[656] Chemistry of the Sun, p. 312; Proc. Roy. Society, vol. lvii., p. 199.
[657] Lockyer’s Chemistry of the Sun, p. 324.
[658] Month. Not., vol. li., p. 76.
[659] Ibid., vol. lviii., p. 370.
[660] Astr. and Astrophysics, vol. xi., p. 615.
[661] Thollon’s estimate (Comptes Rendus, t. xcvii., p. 902) of 300,000 kilometres,
seems considerably too low. Limiting the “average prominence region” to a
shell 54,000 miles deep (2′ of arc as seen from the earth), the visual line will,
at mid-height (27,000 miles from the sun’s surface), travel through (in round
numbers) 320,000 miles of that region.
[662] Liveing and Dewar, Phil. Mag., vol. xvi. (5th ser.), p. 407.
[663] Chemistry of the Sun, p. 260.
[664] Nature, October 14, 1886.
[665] The normal spectrum is that depending exclusively upon wave-length—the
fundamental constant given by nature as regards light. It is obtained by the
interference of rays, in the manner first exemplified by Fraunhofer, and affords
the only unvarying standard for measurement. In the refraction spectrum (upon
which Kirchhoff’s map was founded), the relative positions of the lines vary with
the material of the prisms.
[666] Scheiner, Die Spectralanalyse der Gestirne, p. 168.
[667] Phil. Mag., vol. xxvii., p. 479.
[668] Astr. and Astrophysics, vol. xii., p. 321; Frost-Scheiner, Astr. Spectr., p. 363.
[669] Published in Astroph. Jour., vols. i. to vi.
[670] Astr. and Astrophysics, vol. xi., p. 793.
[671] Astroph. Jour., vol. vi., p. 95.
[672] Annales de l’Observatoire de Nice, t. iii., 1890.
[673] Trans. Royal Society of Edinburgh, vol. xxxvi., p. 99.
[674] Rev. A. L. Cortie, Astr. and Astrophysics, vol. xi., p. 401. Specimens of his
photographs were given by Ranyard in Knowledge, vol. xiii., p. 212.
[675] Ann. d. Phys., Bd. cxvii., p. 296.
[676] Comptes Rendus, t. lxiii., p. 647.
[677] Ibid., t. lxxxvi., p. 317. Some half dozen of these identifications have
proved fallacious.
[678] Chemistry of the Sun, p. 143.
[679] Amer. Jour. of Science, vol. xxxiv., p. 348.
[680] Berlin Abhandlungen, 1889.
[681] Amer. Jour. of Science, vol. xli., p. 243. See Appendix, Table II.
[682] Astrophy. Jour., vol. ix., p. 219; Fowler, Knowledge, vol. xxiii., p. 11.
[683] Amer. Jour, of Science, vol. xiv., p. 89; Nature, vol. xvi., p. 364; Month.
Not., vol. xxxix., p. 440.
[684] Month. Not., vol. xxxviii., p. 473; Trowbridge and Hutchins, Amer. Jour.
of Science, vol. xxxiv., p. 263.
[685] Scheiner, Die Spectralanalyse, p. 180.
[686] Comptes Rendus, t. lxvii., p. 1123.
[687] Rev. A. L. Cortie, Month. Not., vol. li., p. 18.
[688] Young, The Sun, p. 135; Hale, Astr. and Astrophysics, vol. xi., p. 312
Buss, Jour. Brit. Astr. Ass., vol. ix., p. 253.
[689] Phil. Trans., vol. clxx., p. 46.
[690] Comptes Rendus, t. xcvii., p. 555; t. ci., p. 1145.
[691] Liveing and Dewar, Astr. and Astrophysics, vol. xi., p. 705.
[692] Comptes Rendus, t. lx., p. 213; t. lxiii., p. 289.
[693] Ibid., t. cviii., p. 1035.
[694] Ibid., t. cxi., p. 431.
[695] Astroph. Jour., vols. iv., p. 317; vi., p. 426.
[696] Trans. Roy. Soc. Edin., vol. xxxii., p. 452.
[697] Comptes Rendus, t. cxi., p. 941; Huggins, Proc. Roy. Soc., vol. xlvi., p. 168.
CHAPTER V
TEMPERATURE OF THE SUN
Newton was the first who attempted to measure the quantity of
heat received by the earth from the sun. His object in making the
experiment was to ascertain the temperature encountered by the
comet of 1680 at its passage through perihelion. He found it, by
multiplying the observed heating effects of direct sunshine according
to the familiar rule of the “inverse squares of the distances,” to be
about 2,000 times that of red-hot iron.[698]
Determinations of the sun’s thermal power, made with some
scientific exactness, date, however, from 1837. A few days previous
to the beginning of that year, Herschel began observing at the Cape
of Good Hope with an “actinometer,” and obtained results agreeing
quite satisfactorily with those derived by Pouillet from experiments
made in France some months later with a “pyrheliometer.”[699]
Pouillet found that the vertical rays of the sun falling on each
square centimetre of the earth’s surface are competent (apart from
atmospheric absorption) to raise the temperature of 1·7633 grammes
of water one degree Centigrade per minute. This number (1·7633)
he called the “solar constant”; and the unit of heat chosen is
known as the “calorie.” Hence it was computed that the total
amount of solar heat received during a year would suffice to melt a
layer of ice covering the entire earth to a depth of 30·89 metres, or
100 feet; while the heat emitted would melt, at the sun’s surface, a
stratum 11·80 metres thick each minute. A careful series of observations
showed that nearly half the heat incident upon our
atmosphere is stopped in its passage through it.
Herschel got somewhat larger figures, though he assigned only
a third as the spoil of the air. Taking a mean between his own and
Pouillet’s, he calculated that the ordinary expenditure of the sun
per minute would have power to melt a cylinder of ice 184 feet in
diameter, reaching from his surface to that of α Centauri; or,[Pg 217]
putting it otherwise, that an ice-rod 45·3 miles across, continually
darted into the sun with the velocity of light, would scarcely
consume, in dissolving, the thermal supplies now poured abroad
into space.[700] It is nearly certain that this estimate should be
increased by about two-thirds in order to bring it up to the
truth.
Nothing would, at first sight, appear simpler than to pass
from a knowledge of solar emission—a strictly measurable
quantity—to a knowledge of the solar temperature; this being
defined as the temperature to which a surface thickly coated with
lamp-black (that is, of standard radiating power) should be raised to
enable it to send us, from the sun’s distance, the amount of heat
actually received from the sun. Sir John Herschel showed that
heat-rays at the sun’s surface must be 92,000 times as dense as
when they reach the earth; but it by no means follows that either
the surface emitting, or a body absorbing those heat-rays must be
92,000 times hotter than a body exposed here to the full power of
the sun. The reason is, that the rate of emission—consequently
the rate of absorption, which is its correlative—increases very much
faster than the temperature. In other words, a body radiates or
cools at a continually accelerated pace as it becomes more and more
intensely heated above its surroundings.
Newton, however, took it for granted that radiation and
temperature advance pari passu—that you have only to ascertain
the quantity of heat received from, and the distance of a remote
body in order to know how hot it is.[701] And the validity of this
principle, known as “Newton’s Law” of cooling, was never
questioned until De la Roche pointed out, in 1812,[702] that it was
approximately true only over a low range of temperature; while
five years later, Dulong and Petit generalised experimental results
into the rule, that while temperature grows by arithmetical,
radiation increases by geometrical progression.[703] Adopting this
formula, Pouillet derived from his observations on solar heat a solar
temperature of somewhere between 1,461° and 1,761° C. Now, the
higher of these points—which is nearly that of melting platinum—is
undoubtedly surpassed at the focus of certain burning-glasses
which have been constructed of such power as virtually
to bring objects placed there within a quarter of a million of miles
of the photosphere. In the rays thus concentrated, platinum and[Pg 218]
diamond become rapidly vaporised, notwithstanding the great loss
of heat by absorption, first in passing through the air, and again
in traversing the lens. Pouillet’s maximum is then manifestly too
low, since it involves the absurdity of supposing a radiating mass
capable of heating a distant body more than it is itself heated.
Less demonstrably, but scarcely less surely, Mr. J. J. Waterston,
who attacked the problem in 1860, erred in the opposite direction.
Working up, on Newton’s principle, data collected by himself in
India and at Edinburgh, he got for the “potential temperature” of
the sun 12,880,000° Fahr.,[704] equivalent to 7,156,000° C. The phrase
potential temperature (for which Violle substituted, in 1876, effective
temperature) was designed to express the accumulation in a single
surface, postulated for the sake of simplicity, of the radiations
not improbably received from a multitude of separate solar layers
reinforcing each other; and might thus (it was explained) be considerably
higher than the actual temperature of any one stratum.
At Rome, in 1861, Father Secchi repeated Waterston’s experiments,
and reaffirmed his conclusion;[705] while Soret’s observations,
made on the summit of Mont Blanc in 1867,[706] furnished him with
materials for a fresh and even higher estimate of ten million degrees
Centigrade.[707] Yet from the very same data, substituting Dulong
and Petit’s for Newton’s law, Vicaire deduced in 1872 a provisional
solar temperature of 1,398°.[708] This is below that at which iron
melts, and we know that iron-vapour exists high up in the sun’s
atmosphere. The matter was taken into consideration on the
other side of the Atlantic by Ericsson in 1871. He attempted to
re-establish the shaken credit of Newton’s principle, and arrived,
by its means, at a temperature of 4,000,000° Fahrenheit.[709] Subsequently,
an “underrated computation,” based upon observation of the
quantity of heat received by his “sun motor,” gave him 3,000,000°.
And the result, as he insisted, followed inevitably from the principle
that the temperature produced by radiant heat is proportional to its
density, or inversely as its diffusion.[710] The principle, however, is
demonstrably unsound.
In 1876 the sun’s temperature was proposed as the subject of a
prize by the Paris Academy of Sciences; but although the essay of
M. Jules Violle was crowned, the problem was declared to remain
unsolved. Violle (who adhered to Dulong and Petit’s formula)[Pg 219]
arrived at an effective temperature of 1,500° C., but considered that
it might actually reach 2,500° C., if the emissive power of the photospheric
clouds fell far short (as seemed probable) of the lamp-black
standard.[711] Experiments made in April and May, 1881, giving a
somewhat higher result, he raised this figure to 3,000° C.[712]
Appraisements so outrageously discordant as those of Waterston,
Secchi, and Ericsson on the one hand, and those of the French
savants on the other, served only to show that all were based upon a
vicious principle. Professor F. Rosetti,[713] accordingly, of the Paduan
University, at last perceived the necessity for getting out of
the groove of “laws” plainly in contradiction with facts. The
temperature, for instance, of the oxy-hydrogen flame was fixed by
Bunsen at 2,800° C.—an estimate certainly not very far from the
truth. But if the two systems of measurement applied to the sun
be used to determine the heat of a solid body rendered incandescent
in this flame, it comes out, by Newton’s mode of calculation, 45,000°
C.; by Dulong and Petit’s, 870° C.[714] Both, then, are justly discarded,
the first as convicted of exaggeration, the second of undervaluation.
The formula substituted by Rosetti in 1878 was tested
successfully up to 2,000° C.; but since, like its predecessors, it was
a purely empirical rule, guaranteed by no principle, and hence not
to be trusted out of sight, it was, like them, liable to break down at
still higher elevations. Radiation by this new prescription increases
as the square of the absolute temperature—that is, of the number of
degrees counted from the “absolute zero” of -273° C. Its employment
gave for the sun’s radiating surface an effective temperature
of 20,380° C. (including a supposed loss of one-half in the solar
atmosphere); and setting a probable deficiency in emission (as
compared with lamp-black) against a probable mutual reinforcement
of superposed strata, Professor Rosetti considered “effective” as
nearly equivalent to “actual” temperature. A “law of cooling,”
proposed by M. Stefan at Vienna in 1879,[715] was shown by Boltzmann,
many years later, to have a certain theoretical validity.[716] It
is that emission grows as the fourth power of absolute temperature.
Hence the temperature of the photosphere would be proportional to
the square root of the square root of its heating effects at a distance,
and appeared, by Stefan’s calculations from Violle’s measures of solar
radiative intensity, to be just 6,000° C.; while M. H. Le Chatelier[717][Pg 220]
derived 7,600° from a formula, conveying an intricate and unaccountable
relation between the temperature of an incandescent body and
the intensity of its red radiations.
From a series of experiments carefully conducted at Daramona,
Ireland, with a delicate thermal balance, of the kind invented by
Boys and designated a “radio-micrometer,” Messrs. Wilson and
Gray arrived in 1893, with the aid of Stefan’s Law, at a photospheric
temperature of 7,400° C.,[718] reduced by the first-named investigator
in 1901 to 6,590°.[719] Dr. Paschen, of Hanover, on the
other hand, ascribed to the sun a temperature of 5,000° from
comparisons between solar radiative intensity and that of glowing
platinum;[720] while F. W. Very showed in 1895[721] that a minimum
value of 20,000° C. for the same datum resulted from Paschen’s
formula connecting temperature with the position of maximum
spectral energy.
A new line of inquiry was struck out by Zöllner in 1870. Instead
of tracking the solar radiations backward with the dubious guide
of empirical formulæ, he investigated their intensity at their source.
He showed[722] that, taking prominences to be simple effects of the
escape of powerfully compressed gases, it was possible, from the
known mechanical laws of heat and gaseous constitution, to deduce
minimum values for the temperatures prevailing in the area
of their development. These came out 27,700° C. for the strata
lying immediately above, and 68,400° C. for the strata lying immediately
below the photosphere, the former being regarded as the
region into which, and the latter as the region from which the
eruptions took place. In this calculation, no prominences exceeding
40,000 miles (1·5′) in height were included. But in 1884, G. A. Hirn
of Colmar, having regard to the enormous velocities of projection
observed in the interim, fixed two million degrees Centigrade as the
lowest internal temperature by which they could be accounted for;
although admitting the photospheric condensations to be incompatible
with a higher external temperature than 50,000° to 100,000° C.[723]
This method of going straight to the sun itself, observing what
goes on there, and inferring conditions, has much to recommend it;
but its profitable use demands knowledge we are still very far from
possessing. We are quite ignorant, for instance, of the actual
circumstances attending the birth of the solar flames. The assumption
that they are nothing but phenomena of elasticity is a purely
gratuitous one. Spectroscopic indications, again, give hope of
eventually affording a fixed point of comparison with terrestrial[Pg 221]
heat sources; but their interpretation is still beset with uncertainties;
nor can, indeed, the expression of transcendental temperatures
in degrees of impossible thermometers be, at the best, other
than a futile attempt to convey notions respecting a state of things
altogether outside the range of our experience.
A more tangible, as well as a less disputable proof of solar radiative
intensity than any mere estimates of temperature, was provided
in some experiments made by Professor Langley in 1878.[724] Using
means of unquestioned validity, he found the sun’s disc to radiate
87 times as much heat, and 5,300 times as much light as an equal
area of metal in a Bessemer converter after the air-blast had continued
about twenty minutes. The brilliancy of the incandescent
steel, nevertheless, was so blinding, that melted iron, flowing in a
dazzling white-hot stream into the crucible, showed “deep brown by
comparison, presenting a contrast like that of dark coffee poured
into a white cup.” Its temperature was estimated (not quite
securely)[725] at about 2,000° C.; and no allowances were made, in
computing relative intensities, for atmospheric ravages on sunlight,
for the extra impediments to its passage presented by the smoke-laden
air of Pittsburgh, or for the obliquity of its incidence. Thus,
a very large balance of advantage lay on the side of the metal.
A further element of uncertainty in estimating the intrinsic
strength of the sun’s rays has still to be considered. From the
time that his disc first began to be studied with the telescope, it
was perceived to be less brilliant near the edges. Lucas Valerius,
of the Lyncean Academy, seems to have been the first to note this
fact, which, strangely enough, was denied by Galileo in a letter to
Prince Cesi of January 25, 1613.[726] Father Scheiner, however, fully
admitted it, and devoted some columns of his bulky tome to the
attempt to find its appropriate explanation.[727] In 1729 Bouguer
measured, with much accuracy, the amount of this darkening; and
from his data, Laplace, adopting a principle of emission now known
to be erroneous, concluded that the sun loses eleven-twelfths of his
light through absorption in his own atmosphere.[728] The real existence
of this atmosphere, which is totally distinct from the beds of
ignited vapours producing the Fraunhofer lines, is not open to
doubt, although its nature is still a matter of conjecture. The
separate effects of its action on luminous, thermal, and chemical
rays were carefully studied by Father Secchi, who in 1870[729] inferred
the total absorption to be 88/100 of all radiations taken together, and
added the important observation that the light from the limb is no[Pg 222]
longer white, but reddish-brown. Absorptive effects were thus seen
to be unequally distributed; and they could evidently be studied
to advantage only by taking the various rays of the spectrum
separately, and finding out how much each had suffered in transmission.
This was done by H. C. Vogel in 1877.[730] Using a polarising
photometer, he found that only 13 per cent. of the violet rays
escape at the edge of the solar disc, 16 of the blue and green,
25 of the yellow, and 30 per cent. of the red. Midway between
centre and limb, 88·7 of violet light and 96·7 of red penetrate the
absorbing envelope, the abolition of which would increase the
intensity of the sun’s visible spectrum above two and a half times
in the most, and once and a half times in the least refrangible parts.
The nucleus of a small spot was ascertained to be of the same
luminous intensity as a portion of the unbroken surface about two
and a half minutes from the limb. These experiments having been
made during a spot-minimum when there is reason to think that
absorption is below its average strength, Vogel suggested their
repetition at a time of greater activity. They were extended to the
heat-rays by Edwin B. Frost. Detailed inquiries made at Potsdam
in 1892[731] went to show that, were the sun’s atmosphere removed,
his thermal power, as regards ourselves, would be increased 1·7
times. They established, too, the practical uniformity in radiation
of all parts of his disc. A confirmatory result was obtained about
the same time by Wilson and Rambaut, who found that the unveiled
sun would be once and a half times hotter than the actual sun.[732]
Professor Langley, now of Washington, gave to measures of the
kind a refinement previously undreamt of. Reliable determinations
of the “energy” of the individual spectral rays were, for the first
time, rendered possible by his invention of the “bolometer” in
1880.[733] This exquisitely sensitive instrument affords the means of
measuring heat, not directly, like the thermopile, but in its effects
upon the conduction of electricity. It represents, in the phrase of
the inventor, the finger laid upon the throttle-valve of a steam-engine.
A minute force becomes the modulator of a much greater
force, and thus from imperceptible becomes conspicuous. By locally
raising the temperature of an inconceivably fine strip of platinum
serving as the conducting-wire in a circuit, the flow of electricity
is impeded at that point, and the included galvanometer records
a disturbance of the electrical flow. Amounts of heat were thus[Pg 223]
detected in less than ten seconds, which, expended during a thousand
years on the melting of a kilogramme of ice, would leave a part of
the work still undone; and further improvements rendered this
marvellous instrument capable of thrilling to changes of temperature
falling short of one ten-millionth of a degree Centigrade.[734]
The heat contained in the diffraction spectrum is, with equal
dispersions, barely one-tenth of that in the prismatic spectrum. It
had, accordingly, never previously been found possible to measure
it in detail—that is, ray by ray. But it is only from the diffraction,
or normal spectrum that any true idea can be gained as to the real
distribution of energy among the various constituents, visible and
invisible, of a sunbeam. The effect of passage through a prism is
to crowd together the red rays very much more than the blue. To
this prismatic distortion was owing the establishment of a pseudo-maximum
of heat in the infra-red, which disappeared when the
natural arrangement by wave-length was allowed free play.
Langley’s bolometer has shown that the hottest part of the normal
spectrum virtually coincides with its most luminous part, both
lying in the orange, close to the D-line.[735] Thus the last shred of
evidence in favour of the threefold division of solar radiations
vanished, and it became obvious that the varying effects—thermal,
luminous, or chemical—produced by them are due, not to any distinction
of quality in themselves, but to the different properties of
the substances they impinge upon. They are simply bearers of
energy, conveyed in shorter or longer vibrations; the result in each
separate case depending upon the capacity of the material particles
meeting them for taking up those shorter or longer vibrations, and
turning them variously to account in their inner economy.
A long series of experiments at Allegheny was completed in the
summer of 1881 on the crest of Mount Whitney in the Sierra
Nevada. Here, at an elevation of 14,887 feet, in the driest and
purest air, perhaps, in the world, atmospheric absorptive inroads
become less sensible, and the indications of the bolometer, consequently,
surer and stronger. An enormous expansion was at once
given to the invisible region in the solar spectrum below the red.
Captain Abney had got chemical effects from undulations twelve
ten-thousandths of a millimetre in length. These were the longest
recognised as, or indeed believed, on theoretical grounds, to be
capable of existing. Professor Langley now got heating effects from
rays of above twice that wave-length, his delicate thread of platinum
groping its way down nearly to thirty ten-thousandths of a millimetre,[Pg 224]
or three “microns.” The known extent of the solar spectrum
was thus at once more than doubled. Its visible portion covers a
range of about one octave; bolometric indications already in 1884
comprised between three and four. The great importance of the
newly explored region appears from the fact that three-fourths of
the entire energy of sunlight reside in the infra-red, while scarcely
more than one-hundredth part of that amount is found in the better
known ultra-violet space.[736] These curious facts were reinforced, in
1886,[737] by further particulars learned with the help of rock-salt
lenses and prisms, glass being impervious to very slow, as to very
rapid vibrations. Traces were thus detected of solar heat distributed
into bands of transmission alternating with bands of atmospheric
absorption, far beyond the measurable limit of 5·3 microns.
In 1894, Langley described at the Oxford Meeting of the British
Association[738] his new “bolographic” researches, in which the sensitive
plate was substituted for the eye in recording deflections of the
galvanometer responding to variations of invisible heat. Finally,
in 1901,[739] he embodied in a splendid map of the infra-red spectrum
740 absorption-lines of determinate wave-lengths, ranging from
0·76 to 5·3 microns. Their chemical origin, indeed, remains almost
entirely unknown, no extensive investigations having yet been
undertaken of the slower vibrations distinctive of particular substances;
but there is evidence that seven of the nine great bands
crossing the “new spectrum” (as Langley calls it)[740] are telluric, and
subject to seasonal change. Here, then, he thought, might eventually
be found a sure standing-ground for vitally important previsions of
famines, droughts, and bonanza-crops.
Atmospheric absorption had never before been studied with such
precision as it was by Langley on Mount Whitney. Aided by
simultaneous observations from Lone Pine, at the foot of the Sierra,
he was able to calculate the intensity belonging to each ray before
entering the earth’s gaseous envelope—in other words, to construct
an extra-atmospheric curve of energy in the spectrum. The result
showed that the blue end suffered far more than the red, absorption
varying inversely as wave-length. This property of stopping predominantly
the quicker vibrations is shared, as both Vogel and[Pg 225]
Langley[741] have conclusively shown, by the solar atmosphere. The
effect of this double absorption is as if two plates of reddish glass
were interposed between us and the sun, the withdrawal of which
would leave his orb, not only three or four times more brilliant, but
in colour distinctly greenish-blue.[742]
The fact of the uncovered sun being blue has an important
bearing upon the question of his temperature, to afford a somewhat
more secure answer to which was the ultimate object of Professor
Langley’s persevering researches; for it is well known that as
bodies grow hotter, the proportionate representation in their
spectra of the more refrangible rays becomes greater. The lowest
stage of incandescence is the familiar one of red heat. As it gains
intensity, the quicker vibrations come in, and an optical balance of
sensation is established at white heat. The final term of blue heat,
as we now know, is attained by the photosphere. On this ground
alone, then, of the large original preponderance of blue light, we
must raise our estimate of solar heat; and actual measurements
show the same upward tendency. Until quite lately, Pouillet’s
figure of 1.7 calories per minute per square centimetre of terrestrial
surface, was the received value for the “solar constant.” Forbes
had, it is true, got 2.85 from observations on the Faulhorn in 1842;[743]
but they failed to obtain the confidence they merited. Pouillet’s
result was not definitely superseded until Violle, from actinometrical
measures at the summit and base of Mont Blanc in 1875, computed
the intensity of solar radiation at 2.54,[744] and Crova, about the
same time, at Montpellier, showed it to be above two calories.[745]
Langley went higher still. Working out the results of the Mount
Whitney expedition, he was led to conclude atmospheric absorption
to be fully twice as effective as had hitherto been supposed.
Scarcely 60 per cent., in fact, of those solar radiations which strike
perpendicularly through a seemingly translucent sky, were estimated
to attain the sea-level. The rest are reflected, dispersed, or absorbed.
This discovery involved a large addition to the original supply so
mercilessly cut down in transmission, and the solar constant rose at
once to three calories. Nor did the rise stop there. M. Savélieff
deduced for it a value of 3.47 from actinometrical observations
made at Kieff in 1890;[746] and Knut Ångström, taking account of the
arrestive power of carbonic acid, inferred enormous atmospheric
absorption, and a solar constant of four calories.[747] In other words,
the sun’s heat reaching the outskirts of our atmosphere is capable[Pg 226]
of doing without cessation the work of an engine of four-horse
power for each square yard of the earth’s surface. Thus, modern
inquiries tend to render more and more evident the vastness of the
thermal stores contained in the great central reservoir of our system,
while bringing into fair agreement the estimates of its probable temperature.
This is in great measure due to the acquisition of a
workable formula by which to connect temperature with radiation.
Stefan’s rule of a fourth-power relation, if not actually a law of
nature, is a colourable imitation of one; and its employment has
afforded a practical certainty that the sun’s temperature, so far
as it is definable, neither exceeds 12,000° C., nor falls short of
6,500° C.
FOOTNOTES:
[698] Principia, p. 498 (1st ed.).
[699] Comptes Rendus, t. vii., p. 24.
[700] Results of Astr. Observations, p. 446.
[701] “Est enim calor solis ut radiorum densitas, hoc est, reciproce ut quadratum
distantiæ locorum a sole.”—Principia, p. 508 (3d ed., 1726).
[702] Jour. de Physique, t. lxxv., p. 215.
[703] Ann. de Chimie, t. vii., 1817, p. 365.
[704] Phil. Mag., vol. xxiii. (4th ser.), p. 505.
[705] Nuovo Cimento, t. xvi., p. 294.
[706] Comptes Rendus, t. lxv., p. 526.
[707] The direct result of 5-1/3 million degrees was doubled in allowance for absorption
in the sun’s own atmosphere. Comptes Rendus, t. lxxiv., p. 26.
[708] Ibid., p. 31.
[709] Nature, vols. iv., p. 204; v., p. 505.
[710] Ibid., vol. xxx., p. 467.
[711] Ann. de Chim., t. x. (5th ser.), p. 361.
[712] Comptes Rendus, t. xcvi., p. 254.
[713] Phil. Mag., vol. viii., p. 324, 1879.
[714] Ibid., p. 325.
[715] Sitzungsberichte, Wien, Bd. lxxix., ii., p. 391.
[716] Wiedemann’s Annalen, Bd. xxii., p. 291; Scheiner, Strahlung und Temperatur
der Sonne, p. 27.
[717] Comptes Rendus, March 28, 1892; Astr. and Astrophysics, vol. xi., p. 517.
[718] Phil. Trans., vol. clxxxv., p. 361.
[719] Proc. Roy. Society, December 12, 1901.
[720] Scheiner, Temp. der Sonne, p. 36.
[721] Astroph. Jour., vol. ii., p. 318.
[722] Astr. Nach., Nos. 1,815-16.
[723] L’Astronomie, September, 1884, p. 334.
[724] Amer. Jour. of Science, vol. i. (3rd ser.), p. 653.
[725] Young, The Sun, p. 310.
[726] Op., t. vi., p. 198.
[727] Rosa Ursina, lib. iv., p. 618.
[728] Méc. Cél., liv. x., p. 323.
[729] Le Soleil (1st ed.), p. 136.
[730] Monatsber., Berlin, 1877, p. 104.
[731] Astr. Nach., Nos. 3,105-6; Astr. and Astrophysics, vol. xi., p. 720.
[732] Proc. Roy. Irish Acad., vol. ii., No. 2, 1892.
[733] Am. Jour. of Sc., vol. xxi., p. 187.
[734] Amer. Jour. of Science, vol. v., p. 245, 1898.
[735] For J. W. Draper’s partial anticipation of this result, see Ibid. vol. iv.,
1872, p. 174.
[736] Phil. Mag., vol. xiv., p. 179, 1883.
[737] “The Solar and the Lunar Spectrum,” Memoirs National Acad. of Science,
vol. xxxii.; “On hitherto Unrecognised Wave-lengths,” Amer. Jour. of Science,
vol. xxxii., August, 1886.
[738] Astroph. Jour., vol. i., p. 162.
[739] Annals of the Smithsonian Astroph. Observatory, vol. i.; Comptes Rendus,
t. cxxxi., p. 734; Astroph. Jour., vol. iii., p. 63.
[740] Phil. Mag., July, 1901.
[741] Comptes Rendus, t. xcii., p. 701.
[742] Nature, vol. xxvi., p. 589.
[743] Phil. Trans., vol. cxxxii., p. 273.
[744] Ann. de Chim., t. x., p. 321.
[745] Ibid., t. xi., p. 505.
[746] Comptes Rendus, t. cxii., p. 1200.
[747] Wied. Ann., Bd. xxxix., p. 294; Scheiner, Temperatur der Sonne, pp. 36, 38.
CHAPTER VI
THE SUN’S DISTANCE
The question of the sun’s distance arises naturally from the consideration
of his temperature, since the intensity of the radiations
emitted as compared with those received and measured, depends
upon it. But the knowledge of that distance has a value quite
apart from its connection with solar physics. The semi-diameter of
the earth’s orbit is our standard measure for the universe. It is the
great fundamental datum of astronomy—the unit of space, any
error in the estimation of which is multiplied and repeated in a
thousand different ways, both in the planetary and sidereal systems.
Hence its determination was called by Airy “the noblest problem
in astronomy.” It is also one of the most difficult. The quantities
dealt with are so minute that their sure grasp tasks all the resources
of modern science. An observational inaccuracy which would set the
moon nearer to, or farther from us than she really is by one hundred
miles, would vitiate an estimate of the sun’s distance to the extent
of sixteen million![748] What is needed in order to attain knowledge
of the desired exactness is no less than this: to measure an angle
about equal to that subtended by a halfpenny 2,000 feet from the
eye, within a little more than a thousandth part of its value.
The angle thus represented is what is called the “horizontal
parallax” of the sun. By this amount—the breadth of a halfpenny
at 2,000 feet—he is, to a spectator on the rotating earth,
removed at rising and setting from his meridian place in the
heavens. Such, in other terms, would be the magnitude of the
terrestrial radius as viewed from the sun. If we knew this magnitude
with certainty and precision, we should also know with certainty
and precision—the dimensions of the earth being, as they
are, well ascertained—the distance of the sun. In fact, the one
quantity commonly stands for the other in works treating professedly
of astronomy. But this angle of parallax or apparent[Pg 228]
displacement cannot be directly measured—cannot even be perceived
with the finest instruments. Not from its smallness. The
parallactic shift of the nearest of the stars as seen from opposite
sides of the earth’s orbit, is many times smaller. But at the sun’s
limb, and close to the horizon, where the visual angle in question
opens out to its full extent, atmospheric troubles become overwhelming,
and altogether swamp the far more minute effects of
parallax.
There remain indirect methods. Astronomers are well acquainted
with the proportions which the various planetary orbits bear to
each other. They are so connected, in the manner expressed by
Kepler’s Third Law, that the periods being known, it only needs to
find the interval between any two of them in order to infer at once
the distances separating them all from one another and from the
sun. The plan is given; what we want to discover is the scale
upon which it is drawn; so that, if we can get a reliable measure
of the distance of a single planet from the earth, our problem is
solved.
Now some of our fellow-travellers in our unending journey
round the sun, come at times well within the scope of celestial
trigonometry. The orbit of Mars lies at one point not more than
thirty-five million miles outside that of the earth, and when the
two bodies happen to arrive together in or near the favourable spot—a
conjuncture which occurs every fifteen years—the desired
opportunity is granted. Mars is then “in opposition,” or on the
opposite side of us from the sun, crossing the meridian consequently
at midnight.[749] It was from an opposition of Mars, observed in 1672
by Richer at Cayenne in concert with Cassini in Paris, that the first
scientific estimate of the sun’s distance was derived. It appeared
to be nearly eighty-seven millions of miles (parallax 9·5′); while
Flamsteed deduced 81,700,000 (parallax 10′) from his independent
observations of the same occurrence—a difference quite insignificant
at that stage of the inquiry. But Picard’s result was just half Flamsteed’s
(parallax 20′; distance forty-one million miles); and Lahire
considered that we must be separated from the hearth of our system
by an interval of at least 136 million miles.[750] So that uncertainty
continued to have an enormous range.
Venus, on the other hand, comes closest to the earth when she
passes between it and the sun. At such times of “inferior conjunction”
she is, however, still twenty-six million miles, or (in[Pg 229]
round numbers) 109 times as distant as the moon. Moreover, she
is so immersed in the sun’s rays that it is only when her path lies
across his disc that the requisite facilities for measurement are
afforded. These “partial eclipses of the sun by Venus” (as Encke
termed them) are coupled together in pairs,[751] of which the components
are separated by eight years, recurring at intervals alternately
of 105-1/2 and 121-1/2 years. Thus, the first calculated transit
took place in December, 1631, and its companion (observed by
Horrocks) in the same month (N.S.), 1639. Then, after the lapse
of 121-1/2 years, came the June couple of 1761 and 1769; and again
after 105-1/2, the two last observed, December 8, 1874, and December
6, 1882. Throughout the twentieth century there will be no
transit of Venus; but the astronomers of the twenty-first will only
have to wait four years for the first of a June pair. The rarity of
these events is due to the fact that the orbits of the earth and
Venus do not lie in the same plane. If they did, there would be a
transit each time that our twin-planet overtakes us in her more rapid
circling—that is, on an average, every 584 days. As things are
actually arranged, she passes above or below the sun, except when
she happens to be very near the line of intersection of the two
tracks.
Such an occurrence as a transit of Venus seems, at first sight,
full of promise for solving the problem of the sun’s distance. For
nothing would appear easier than to determine exactly either the
duration of the passage of a small, dark orb across a large brilliant
disc, or the instant of its entry upon or exit from it. And the
differences in these times (which, owing to the comparative nearness
of Venus, are quite considerable), as observed from remote parts of
the earth, can be translated into differences of space—that is, into
apparent or parallactic displacements, whereby the distance of Venus
becomes known, and thence, by a simple sum in proportion, the
distance of the sun. But in that word “exactly” what snares and
pitfalls lie hid! It is so easy to think and to say; so indefinitely
hard to realise. The astronomers of the eighteenth century were
full of hope and zeal. They confidently expected to attain, through[Pg 230]
the double opportunity offered them, to something like a permanent
settlement of the statistics of our system. They were grievously
disappointed. The uncertainty as to the sun’s distance, which they
had counted upon reducing to a few hundred thousand miles, remained
at many millions.
In 1822, however, Encke, then director of the Seeberg Observatory
near Gotha, undertook to bring order out of the confusion of discordant,
and discordantly interpreted observations. His combined
result for both transits (1761 and 1769) was published in 1824,[752] and
met universal acquiescence. The parallax of the sun thereby established
was 8·5776′, corresponding to a mean distance[753] of 95-1/4 million
miles. Yet this abolition of doubt was far from being so satisfactory
as it seemed. Serenity on the point lasted exactly thirty years. It
was disturbed in 1854 by Hansen’s announcement[754] that the observed
motions of the moon could be drawn into accord with theory only
on the terms of bringing the sun considerably nearer to us than he
was supposed to be.
Dr. Matthew Stewart, professor of mathematics in the University
of Edinburgh, had made a futile attempt in 1763 to deduce the sun’s
distance from his disturbing power over our satellite.[755] Tobias
Mayer of Göttingen, however, whose short career was fruitful of
suggestions, struck out the right way to the same end; and Laplace,
in the seventh book of the Mécanique Céleste,[756] gave a solar parallax
derived from the lunar “parallactic inequality” substantially
identical with that issuing from Encke’s subsequent discussion of
the eighteenth-century transits. Thus, two wholly independent
methods—the trigonometrical, or method by survey, and the gravitational,
or method by perturbation—seemed to corroborate each the
upshot of the use of the other until the nineteenth century was
well past its meridian. It is singular how often errors conspire to
lead conviction astray.
Hansen’s note of alarm in 1854 was echoed by Leverrier in 1858.[757]
He found that an apparent monthly oscillation of the sun which[Pg 231]
reflects a real monthly movement of the earth round its common
centre of gravity with the moon, and which depends for its amount
solely on the mass of the moon and the distance of the sun,
required a diminution in the admitted value of that distance by
fully four million miles. Three years later he pointed out that
certain perplexing discrepancies between the observed and computed
places both of Venus and Mars, would vanish on the adoption of a
similar measure.[758] Moreover, a favourable opposition of Mars gave
the opportunity in 1862 for fresh observations, which, separately
worked out by Stone and Winnecke, agreed with all the newer
investigations in fixing the great unit at slightly over 91 million
miles. In Newcomb’s hands they gave 92-1/2 million.[759] The accumulating
evidence in favour of a large reduction in the sun’s distance
was just then reinforced by an auxiliary result of a totally different
and unexpected kind.
The discovery that light does not travel instantaneously from
point to point, but takes some short time in transmission, was made
by Olaus Römer in 1675, through observing that the eclipses of
Jupiter’s satellites invariably occurred later, when the earth was on
the far side, than when it was on the near side of its orbit. Half
the difference, or the time spent by a luminous vibration in crossing
the “mean radius” of the earth’s orbit, is called the “light-equation”;
and the determination of its precise value has claimed the minute
care distinctive of modern astronomy. Delambre in 1792 made it
493 seconds. Glasenapp, a Russian astronomer, raised the estimate
in 1874 to 501, Professor Harkness adopts a safe medium value of
498 seconds. Hence, if we had any independent means of ascertaining
how fast light travels, we could tell at once how far off the
sun is.
There is yet another way by which knowledge of the swiftness
of light would lead us straight to the goal. The heavenly bodies
are perceived, when carefully watched and measured, to be pushed
forward out of their true places, in the direction of the earth’s
motion, by a very minute quantity. This effect (already adverted
to) has been known since Bradley’s time as “aberration.” It arises
from a combination of the two movements of the earth round the
sun and of the light-waves through the ether. If the earth stood
still, or if light spent no time on the road from the stars, such an
effect would not exist. Its amount represents the proportion
between the velocities with which the earth and the light-rays
pursue their respective journeys. This proportion is, roughly, one
to ten thousand. So that here again, if we knew the rate per
second of luminous transmission, we should also know the rate per[Pg 232]
second of the earth’s movement, consequently the size of its orbit
and the distance of the sun.
But, until lately, instead of finding the distance of the sun from
the velocity of light, there has been no means of ascertaining the
velocity of light except through the imperfect knowledge possessed
as to the distance of the sun. The first successful terrestrial experiments
on the point date from 1849; and it is certainly no slight
triumph of human ingenuity to have taken rigorous account of the
delay of a sunbeam in flashing from one mirror to another. Fizeau
led the way,[760] and he was succeeded, after a few months, by Léon
Foucault,[761] who, in 1862, had so far perfected Wheatstone’s method
of revolving mirrors, as to be able to announce with authority that
light travelled slower, and that the sun was in consequence nearer
than had been supposed.[762] Thus a third line of separate research
was found to converge to the same point with the two others.
Such a conspiracy of proof was not to be resisted, and at the
anniversary meeting of the Royal Astronomical Society in February,
1864, the correction of the solar distance took the foremost place
in the annals of the year. Lest, however, a sudden bound of four
million miles nearer to the centre of our system should shake
public faith in astronomical accuracy, it was explained that the
change in the solar parallax corresponding to that huge leap,
amounted to no more than the breadth of a human hair 125 feet
from the eye![763] The Nautical Almanac gave from 1870 the altered
value of 8.95′, for which Newcomb’s result of 8.85′, adopted in
1869 in the Berlin Ephemeris, was substituted some ten years
later. In astronomical literature the change was initiated by Sir
Edmund Beckett in the first edition (1865) of his Astronomy without
Mathematics.
If any doubt remained as to the misleading character of Encke’s
deduction, so long implicitly trusted in, it was removed by Powalky’s
and Stone’s rediscussions, in 1864 and 1868 respectively, of the
transit observations of 1769. Using improved determinations of
the longitude of the various stations, and a selective judgment in
dealing with their materials, which, however indispensable, did not
escape adverse criticism, they brought out results confirmatory of
the no longer disputed necessity for largely increasing the solar
parallax, and proportionately diminishing the solar distance. Once[Pg 233]
more in 1890, and this time with better success, the eighteenth-century
transits were investigated by Professor Newcomb.[764] Turning
to account the experience gained in the interim regarding the
optical phenomena accompanying such events, he elicited from the
mass of somewhat discordant observations at his command, a
parallax (8·79′) in close agreement with the value given by sundry
modes of recent research.
Conclusions on the subject, however, were still regarded as purely
provisional. A transit of Venus was fast approaching, and to its
arbitrament, as to that of a court of final appeal, the pending question
was to be referred. It is true that the verdict in the same case
by the same tribunal a century earlier had proved of so indecisive a
character as to form only a starting-point for fresh litigation; but
that century had not passed in vain, and it was confidently anticipated
that observational difficulties, then equally unexpected and
insuperable, would yield to the elaborate care and skill of forewarned
modern preparation.
The conditions of the transit of December 8, 1874, were sketched
out by Sir George Airy, then Astronomer-Royal, in 1857,[765] and
formed the subject of eager discussion in this and other countries
down to the very eve of the occurrence. In these Mr. Proctor took
a leading part; and it was due to his urgent representations that
provision was made for the employment of the method identified
with the name of Halley,[766] which had been too hastily assumed inapplicable
to the first of each transit-pair. It depends upon the
difference in the length of time taken by the planet to cross the
sun’s disc, as seen from various points of the terrestrial surface, and
requires, accordingly, the visibility of both entrance and exit at the
same station. Since these were, in 1874, separated by about three
and a half hours, and the interval may be much longer, the choice
of posts for the successful use of the “method of durations” is a
matter of some difficulty.
The system described by Delisle in 1760, on the other hand,
involves merely noting the instant of ingress or egress (according to
situation) from opposite extremities of a terrestrial diameter; the
disparity in time giving a measure of the planet’s apparent displacement,
hence of its actual rate of travel in miles per minute, from
which its distances severally from earth and sun are immediately
deducible. Its chief attendant difficulty is the necessity for
accurately fixing the longitudes of the points of observation. But
this was much more sensibly felt a century ago than it is now,[Pg 234]
the improved facility and certainty of modern determinations
tending to give the Delislean plan a decided superiority over its
rival.
These two traditional methods were supplemented in 1874 by the
camera and the heliometer. From photography, above all, much
was expected. Observations made by its means would have the
advantages of impartiality, multitude, and permanence. Peculiarities
of vision and bias of judgment would be eliminated; the slow
progress of the phenomenon would permit an indefinite number of
pictures to be taken, their epochs fixed to a fraction of a second;
while subsequent leisurely comparison and measurement could
hardly fail, it was thought, to educe approximate truth from the
mass of accumulated evidence. The use of the heliometer (much
relied on by German observers) was so far similar to that of the
camera that the object aimed at by both was the determination of
the relative positions of the centres of the sun and Venus viewed, at
the same absolute instant, from opposite sides of the globe. So
that the principle of the two older methods was to ascertain the
exact times of meeting between the solar and planetary limbs;
that of the two modern to determine the position of the dark body
already thrown into complete relief by its shining background.
The former are “methods by contact,” the latter “methods by projection.”
Every country which had a reputation to keep or to gain for
scientific zeal was forward to co-operate in the great cosmopolitan
enterprise of the transit. France and Germany each sent out six
expeditions; twenty-six stations were in Russian, twelve in English,
eight in American, three in Italian, one in Dutch occupation. In
all, at a cost of nearly a quarter of a million, some fourscore distinct
posts of observation were provided; among them such inhospitable,
and all but inaccessible rocks in the bleak Southern Ocean, as
St. Paul’s and Campbell Islands, swept by hurricanes, and fitted
only for the habitation of seabirds, where the daring votaries of
science, in the wise prevision of a long leaguer by the elements,
were supplied with stores for many months, or even a whole year.
Siberia and the Sandwich Islands were thickly beset with observers;
parties of three nationalities encamped within the mists of Kerguelen
Island, expressively termed the “Land of Desolation,” in the
sanguine, though not wholly frustrated hope of a glimpse of the
sun at the right moment. M. Janssen narrowly escaped destruction
from a typhoon in the China seas on his way to Nagasaki; Lord
Lindsay (now Earl of Crawford and Balcarres) equipped, at his
private expense, an expedition to Mauritius, which was in itself an
epitome of modern resource and ingenuity.[Pg 235]
During several years, the practical methods best suited to insure
success for the impending enterprise formed a subject of European
debate. Official commissions were appointed to receive and decide
upon evidence; and experiments were in progress for the purpose
of defining the actual circumstances of contacts, the precise determination
of which constituted the only tried, though by no means
an assuredly safe road to the end in view. In England, America,
France, and Germany, artificial transits were mounted, and the
members of the various expeditions were carefully trained to
unanimity in estimating the phases of junction and separation
between a moving dark circular body and a broad illuminated disc.
In the previous century, a formidable and prevalent phenomenon,
which acquired notoriety as the “Black Drop” or “Black Ligament,”
had swamped all pretensions to rigid accuracy. It may be
described as substituting adhesion for contact, the limbs of the sun
and planet, instead of meeting and parting with the desirable clean
definiteness, clinging together as if made of some glutinous material,
and prolonging their connection by means of a dark band or dark
threads stretched between them. Some astronomers ascribed this
baffling appearance entirely to instrumental imperfections; others
to atmospheric agitation; others again to the optical encroachment
of light upon darkness known as “irradiation.” It is probable
that all these causes conspired, in various measure, to produce it;
and it is certain that its conspicuous appearance may, by suitable
precautions, be obviated.
The organisation of the British forces reflected the utmost credit
on the energy and ability of Lieutenant-Colonel Tupman, who was
responsible for the whole. No useful measure was neglected. Each
observer went out ticketed with his “personal equation,” his senses
drilled into a species of martial discipline, his powers absorbed, so
far as possible, in the action of a cosmopolitan observing machine.
Instrumental uniformity and uniformity of method were obtainable,
and were attained; but diversity of judgment unhappily survived
the best-directed efforts for its extirpation.
The eventful day had no sooner passed than telegrams began to
pour in, announcing an outcome of considerable, though not unqualified
success. The weather had proved generally favourable;
the manifold arrangements had worked well; contacts had been
plentifully observed; photographs in lavish abundance had been
secured; a store of materials, in short, had been laid up, of which it
would take years to work out the full results by calculation. Gradually,
nevertheless, it came to be known that the hope of a definitive
issue must be abandoned. Unanimity was found to be as remote
as ever. The dreaded “black ligament” gave, indeed, less trouble[Pg 236]
than was expected; but another appearance supervened which took
most observers by surprise. This was the illumination due to the
atmosphere of Venus. Astronomers, it is true, were not ignorant
that the planet had, on previous occasions, been seen girdled with
a lucid ring; but its power to mar observations by the distorting
effect of refraction had scarcely been reckoned with. It proved,
however, to be very great. Such was the difficulty of determining
the critical instant of internal contact, that (in Colonel Tupman’s
words) “observers side by side, with adequate optical means,
differed as much as twenty or thirty seconds in the times they
recorded for phenomena which they have described in almost
identical language.”[767]
Such uncertainties in the data admitted of a corresponding
variety in the results. From the British observations of ingress
and egress Sir George Airy[768] derived, in 1877, a solar parallax of
8·76′ (corrected to 8·754′), indicating a mean distance of 93,375,000
miles. Mr. Stone obtained a value of ninety-two millions (parallax
8·88′), and held any parallax less than 8·84′ or more than 8·93′ to
be “absolutely negatived” by the documents available.[769] Yet, from
the same, Colonel Tupman deduced 8·81′,[770] implying a distance
700,000 miles greater than Stone had obtained. The best French
observations of contacts gave a parallax of about 8·88′; French
micrometric measures the obviously exaggerated one of 9·05′.[771]
Photography, as practised by most of the European parties, was
a total failure. Utterly discrepant values of the microscopic displacements
designed to serve as sounding lines for the solar system,
issued from attempts to measure even the most promising pictures.
“You might as well try to measure the zodiacal light,” it was
remarked to Sir George Airy. Those taken on the American plan
of using telescopes of so great focal length as to afford, without
further enlargement, an image of the requisite size, gave notably
better results. From an elaborate comparison of those dating from
Vladivostock, Nagasaki, and Pekin, with others from Kerguelen
and Chatham Islands, Professor D. P. Todd, of Amherst College,
deduced a solar distance of about ninety-two million miles (parallax
8·883′ ±0·034′),[772] and the value was much favoured by concurrent
evidence.
On the whole, estimates of the great spatial unit cannot be said
to have gained any security from the combined effort of 1874. A
few months before the transit, Mr. Proctor considered that the
uncertainty then amounted to 1,448,000 miles;[773] five years after the[Pg 237]
transit, Professor Harkness judged it to be still 1,575,950 miles;[774]
yet it had been hoped that it would have been brought down to
100,000. As regards the end for which it had been undertaken,
the grand campaign had come to nothing. Nevertheless, no sign of
discouragement was apparent. There was a change of view, but no
relaxation of purpose. The problem, it was seen, could be solved
by no single heroic effort, but by the patient approximation of
gradual improvements. Astronomers, accordingly, looked round
for fresh means or more refined expedients for applying those
already known. A new phase of exertion was entered upon.
On September 5, 1877, Mars came into opposition near the part
of his orbit which lies nearest to that of the earth, and Dr. Gill (now
Sir David) took advantage of the circumstance to appeal once more
to him for a decision on the quæstio vexata of the sun’s distance. He
chose, as the scene of his labours, the Island of Ascension, and for
their plan a method recommended by Airy in 1857,[775] but never before
fairly tried. This is known as the “diurnal method of parallaxes.”
Its principle consists in substituting successive morning and evening
observations from the same spot, for simultaneous observations from
remote spots, the rotation of the earth supplying the necessary
difference in the points of view. Its great advantage is that of
unity in performance. A single mind, looking through the same
pair of eyes, reinforced with the same optical appliances, is employed
throughout, and the errors inseparable from the combination of data
collected under different conditions are avoided. There are many cases
in which one man can do the work of two better than two men can
do the work of one. The result of Gill’s skilful determinations
(made with Lord Lindsay’s heliometer) was a solar parallax of 8·78′,
corresponding to a distance of 93,080,000 miles.[776] The bestowal
of the Royal Astronomical Society’s gold medal stamped the merit
of this distinguished service.
But there are other subjects for this kind of inquiry besides
Mars and Venus. Professor Galle of Breslau suggested in 1872[777]
that some of the minor planets might be got to repay astronomers
for much disinterested toil spent in unravelling their motions, by
lending aid to their efforts towards a correct celestial survey. Ten
or twelve come near enough, and are bright enough for the purpose;
in fact, the absence of sensible magnitude is one of their chief
recommendations, since a point of light offers far greater facilities
for exact measurement than a disc. The first attempt to work this
new vein was made at the opposition of Phocæa in 1872; and from
observations of Flora in the following year at twelve observatories[Pg 238]
in the northern and southern hemispheres, Galle deduced a solar
parallax of 8·87′.[778] At Mauritius in 1874, Lord Lindsay and Sir
David Gill applied the “diurnal method” to Juno, then conveniently
situated for the purpose; and the continued use of similar occasions
affords an unexceptionable means for improving knowledge of the
sun’s distance. They frequently recur; they need no elaborate
preparation; a single astronomer armed with a heliometer can do
all the requisite work. Dr. Gill, however, organized a more
complex plan of operations upon Iris in 1888, and upon Victoria
and Sappho in 1889. A novel method was adopted. Its object
was to secure simultaneous observations made from opposite
sides of the globe just when the planet lay in the plane passing
through the centre of the earth and the two observers, the same
pair of reference-stars being used on each occasion. The displacements
caused by parallax were thus in a sense doubled, since the
star to which the planet seemed approximated in the northern
hemisphere, showed as if slightly removed from it in the southern,
and vice versâ. As the planet pursued its course, fresh star-couples
came into play, during the weeks that the favourable period lasted.
In these determinations, only heliometers were employed. Dr.
Elkin, of Yale college, co-operated throughout, and the heliometers
of Dresden, Göttingen, Bamberg, and Leipzig, shared in the work,
while Dr. Auwers of Berlin was Sir David Gill’s personal coadjutor
at the Cape. Voluminous data were collected; meridian observations
of the stars of reference for Victoria occupied twenty-one
establishments during four months; the direct work of triangulation
kept four heliometers in almost exclusive use for the best
part of a year; and the ensuing toilsome computations, carried
out during three years at the Cape Observatory, filled two bulky
tomes[779] with their details. Gill’s final result, published in 1897, was
a parallax of 8·802′, equivalent to a solar distance of 92,874,000;
and it was qualified by a probable error so small that the value
might well have been accepted as definitive but for an unlooked-for
discovery. The minor planet Eros, detected August 14,
1898, was found to pursue a course rendering it an almost ideal
intermediary in solar parallax-determinations. Once in thirty years,
it comes within fifteen million miles of the earth; and although the
next of these choice epochs must be awaited for some decades, an
opposition too favourable to be neglected occurred in 1900. At an
International Conference, accordingly, held at Paris in July of that
year, a plan of photographic operations was concerted between the[Pg 239]
representatives of no less than 58 observatories.[780] Its primary object
was to secure a large stock of negatives showing the planet with
the comparison-stars along the route traversed by it from October,
1900, to March, 1901,[781] and this at least was successfully attained.
Their measurement will in due time educe the apparent displacements
of the moving object as viewed simultaneously from remote
parts of the earth; and the upshot should be a solar parallax
adequate in accuracy to the exigent demands of the twentieth
century.
The second of the nineteenth-century pair of Venus-transits
was looked forward to with much abated enthusiasm. Russia
refused her active co-operation in observing it, on the ground that
oppositions of the minor planets were trigonometrically more useful,
and financially far less costly; and her example was followed by
Austria; while Italian astronomers limited their sphere of action to
their own peninsula. Nevertheless, it was generally held that a
phenomenon which the world could not again witness until it was
four generations older should, at the price of any effort, not be
allowed to pass in neglect.
The persuasion of its importance justified the summoning of an
International Conference at Paris in 1881, from which, however,
America, preferring independent action, held aloof. It was decided
to give Delisle’s method another trial; and the ambiguities attending
and marring its use were sought to be obviated by careful regulations
for insuring agreement in the estimation of the critical
moments of ingress and egress.[782] But in fact (as M. Puiseux had
shown[783]), contacts between the limbs of the sun and planet, so far
from possessing the geometrical simplicity attributed to them, are
really made up of a prolonged succession of various and varying
phases, impossible either to predict or identify with anything like
rigid exactitude. Sir Robert Ball compared the task of determining
the precise instant of their meeting or parting, to that of telling the
hour with accuracy on a watch without a minute hand; and the
comparison is admittedly inadequate. For not only is the apparent
movement of Venus across the sun extremely slow, being but the
excess of her real motion over that of the earth; but three distinct
atmospheres—the solar, terrestrial, and Cytherean—combine to
deform outlines and mask the geometrical relations which it is
desired to connect with a strict count of time.
The result was very much what had been expected. The[Pg 240]
arrangements were excellent, and were only in a few cases disconcerted
by bad weather. The British parties, under the experienced
guidance of Mr. Stone, the late Radcliffe observer, took up positions
scattered over the globe, from Queensland to Bermuda; the Americans
collected a whole library of photographs; the Germans and
Belgians trusted to the heliometer; the French used the camera as
an adjunct to the method of contacts. Yet little or no approach
was made to solving the problem. Thus, from 606 measures of
Venus on the sun, taken with a new kind of heliometer at Santiago
in Chili, M. Houzeau, of the Brussels Observatory, derived a solar
parallax of 8.907′, and a distance of 91,727,000 miles.[784] But the
“probable errors” of this determination amounted to 0.084′ either
way: it was subject to a “more or less” of 900,000, or to a total
uncertainty of 1,800,000 miles. The “probable error” of the
English result, published in 1887, was less formidable,[785] yet the
details of the discussion showed that no great confidence could be
placed in it. The sun’s distance came out 92,560,000 miles; while
92,360,000 was given by Professor Harkness’s investigation of 1,475
American photographs.[786] Finally, Dr. Auwers deduced from the
German heliometric measures the unsatisfactorily small value of
92,000,000 miles.[787] The transit of 1882 had not, then, brought about
the desired unanimity.
The state and progress of knowledge on this important topic were
summed up by Faye and Harkness in 1881.[788] The methods employed
in its investigation fall (as we have seen) into three separate classes—the
trigonometrical, the gravitational, and the “phototachymetrical”—an
ungainly adjective used to describe the method by
the velocity of light. Each has its special difficulties and sources
of error; each has counter-balancing advantages. The only trustworthy
result from celestial surveys, was at that time furnished
by Gill’s observations of Mars in 1877. But the method by lunar
and planetary disturbances is unlike all the others in having time
on its side. It is this which Leverrier declared with emphasis
must inevitably prevail, because its accuracy is continually growing.[789]
The scarcely perceptible errors which still impede its application are
of such a nature as to accumulate year by year; eventually, then,
they will challenge, and must receive, a more and more perfect
correction. The light-velocity method, however, claimed, and for
some years justified, M. Faye’s preference.
By a beautiful series of experiments on Foucault’s principle,[Pg 241]
Michelson fixed in 1879 the rate of luminous transmission at
299,930 (corrected later to 299,910) kilometres a second.[790] This
determination was held by Professor Todd to be entitled to four
times as much confidence as any previous one; and if the solar
parallax of 8·758′ deduced from it by Professor Harkness errs
somewhat by defect, it is doubtless because Glasenapp’s “light-equation,”
with which it was combined, errs slightly by excess.
But all earlier efforts of the kind were thrown into the shade by
Professor Newcomb’s arduous operations at Washington in 1880-1882.[791]
The scale upon which they were conducted was in itself
impressive. Foucault’s entire apparatus in 1862 had been enclosed
in a single room; Newcomb’s revolving and fixed mirrors, between
which the rays of light were to run their timed course, were set up
on opposite shores of the Potomac, at a distance of nearly four
kilometres. This advantage was turned to the utmost account by
ingenuity and skill in contrivance and execution; and the deduced
velocity of 299,860 kilometres = 186,328 miles a second, had an
estimated error (30 kilometres) only one-tenth that ascribed by
Cornu to his own result in 1874.
Just as these experiments were concluded in 1882, M. Magnus
Nyrén, of St. Petersburg, published an elaborate investigation of
the small annular displacements of the stars due to the successive
transmission of light, involving an increase of Struve’s “constant of
aberration” from 20·445′ to 20·492′. And from the new value,
combined with Newcomb’s light-velocity, was derived a valuable
approximation to the sun’s distance, concluded at 92,905,021 miles
(parallax = 8·794′). Yet it is not quite certain that Nyrén’s correction
was an improvement. A differential method of determining
the amount of aberration, struck out by M. Loewy of Paris,[792] avoids
most of the objections to the absolute method previously in vogue;
and the upshot of its application in 1891 was to show that Struve’s
constant might better be retained than altered, Loewy’s of 20·447′
varying from it only to an insignificant extent. Professor Hall
had, moreover, deduced nearly the same value (20·454′) from the
Washington observations since 1862, of α Lyræ (Vega); whence, in
conjunction with Newcomb’s rate of light transmission, he arrived
at a solar parallax of 8·81′.[793] Inverting the process, Sir David Gill
in 1897 derived the constant from the parallax. If the earth’s
orbit have a mean radius, as found by him, of 92,874,000 miles,
then, he calculated, the aberration of light—Newcomb’s measures of[Pg 242]
its velocity being supposed exact—amounts to 20.467′. This figure
can need very slight correction.
Professor Harkness surveyed in 1891,[794] from an eclectic point of
view, the general situation as regarded the sun’s parallax. Convinced
that no single method deserved an exclusive preference, he
reached a plausible result through the combination, on the principle
of least squares—that is, by the mathematical rules of probability—of
all the various quantities upon which the great datum depends.
It thus summed up and harmonised the whole of the multifarious
evidence bearing upon the point, and, as modified in 1894,[795] falls
very satisfactorily into line with the Cape determination. We may,
then, at least provisionally, accept 92,870,000 miles as the length of
our measuring-rod for space. Nor do we hazard much in fixing
100,000 miles as the outside limit of its future correction.
FOOTNOTES:
[748] Airy, Month. Not., vol. xvii., p. 210.
[749] Mars comes into opposition once in about 780 days; but owing to the
eccentricity of both orbits, his distance from the earth at those epochs varies
from thirty-five to sixty-two million miles.
[750] J. D. Cassini, Hist. Abrégée de la Parallaxe du Soleil, p. 122, 1772.
[751] The present period of coupled eccentric transits will, in the course of ages,
be succeeded by a period of single, nearly central transits. The alignments by
which transits are produced, of the earth, Venus, and the sun, close to the place
of intersection of the two planetary orbits, now occur, the first a little in front of,
the second, after eight years less two and a half days, a little behind the node.
But when the first of these two meetings takes place very near the node, giving
a nearly central transit, the second falls too far from it, and the planet escapes
projection on the sun. The reason of the liability to an eight-yearly recurrence
is that eight revolutions of the earth are accomplished in only a very little more
time than thirteen revolutions of Venus.
[752] Die Entfernung der Sonne: Fortsetzung, p. 108. Encke slightly corrected
his results of 1824 in Berlin Abh., 1835, p. 295.
[753] Owing to the ellipticity of its orbit, the earth is nearer to the sun in January
than in June by 3,100,000 miles. The quantity to be determined, or “mean
distance,” is that lying midway between these extremes—is, in other words, half
the major axis of the ellipse in which the earth travels.
[754] Month. Not., vol. xv., p. 9.
[755] The Distance of the Sun from the Earth determined by the Theory of Gravity,
Edinburgh, 1763.
[756] Opera, t. iii., p. 326.
[757] Comptes Rendus, t. xlvi., p. 882. The parallax 8·95′ derived by Leverrier
from the above-described inequality in the earth’s motion, was corrected by
Stone to 8·91′. Month. Not., vol. xxviii., p. 25.
[758] Month. Not., vol. xxxv., p. 156.
[759] Wash. Obs., 1865, App. ii., p. 28.
[760] Comptes Rendus, t. xxix., p. 90.
[761] Ibid., t. xxx., p. 551.
[762] Ibid., t. lv., p. 501. The previously admitted velocity was 308 million
metres per second; Foucault reduced it to 298 million. Combined with Struve’s
“constant of aberration” this gave 8.86′ for the solar parallax, which exactly
agreed with Cornu’s result from a repetition of Fizeau’s experiments in 1872.
Comptes Rendus, t. lxxvi., p. 338.
[763] Month. Not., vol. xxiv., p. 103.
[764] Astr. Papers of the American Ephemeris, vol. ii., p. 263.
[765] Month. Not., vol. xvii., p. 208.
[766] Because closely similar to that proposed by him in Phil. Trans. for 1716.
[767] Month. Not., vol. xxxviii., p. 447.
[768] Ibid., p. 11.
[769] Ibid., p. 294.
[770] Ibid., p. 334.
[771] Comptes Rendus, t. xcii., p. 812.
[772] Observatory, vol. v., p. 205.
[773] Transits of Venus, p. 89 (1st ed.).
[774] Am. Jour. of Sc., vol. xx., p. 393.
[775] Month. Not., vol. xvii., p. 219.
[776] Mem. Roy. Astr. Soc., vol. xlvi., p. 163.
[777] Astr. Nach., No. 1,897.
[778] Hilfiker, Bern Mittheilungen, 1878, p. 109.
[779] Annals of the Cape Observatory, vols. vi., vii.
[780] Rapport sur l’État de l’Observatoire de Paris pour l’Année 1900, p. 7.
[781] Observatory, vol. xxiii., p. 311; Newcomb, Astr. Jour., No. 480.
[782] Comptes Rendus, t. xciii., p. 569.
[783] Ibid., t. xcii., p. 481.
[784] Bull. de l’Acad., t. vi., p. 842.
[785] Month. Not., vol. xlviii., p. 201.
[786] Astr. Jour., No. 182.
[787] Astr. Nach., No. 3,066.
[788] Comptes Rendus, t. xcii., p. 375; Am. Jour. of Sc., vol. xxii., p. 375.
[789] Month. Not., vol. xxxv., p. 401.
[790] Am. Jour. of Sc., vol. xviii., p. 393.
[791] Nature, vol. xxxiv., p. 170; Astron. Papers of the American Ephemeris,
vol. ii., p. 113.
[792] Comptes Rendus, t. cxii., p. 549.
[793] Astr. Journ., Nos. 169, 170
[794] The Solar Parallax and its Related Constants, Washington, 1891.
[795] Astr. and Astrophysics, vol. xiii., p. 626.
CHAPTER VII
PLANETS AND SATELLITES
Johann Hieronymus Schröter was the Herschel of Germany.
He did not, it is true, possess the more brilliant gifts of his rival.
Herschel’s piercing discernment, comprehensive intelligence, and
inventive splendour were wanting to him. He was, nevertheless,
the founder of descriptive astronomy in Germany, as Herschel was
in England.
Born at Erfurt in 1745, he prosecuted legal studies at Göttingen,
and there imbibed from Kästner a life-long devotion to science.
From the law, however, he got the means of living, and, what was
to the full as precious to him, the means of observing. Entering
the sphere of Hanoverian officialism in 1788, he settled a few years
later at Lilienthal, near Bremen, as “Oberamtmann,” or chief magistrate.
Here he built a small observatory, enriched in 1785 with
a seven-foot reflector by Herschel, then one of the most powerful
instruments to be found anywhere out of England. It was soon
surpassed, through his exertions, by the first-fruits of native industry
in that branch. Schrader of Kiel transferred his workshops to
Lilienthal in 1792, and constructed there, under the superintendence
and at the cost of the astronomical Oberamtmann, a thirteen-foot
reflector, declared by Lalande to be the finest telescope in existence,
and one twenty-seven feet in focal length, probably as inferior to its
predecessor in real efficiency as it was superior in size.
Thus, with instruments of gradually increasing power, Schröter
studied during thirty-four years the topography of the moon and
planets. The field was then almost untrodden; he had but few
and casual predecessors, and has since had no equal in the sustained
and concentrated patience of his hourly watchings. Both their
prolixity and their enthusiasm are faithfully reflected in his various
treatises. Yet the one may be pardoned for the sake of the other,
especially when it is remembered that he struck out a substantially
new line, and that one of the main lines of future advance. Moreover,[Pg 244]
his infectious zeal communicated itself; he set the example of
observing when there was scarcely an observer in Germany; and
under his roof Harding and Bessel received their training as practical
astronomers.
But he was reserved to see evil days. Early in 1813 the French
under Vandamme occupied Bremen. On the night of April 20, the
Vale of Lilies was, by their wanton destructiveness, laid waste with
fire; the Government offices were destroyed, and with them the
chief part of Schröter’s property, including the whole stock of his
books and writings. There was worse behind. A few days later,
his observatory, which had escaped the conflagration, was broken
into, pillaged, and ruined. His life was wrecked with it. He survived
the catastrophe three years without the means to repair, or
the power to forget it, and gradually sank from disappointment into
decay, terminated by death, August 29, 1816. He had, indeed,
done all the work he was capable of; and though not of the first
quality, it was far from contemptible. He laid the foundation of the
comparative study of the moon’s surface, and the descriptive particulars
of the planets laboriously collected by him constituted a
store of more or less reliable information hardly added to during
the ensuing half century. They rested, it is true, under some
shadow of doubt; but the most recent observations have tended
on several points to rehabilitate the discredited authority of the
Lilienthal astronomer. We may now briefly resume, and pursue in
its further progress, the course of his studies, taking the planets in
the order of their distances from the sun.
In April, 1792, Schröter saw reason to conclude, from the gradual
degradation of light on its partially illuminated disc, that Mercury
possesses a tolerably dense atmosphere.[796] During the transit of
May 7, 1799, he was, moreover, struck with the appearance of a
ring of softened luminosity encircling the planet to an apparent
height of three seconds, or about a quarter of its own diameter.[797]
Although a “mere thought” in texture, it remained persistently
visible both with the seven-foot and the thirteen-foot reflectors,
armed with powers up to 288. It had a well-marked grayish
boundary, and reminded him, though indefinitely fainter, of the
penumbra of a sun-spot. A similar appendage had been noticed by
De Plantade at Montpellier, November 11, 1736, and again in 1786
and 1789 by Prosperin and Flaugergues; but Herschel, on November
9, 1802, saw the preceding limb of the planet projected on the
sun cut the luminous solar clouds with the most perfect sharpness.[798]
The presence, however, of a “halo” was unmistakable in 1832,[Pg 245]
when Professor Moll, of Utrecht, described it as a “nebulous ring
of a darker tinge approaching to the violet colour.”[799] Again, to
Huggins and Stone, November 5, 1868, it showed as lucid and most
distinct. No change in the colour of the glasses used, or the powers
applied, could get rid of it, and it lasted throughout the transit.[800]
It was next seen by Christie and Dunkin at Greenwich, May 6,
1878,[801] and with much precision of detail by Trouvelot at Cambridge
(U.S.).[802] Professor Holden, on the other hand, noted at Hastings-on-Hudson
the total absence of all anomalous appearances.[803] Nor
could any vestige of them be perceived by Barnard at Lick on
November 10, 1894.[804] Various effects of irradiation and diffraction
were, however, observed by Lowell and W. H. Pickering at Flagstaff;[805]
and Davidson was favoured at San Francisco with glimpses
of the historic aureola,[806] as well as of a central whitish spot, which
often accompanies it. That both are somehow of optical production
can scarcely be doubted.
Nothing can be learned from them regarding the planet’s physical
condition. Airy showed that refraction in a Mercurian atmosphere
could not possibly originate the noted aureola, which must accordingly
be set down as “strictly an ocular nervous phenomenon.”[807] It
is the less easy to escape from this conclusion that we find the
virtually airless moon capable of exhibiting a like appendage. Professor
Stephen Alexander, of the United States Survey, with two
other observers, perceived, during the eclipse of the sun of July 18,
1860, the advancing lunar limb to be bordered with a bright band;[808]
and photographic effects of the same kind appear in pictures of
transits of Venus and partial solar eclipses.
The spectroscope affords little information as to the constitution
of Mercury. Its light is of course that of the sun reflected, and
its spectrum is consequently a faint echo of the Fraunhofer spectrum.
Dr. H. C. Vogel, who first examined it in April, 1871, suspected
traces of the action of an atmosphere like ours,[809] but, it would seem,
on slight grounds. It is, however, certainly very poor in blue rays.
More definite conclusions were, in 1874,[810] derived by Zöllner from
photometric observations of Mercurian phases. A similar study of
the waxing and waning moon had afforded him the curious discovery[Pg 246]
that light-changes dependent upon phase vary with the nature of
the reflecting surface, following a totally different law on a smooth
homogeneous globe and on a rugged and mountainous one. Now
the phases of Mercury—so far as could be determined from only
two sets of observations—correspond with the latter kind of
structure. Strictly analogous to those of the moon, they seem to
indicate an analogous mode of surface-formation. This conclusion
was fully borne out by Müller’s more extended observations at
Potsdam during the years 1885-1893.[811] Practical assurance was
gained from them that the innermost planet has a rough rind of
dusky rock, absorbing all but 17 per cent. of the light poured
upon it by the fierce adjacent sun. Its “albedo,” in other words, is
0·17,[812] which is precisely that ascribed to the moon. The absence
of any appreciable Mercurian atmosphere followed almost necessarily
from these results.
On March 26, 1800, Schröter, observing with his 13-foot reflector
in a peculiarly clear sky, perceived the southern horn of Mercury’s
crescent to be quite distinctly blunted.[813] Interception of sunlight
by a Mercurian mountain rather more than eleven English miles
high explained the effect to his satisfaction. By carefully timing
its recurrence, he concluded rotation on an axis in a period of 24
hours 4 minutes. The first determination of the kind rewarded
twenty years of unceasing vigilance. It received ostensible confirmation
from the successive appearances of a dusky streak and
blotch in May and June, 1801.[814] These, however, were inferred to
be no permanent markings on the body of the planet, but atmospheric
formations, the streak at times drifting forwards (it was thought)
under the fluctuating influence of Mercurian breezes. From a
rediscussion of these somewhat doubtful observations Bessel inferred
that Mercury rotates on an axis inclined 70° to the plane of its orbit
in 24 hours 53 seconds.
The rounded appearance of the southern horn seen by Schröter
was more or less doubtfully caught by Noble (1864), Burton, and
Franks (1877);[815] but was obvious to Mr. W. F. Denning at Bristol
on the morning of November 5, 1882.[816] That the southern polar
regions are usually less bright than the northern is well ascertained;
but the cause of the deficiency remains dubious. If inequalities of[Pg 247]
surface are in question, they must be on a considerable scale; and
a similar explanation might be given of the deformations of the
“terminator”—or dividing-line between darkness and light in the
planet’s phases—first remarked by Schröter, and again clearly seen
by Trouvelot in 1878 and 1881.[817] The displacement, during four
days, of certain brilliant and dusky spaces on the disc indicated to
Mr. Denning in 1882 rotation in about twenty-five hours; while the
general aspect of the planet reminded him of that of Mars.[818] But
the difficulties in the way of its observation are enormously enhanced
by its constant close attendance on the sun.
In his sustained study of the features of Mercury, Schröter had
no imitator until Schiaparelli took up the task at Milan in 1882.
His observations were made in daylight. It was found that much
more could be seen, and higher magnifying powers used, high up
in the sky near the sun, than at low altitudes, through the agitated
air of morning or evening twilight. A notable discovery ensued.[819]
Following the planet hour by hour, instead of making necessarily
brief inspections at intervals of about a day, as previous observers
had done, it was found that the markings faintly visible remained
sensibly fixed, hence, that there was no rotation in a period at all
comparable with that of the earth. And after long and patient
watching, the conclusion was at last reached that Mercury turns on
his axis in the same time needed to complete a revolution in his
orbit. One of his hemispheres, then, is always averted from the
sun, as one of the moon’s hemispheres from the earth, while the
other never shifts from beneath his torrid rays. The “librations,”
however, of Mercury are on a larger scale than those of the moon,
because he travels in a more eccentric path. The temporary inequalities
arising between his “even pacing” on an axis and his
alternately accelerated and retarded elliptical movement occasion, in
fact, an oscillation to and fro of the boundaries of light and darkness
on his globe over an arc of 47° 22′, in the course of his year of 88
days. Thus the regions of perpetual day and perpetual night are
separated by two segments, amounting to one-fourth of the entire
surface, where the sun rises and sets once in 88 days. Else there is
no variation from the intense glare on one side of the globe, and
the nocturnal blackness on the other.
To Schiaparelli’s scrutiny, Mercury appeared as a “spotty globe,”
enveloped in a tolerably dense atmosphere. The brownish stripes
and streaks, discerned on his rose-tinged disc, and judged to be
permanent, were made the basis of a chart. They were not indeed[Pg 248]
always equally well seen. They disappeared regularly near the
limb, and were at times veiled even when centrally situated. Some
of them had been clearly perceived by De Ball at Bothkamp in
1882.[820]
Mr. Lowell followed Schiaparelli’s example by observing Mercury
in the full glare of noon. “The best time to study him,” he remarked,
“is when planetary almanacs state ‘Mercury invisible.'”
A remarkable series of drawings executed, some at Flagstaff in
1896, the remainder at Mexico in 1897, supplied grounds for the
following, among other, conclusions.[821] Mercury rotates synchronously
with its revolution—that is, once in 88 days—on an
axis sensibly perpendicular to its orbital plane. No certain signs
of a Mercurian atmosphere are visible. The globe is seamed and
furrowed with long narrow markings, explicable as cracks in
cooling. It is, and always was, a dead world. From micrometrical
measures, moreover, the inferences were drawn that the planet’s
mass has a probable value about 1/20 that of the earth, while its mean
density falls considerably short of the terrestrial standard.
The theory of Mercury’s movements has always given trouble.
In Lalande’s,[822] as in Mästlin’s time, the planet seemed to exist for
no other purpose than to throw discredit on astronomers; and
even to Leverrier’s powerful analysis it long proved recalcitrant.
On the 12th of September, 1869, however, he was able to
announce before the Academy of Sciences[823] the terms of a compromise
between observation and calculation. They involved
the addition of a new member to the solar system. The hitherto
unrecognised presence of a body about the size of Mercury itself
revolving at somewhat less than half its mean distance from the
sun (or, if farther, then of less mass, and vice versâ), would, it
was pointed out, produce exactly the effect required, of displacing
the perihelion of the former planet 38′ a century more
than could otherwise be accounted for. The planes of the two
orbits, however, should not lie far apart, as otherwise a nodal
disturbance would arise not perceived to exist. It was added
that a ring of asteroids similarly placed would answer the purpose
equally well, and was more likely to have escaped notice.
Upon the heels of this forecast followed promptly a seeming
verification. Dr. Lescarbault, a physician residing at Orgères,
whose slender opportunities had not blunted his hopes of achievement,
had, ever since 1845, when he witnessed a transit of
Mercury, cherished the idea that an unknown planet might
be caught thus projected on the solar background. Unable to[Pg 249]
observe continuously until 1858, he, on March 26, 1859, saw
what he had expected—a small perfectly round object slowly
traversing the sun’s disc. The fruitless expectation of reobserving
the phenomenon, however, kept him silent, and it was not until
December 22, after the news of Leverrier’s prediction had reached
him, that he wrote to acquaint him with his supposed discovery.[824]
The Imperial Astronomer thereupon hurried down to Orgères, and
by personal inspection of the simple apparatus used, by searching
cross-examination and local inquiry, convinced himself of the
genuine character and substantial accuracy of the reported observation.
He named the new planet “Vulcan,” and computed elements
giving it a period of revolution slightly under twenty days.[825] But
it has never since been seen. M. Liais, director of the Brazilian
Coast Survey, thought himself justified in asserting that it never
had been seen. Observing the sun for twelve minutes after the
supposed ingress recorded at Orgères, he noted those particular
regions of its surface as “très uniformes d’intensité.”[826] He subsequently,
however, admitted Lescarbault’s good faith, at first rashly
questioned. The planet-seeking doctor was, in truth, only one
among many victims of similar illusions.
Waning interest in the subject was revived by a fresh announcement
of a transit witnessed, it was asserted, by Weber at Peckeloh,
April 4, 1876.[827] The pseudo-planet, indeed, was detected shortly
afterwards on the Greenwich photographs, and was found to have
been seen by M. Ventosa at Madrid in its true character of a sun-spot
without penumbra; but Leverrier had meantime undertaken
the investigation of a list of twenty similar dubious appearances,
collected by Haase, and republished by Wolf in 1872.[828] From these,
five were picked out as referring in all likelihood to the same body,
the reality of whose existence was now confidently asserted, and of
which more or less probable transits were fixed for March 22, 1877,
and October 15, 1882.[829] But, widespread watchfulness notwithstanding,
no suspicious object came into view at either epoch.
The next announcement of the discovery of “Vulcan” was
on the occasion of the total solar eclipse of July 29, 1878.[830] This
time it was stated to have been seen at some distance south-west of
the obscured sun, as a ruddy star with a minute planetary disc; and
its simultaneous detection by two observers—the late Professor[Pg 250]
James C. Watson, stationed at Rawlins (Wyoming Territory), and
Professor Lewis Swift at Denver (Colorado)—was at first readily
admitted. But their separate observations could, on a closer
examination, by no possibility be brought into harmony, and, if
valid, certainly referred to two distinct objects, if not to four; each
astronomer eventually claiming a pair of planets. Nor could any
one of the four be identified with Lescarbault’s and Leverrier’s
Vulcan, which, if a substantial body revolving round the sun, must
then have been found on the east side of that luminary.[831] The most
feasible explanation of the puzzle seems to be that Watson and
Swift merely saw each the same two stars in Cancer: haste and
excitement doing the rest.[832] Nevertheless, they strenuously maintained
their opposite conviction.[833]
Intra-Mercurian planets have since been diligently searched for
when the opportunity of a total eclipse offered, especially during
the long obscuration at Caroline Island. Not only did Professor
Holden “sweep” in the solar vicinity, but Palisa and Trouvelot
agreed to divide the field of exploration, and thus make sure of
whatever planetary prey there might be within reach; yet with only
negative results. Photographic explorations during recent eclipses
have been equally fruitless. Belief in the presence of any considerable
body or bodies within the orbit of Mercury is, accordingly, at
a low ebb. Yet the existence of the anomaly in the Mercurian
movements indicated by Leverrier has been made only surer by
further research.[834] Its elucidation constitutes one of the “pending
problems” of astronomy.
From the observation at Bologna in 1666-67 of some very faint
spots, Domenico Cassini concluded a rotation or libration of Venus—he
was not sure which—in about twenty-three hours.[835] By
Bianchini in 1726 the period was augmented to twenty-four days
eight hours. J. J. Cassini, however, in 1740, showed that the data
collected by both observers were consistent with rotation in twenty-three
hours twenty minutes.[836] So the matter rested until Schröter’s[Pg 251]
time. After watching nine years in vain, he at last, February 28,
1788, perceived the ordinarily uniform brightness of the planet’s
disc to be marbled with a filmy streak, which returned periodically
to the same position in about twenty-three hours twenty-eight
minutes. This approximate estimate was corrected by the application
of a more definite criterion. On December 28, 1789, the
southern horn of the crescent Venus was seen truncated, an outlying
lucid point interrupting the darkness beyond. Precisely the same
appearance recurred two years later, giving for the planet’s rotation
a period of 23h. 21m.[837] To this only twenty-two seconds were
added by De Vico, as the result of over 10,000 observations made
with the Cauchoix refractor of the Collegio Romano, 1839-41.[838] The
axis of rotation was found to be much more bowed towards the
orbital plane than that of the earth, the equator making with it an
angle of 53° 11′.
These conclusions inspired, it is true, much distrust, consequently
there were no received ideas on the subject to be subverted. Nevertheless,
a shock of surprise was felt at Schiaparelli’s announcement,
early in 1890,[839] that Venus most probably rotates after the fashion
just previously ascribed to Mercury. A continuous series of observations,
from November, 1877, to February, 1878, with their records
in above a hundred drawings, supplied the chief part of the data
upon which he rested his conclusions. They certainly appeared
exceptionally well-grounded; and the doubts at first qualifying
them were removed by a fresh set of determinations in July, 1895.[840]
Most observers had depended, in their attempts to ascertain the
rotation-period of Venus, upon evanescent shadings, most likely
of atmospheric origin, and scarcely recognisable from day to day.
Schiaparelli fixed his attention upon round, defined, lustrously white
spots, the presence of which near the cusps of the illuminated
crescent has been attested for close upon two centuries. His steady
watch over them showed the invariability of their position with
regard to the terminator; and this is as much as to say that the
regions of day and night do not shift on the surface of the planet.
In other words, she keeps the same face always turned towards the
sun. Moreover, since her orbit is nearly circular, libratory effects
are very small. They amount in fact to only just one-thirtieth of
those serving to modify the severe contrasts of climate in Mercury.
Confirmatory evidence of Schiaparelli’s result for Venus is not
wanting. Thus, observations irreconcilable with a swift rate of
rotation were made at Bothkamp in 1871 by Vogel and Lohse;[841]
and a drawing executed by Professor Holden with the great
Washington reflector, December 15, 1877, showed the same markings
in the positions recorded at Milan to have been occupied by
them eight hours previously. Further, a series of observations,
carried out by M. Perrotin at Nice, May 15 to October 4, 1890, and
from Mount Mounier in 1895-6, with the special aim of testing the
inference of synchronous rotation and revolution, proved strongly
corroborative of it.[842] A remarkable collection of drawings made by
Mr. Lowell in 1896 appeared decisive in its favour;[843] Tacchini at
Rome,[844] Mascari at Catania and Etna,[845] Cerulli at Terano,[846] obtained
in 1892-6 evidence similar in purport. On the other hand, Niesten
of Brussels found reason to revert to Vico’s discarded elements for
the planet’s rotation;[847] and Trouvelot,[848] Stanley Williams,[849] Villiger,[850]
and Leo Brenner,[851] so far agreed with him as to adopt a period of
approximately twenty-four hours. Finally, E. Von Oppolzer suggested
an appeal to the spectroscope;[852] and Bélopolsky secured in
1900[853] spectrograms apparently marked by the minute displacements
corresponding to a rapid rate of axial movement. But they
were avowedly taken only as an experiment, with unsuitable
apparatus; and the desirable verification of their supposed import
is not yet forthcoming. Until it is, Schiaparelli’s period of 225 days
must be allowed to hold the field.
Effects attributed to great differences of level in the surface of
Venus have struck many observers. Francesco Fontana at Naples
in 1643 noticed irregularities along the inner edge of the crescent.[854]
Lahire in 1700 considered them—regard being had to difference of
distance—to be much more strongly marked than those visible in
the moon.[855] Schröter’s assertions to the same effect, though scouted
with some unnecessary vehemence by Herschel,[856] have since been[Pg 253]
repeatedly confirmed; amongst others by Mädler, De Vico, Langdon,
who in 1873 saw the broken line of the terminator with peculiar
distinctness through a veil of auroral cloud;[857] by Denning,[858] March
30, 1881, despite preliminary impressions to the contrary, as well
as by C. V. Zenger at Prague, January 8, 1883. The great mountain
mass, presumed to occasion the periodical blunting of the southern
horn, was precariously estimated by the Lilienthal observer to rise
to the prodigious height of nearly twenty-seven miles, or just five
times the elevation of Mount Everest! Yet the phenomenon
persists, whatever may be thought of the explanation. Moreover,
the speck of light beyond, interpreted as the visible sign of a
detached peak rising high enough above the encircling shadow to
catch the first and last rays of the sun, was frequently discerned by
Baron Van Ertborn in 1876;[859] while an object near the northern
horn of the crescent, strongly resembling a lunar ring-mountain, was
delineated both by De Vico in 1841 and by Denning forty years
later.
We are almost equally sure that Venus, as that the earth is
encompassed with an atmosphere. Yet, notwithstanding luminous
appearances plainly due to refraction during the transits both of
1761 and 1769, Schröter, in 1792, took the initiative in coming to
a definite conclusion on the subject.[860] It was founded, first, on the
rapid diminution of brilliancy towards the terminator, attributed to
atmospheric absorption; next, on the extension beyond a semicircle
of the horns of the crescent; lastly, on the presence of a bluish
gleam illuminating the early hours of the Cytherean night with what
was taken to be genuine twilight. Even Herschel admitted that
sunlight, by the same effect through which the heavenly bodies show
visibly above our horizons while still geometrically below them, appeared
to be bent round the shoulder of the globe of Venus. Ample confirmation
of the fact has since been afforded. At Dorpat in May,
1849, the planet being within 3° 26′ of inferior conjunction, Mädler
found the arms of waning light upon the disc to embrace no less than
240° of its extent;[861] and in December, 1842, Mr. Guthrie, of Bervie,
N.B., actually observed, under similar conditions, the whole circumference
to be lit up with a faint nebulous glow.[862] The same curious
phenomenon was intermittently seen by Mr. Leeson Prince at
Uckfield in September, 1861;[863] but with more satisfactory distinctness[Pg 254]
by Mr. C. S. Lyman of Yale College,[864] before and after the
conjunction of December 11, 1866, and during nearly five hours
previous to the transit of 1874, when the yellowish ring of refracted
light showed at one point an approach to interruption, possibly
through the intervention of a bank of clouds. Again, on December
2, 1898, Venus being 1° 45′ from the sun’s centre, Mr. H. N. Russell,
of the Halsted Observatory, descried the coalescence of the cusps,
and founded on the observation a valuable discussion of such effects.[865]
Taking account of certain features in the case left unnoticed by
Neison[866] and Proctor,[867] he inferred from them the presence of a
Cytherean atmosphere considerably less refractive than our own,
although possibly, in its lower strata, encumbered with dust or
haze.
Similar appearances are conspicuous during transits. But while
the Mercurian halo is characteristically seen on the sun, the “silver
thread” round the limb of Venus commonly shows on the part off
the sun. There are, however, instances of each description in both
cases. Mr. Grant, in collecting the records of physical phenomena
accompanying the transits of 1761 and 1769, remarks that no one
person saw both kinds of annulus, and argues a dissimilarity in their
respective modes of production.[868] Such a dissimilarity probably
exists, in the sense that the inner section of the ring is illusory, the
outer, a genuine result of the bending of light in a gaseous
envelope; but the distinction of separate visibility has not been
borne out by recent experience. Several of the Australian observers
during the transit of 1874 witnessed the complete phenomenon.
Mr. J. Macdonnell, at Eden, saw a “shadowy nebulous ring”
surround the whole disc when ingress was two-thirds accomplished;
Mr. Tornaghi, at Goulburn, perceived a halo, entire and unmistakable,
at half egress.[869] Similar observations were made at Sydney,[870]
and were renewed in 1882 by Lescarbault at Orgères, by Metzger
in Java, and by Barnard at Vanderbilt University.[871]
Spectroscopic indications of aqueous vapour as present in the
atmosphere of Venus, were obtained in 1874 and 1882, by Tacchini
and Riccò in Italy, and by Young in New Jersey.[872] Janssen, however,
who made a special study of the point subsequently to the
transit of 1882, found them much less certain than he had[Pg 255]
anticipated;[873] and Vogel, by repeated examinations, 1871-73, could detect
only the very slightest variations from the pattern of the solar
spectrum. Some additions there indeed seem to be in the thickening
of a few water and oxygen-lines; but so nearly evanescent as
to induce the persuasion that most of the light we receive from
Venus has traversed only the tenuous upper portion of its atmosphere.[874]
It is reflected, at any rate, with comparatively slight
diminution. On the 26th and 27th of September, 1878, a close
conjunction gave Mr. James Nasmyth the rare opportunity of
watching Venus and Mercury for several hours side by side in the
field of his reflector; when the former appeared to him like clean
silver, the latter as dull as lead or zinc.[875] Yet the light incident
upon Mercury is, on an average, three and a half times as strong
as the light reaching Venus. Thus, the reflective power of Venus
must be singularly strong. And we find, accordingly, from a combination
of Zöllner’s with Müller’s results, that its albedo is but
little inferior to that of new-fallen snow; in other words, it gives
back 77 per cent. of the luminous rays impinging upon it.
This extraordinary brilliancy would be intelligible were it permissible
to suppose that we see nothing of the planet but a dense
canopy of clouds. But the hypothesis is discountenanced by the
Flagstaff observations, and is irreconcilable with the visibility of
mountainous elevations, and permanent surface-markings. To
Mr. Lowell these were so distinct and unchanging as to furnish data
for a chart of the Cytherean globe, and the peculiar arrangement of
divergent shading exhibited in it cannot off-hand be set down as
unreal, in view of Perrotin’s earlier discernment of analogous linear
traces. Gruithuisen’s “snow-caps,”[876] however—it is safe to say—do
not exist as such; although shining regions near the poles form a
well-attested trait of the strange Cytherean landscape.
The “secondary,” or “ashen light,” of Venus was first noticed
by Riccioli in 1643; it was seen by Derham about 1715, by Kirch
in 1721, by Schröter and Harding in 1806;[877] and the reality of the
appearance has since been authenticated by numerous and trustworthy
observations. It is precisely similar to that of the “old
moon in the new moon’s arms”; and Zenger, who witnessed it with
unusual distinctness, January 8, 1883,[878] supposes it due to the same
cause—namely, to the faint gleam of reflected earth-light from the
night-side of the planet. When we remember, however, that “full[Pg 256]
earth-light” on Venus, at its nearest, has little more than 1/12000 its
intensity on the moon, we see at once that the explanation is inadequate.
Nor can Professor Safarik’s,[879] by phosphorescence of the
warm and teeming oceans with which Zöllner[880] regarded the globe
of Venus as mainly covered, be seriously entertained. Vogel’s
suggestion is more plausible. He and O. Lohse, at Bothkamp,
November 3 to 11, 1871, saw the dark hemisphere partially illuminated
by secondary light, extending 30° from the terminator, and
thought the effect might be produced by a very extensive twilight.[881]
Others have had recourse to the analogy of our auroræ, and J. Lamp
suggested that the grayish gleam, visible to him at Bothkamp,
October 21 and 26, 1887,[882] might be an accompaniment of electrical
processes connected with the planet’s meteorology. Whatever the
origin of the phenomenon, it may serve, on a night-enwrapt hemisphere,
to dissipate some of the thick darkness otherwise encroached
upon only by “the pale light of stars.”
Venus was once supposed to possess a satellite. But belief in its
existence has died out. No one, indeed, has caught even a deceptive
glimpse of such an object during the last 125 years. Yet it was
repeatedly and, one might have thought, well observed in the seventeenth
and eighteenth centuries. Fontana “discovered” it in 1645;
Cassini—an adept in the art of seeing—recognised it in 1672, and
again in 1686; Short watched it for a full hour in 1740 with varied
instrumental means; Tobias Mayer in 1759, Montaigne in 1761;
several astronomers at Copenhagen in March, 1764, noted what they
considered its unmistakable presence; as did Horrebow in 1768.
But M. Paul Stroobant,[883] who in 1887 submitted all the available
data on the subject to a searching examination, identified Horrebow’s
satellite with θ Libræ, a fifth-magnitude star; and a few other
apparitions were, by his industry, similarly explained away. Nevertheless,
several withstood all efforts to account for them, and together
form a most curious case of illusion. For it is quite certain that
Venus has no such conspicuous attendant.
The third planet encountered in travelling outward from the sun
is the abode of man. He has in consequence opportunities for
studying its physical habitudes altogether different from the baffling
glimpse afforded to him of the other members of the solar family.
Regarding the earth, then, a mass of knowledge so varied and comprehensive
has been accumulated as to form a science—or rather
several sciences—apart. But underneath all lie astronomical relations,
the recognition and investigation of which constitute one of
the most significant intellectual events of the present century.
It is indeed far from easy to draw a line of logical distinction
between items of knowledge which have their proper place here,
and those which should be left to the historian of geology. There
are some, however, of which the cosmical connections are so close
that it is impossible to overlook them. Among these is the ascertainment
of the solidity of the globe. At first sight it seems difficult
to conceive what the apparent positions of the stars can have to do
with subterranean conditions; yet it was from star measurements
alone that Hopkins, in 1839, concluded the earth to be solid to a
depth of at least 800 or 1,000 miles.[884] His argument was, that if it
were a mere shell filled with liquid, precession and nutation would
be much larger than they are observed to be. For the shell alone
would follow the pull of the sun and moon on its equatorial girdle,
leaving the liquid behind; and being thus so much the lighter, would
move the more readily. There is, it is true, grave reason to doubt
whether this reasoning corresponds with the actual facts of the
case;[885] but the conclusion to which it led has been otherwise affirmed
and extended.
Indications of an identical purport have been derived from another
kind of external disturbance, affecting our globe through the same
agencies. Lord Kelvin (then Sir William Thomson) pointed out in
1862[886] that tidal influences are brought to bear on land as well as on
water, although obedience to them is perceptible only in the mobile
element. Some bodily distortion of the earth’s figure must, however,
take place, unless we suppose it of absolute or “preternatural”
rigidity, and the amount of such distortion can be determined
from its effect in diminishing oceanic tides below their calculated
value. For if the earth were perfectly plastic to the stresses
of solar and lunar gravity, tides—in the ordinary sense—would
not exist. Continents and oceans would swell and subside together.
It is to the difference in the behaviour of solid and liquid[Pg 258]
terrestrial constituents that the ebb and flow of the waters
are due.
Six years later, the distinguished Glasgow professor suggested
that this criterion might, by the aid of a prolonged series of exact
tidal observations, be practically applied to test the interior
condition of our planet.[887] In 1882, accordingly, suitable data extending
over thirty-three years having at length become available,
Mr. G. H. Darwin performed the laborious task of their analysis,
with the general result that the “effective rigidity” of the earth’s
mass must be at least as great as that of steel.[888]
Ratification from an unexpected quarter has lately been brought
to this conclusion. The question of a possible mobility in the
earth’s axis of rotation has often been mooted. Now at last it has
received an affirmative reply. Dr. Küstner detected, in his observations
of 1884-85, effects apparently springing from a minute variation
in the latitude of Berlin. The matter having been brought before
the International Geodetic Association in 1888, special observations
were set on foot at Berlin, Potsdam, Prague, and Strasbourg, the
upshot of which was to bring plainly to view synchronous, and
seemingly periodic fluctuations of latitude to the extent of half a
second of arc. The reality of these was verified by an expedition to
Honolulu in 1891-92, the variations there corresponding inversely to
those simultaneously determined in Europe.[889] Their character was
completely defined by Mr. S. C. Chandler’s discussion in October,
1891.[890] He showed that they could be explained by supposing the
pole of the earth to describe a circle with a radius of thirty feet in
a period of fourteen months. Confirmation of this hypothesis was
found by Dr. B. A. Gould in the Cordoba observations,[891] and it was
provided with a physical basis through the able co-operation of
Professor Newcomb.[892] The earth, owing to its ellipsoidal shape,
should, apart from disturbance, rotate upon its “axis of figure,” or
shortest diameter; since thus alone can the centrifugal forces
generated by its spinning balance each other. Temporary causes,
however, such as heavy falls of snow or rain limited to one continental
area, the shifting of ice-masses, even the movements of winds,
may render the globe slightly lop-sided, and thus oblige it to forsake
its normal axis, and rotate on one somewhat divergent from it.
This “instantaneous axis” (for it is incessantly changing) must, by
mathematical theory, revolve round the axis of figure in a period of
306 days. Provided, that is to say, the earth were a perfectly
rigid body. But it is far from being so; it yields sensibly to every[Pg 259]
strain put upon it; and this yielding tends to protract the time of
circulation of the displaced pole. The length of its period, then,
serves as a kind of measure of the plasticity of the globe; which,
according to Newcomb’s and S. S. Hough’s independent calculations,[893]
seems to be a little less than that of steel. In an earth compacted
of steel, the instantaneous axis would revolve in 441 days; in the
actual earth, the process is accomplished in 428 days. By this new
path, accordingly, astronomers have been led to an identical estimate
of the consistence of our globe with that derived from tidal investigations.
Variations of latitude are intrinsically complex. To produce
them, an incalculable interplay of causes must be at work, each
with its proper period and law of action.[894] All the elements of
the phenomenon are then in a perpetual state of flux,[895] and absorb for
their continual redetermination, the arduous and combined labours
of many astronomers. Nor is this trouble superfluous. Minute in
extent though they be, the shiftings of the pole menace the very
foundations of exact celestial science; their neglect would leave the
entire fabric insecure. Just at the beginning of the present century
they reached a predicted minimum, but are expected again to
augment their range after the year 1902. The interesting suggestion
has been made by Mr. J. Halm that such fluctuations are, in some
obscure way, affected by changes in solar activity, and conform like
them to an eleven-year cycle.[896]
In a paper read before the Geological Society, December 15, 1830,[897]
Sir John Herschel threw out the idea that the perplexing changes
of climate revealed by the geological record might be explained
through certain slow fluctuations in the eccentricity of the earth’s
orbit, produced by the disturbing action of the other planets.
Shortly afterwards, however, he abandoned the position as untenable;[898]
and it was left to the late Dr. James Croll, in 1864[899] and
subsequent years, to reoccupy and fortify it. Within restricted
limits (as Lagrange and, more certainly and definitely, Leverrier
proved), the path pursued by our planet round the sun alternately
contracts, in the course of ages, into a moderate ellipse, and expands
almost to a circle, the major axis, and consequently the mean[Pg 260]
distance, remaining invariable. Even at present, when the eccentricity
approaches a minimum, the sun is nearer to us in January than in
July by above three million miles, and some 850,000 years ago this
difference was more than four times as great. Dr. Croll brought
together[900] a mass of evidence to support the view, that, at epochs of
considerable eccentricity, the hemisphere of which the winter, occurring
at aphelion, was both intensified and prolonged, must have
undergone extensive glaciation; while the opposite hemisphere, with
a short, mild winter, and long, cool summer, enjoyed an approach
to perennial spring. These conditions were exactly reversed at the
end of 10,500 years, through the shifting of the perihelion combined
with the precession of the equinoxes, the frozen hemisphere blooming
into a luxuriant garden as its seasons came round to occur at the
opposite sites of the terrestrial orbit, and the vernal hemisphere
subsiding simultaneously into ice-bound rigour.[901] Thus a plausible
explanation was offered of the anomalous alternations of glacial and
semi-tropical periods, attested, on incontrovertible geological
evidence, as having succeeded each other in times past over what
are now temperate regions. They succeeded each other, it is true,
with much less frequency and regularity than the theory demanded;
but the discrepancy was overlooked or smoothed away. The most
recent glacial epoch was placed by Dr. Croll about 200,000 years
ago, when the eccentricity of the earth’s orbit was 3·4 times as great
as it is now. At present a faint representation of such a state of
things is afforded by the southern hemisphere. One condition of
glaciation in the coincidence of winter with the maximum of remoteness
from the sun, is present; the other—a high eccentricity—is
deficient. Yet the ring of ice-bound territory hemming in the
southern pole is well known to be far more extensive than the corresponding
region in the north.
The verification of this ingenious hypothesis depends upon a
variety of intricate meteorological conditions, some of which have
been adversely interpreted by competent authorities.[902] What is still
more serious, its acceptance seems precluded by time-relations of a
simple kind. Dr. Wright[903] has established with some approach to
certainty that glacial conditions ceased in Canada and the United
States about ten or twelve thousand years ago. The erosive action
of the Falls of Niagara qualifies them to serve as a clepsydra, or
water-clock on a grand scale; and their chronological indications
have been amply corroborated elsewhere and otherwise on the same[Pg 261]
continent. The astronomical Ice Age, however, should have been
enormously more antique. No reconciliation of the facts with the
theory appears possible.
The first attempt at an experimental estimate of the “mean
density” of the earth was Maskelyne’s observation in 1774 of the
deflection of a plumb-line through the attraction of Schehallien.
The conclusion thence derived, that our globe weighs 4-1/2 times as
much as an equal bulk of water,[904] was not very exact. It was considerably
improved upon by Cavendish, who, in 1798, brought into
use the “torsion-balance” constructed for the same purpose by John
Michell. The resulting estimate of 5·48 was raised to 5·66 by
Francis Baily’s elaborate repetition of the process in 1838-42. From
experiments on the subject made in 1872-73 by Cornu and Baille
the slightly inferior value of 5·56 was derived; and it was further
shown that the data collected by Baily, when corrected for a systematic
error, gave practically the same result (5·55).[905] M. Wilsing’s
of 5·58, obtained at Potsdam in 1889,[906] nearly agreed with it; while
Professor Poynting, by means of a common balance, arrived at a
terrestrial mean density of 5·49.[907] Professor Boys next entered the
field with an exquisite apparatus, in which a quartz fibre performed
the functions of a torsion-rod; and the figure 5·53 determined by
him, and exactly confirmed by Dr. Braun’s research at Mariaschein,
Bohemia, in 1896,[908] may be called the standard value of the required
datum. Newton’s guess at the average weight of the earth as five
or six times that of water has thus been curiously verified.
Operations for determining the figure of the earth were carried
out during the last century on an unprecedented scale. The Russo-Scandinavian
arc, of which the measurement was completed under
the direction of the elder Struve in 1855, reached from Hammerfest
to Ismailia on the Danube, a length of 25° 20′. But little inferior
to it was the Indian arc, begun by Lambton in the first years of the
century, continued by Everest, revised and extended by Walker.
Both were surpassed in compass by the Anglo-French arc, which
embraced 28°; and considerable segments of meridians near the
Atlantic and Pacific shores of North America were measured under
the auspices of the United States Coast Survey. But these operations
shrink into insignificance by comparison with Sir David Gill’s
grandiose scheme for uniting two hemispheres by a continuous
network of triangulation. The history of geodesy in South Africa[Pg 262]
began with Lacaille’s measurements in 1752. They were repeated
and enlarged in scope by Sir Thomas Maclear in 1841-48; and his
determinations prepared the way for a complete survey of Cape
Colony and Natal, executed during the ten years 1883-92 by Colonel
Morris, R.E., under the direction of Sir David Gill.[909] Bechuanaland
and Rhodesia were subsequently included in the work; and the
Royal Astronomer obtained, in 1900, the support of the International
Geodetic Association for its extension to the mouth of the Nile.
Nor was this the limit of his design. By carrying the survey along
the Levantine coast, connection can be established with Struve’s
system, and the magnificent amplitude of 105° will be given to the
conjoined African and European arcs. Meantime, the French have
undertaken the remeasurement of Bouguer’s Peruvian arc, and a
corresponding Russo-Swedish[910] enterprise is progressing in Spitzbergen;
so that abundant materials will ere long be provided for
fresh investigations of the shape and size of our planet. The smallness
of the outstanding uncertainty can be judged of by comparing
J. B. Listing’s[911] with General Clarke’s[912] results, published in the same
year (1878). Listing stated the dimensions of the terrestrial spheroid
as follows: Equatorial radius = 3,960 miles; polar radius = 3,947
miles; ellipticity = 1/288·5. Clarke’s corresponding figures were:
3,963 and 3,950 miles, giving an ellipticity of 1/293·5. The value of
the latter fraction at present generally adopted is 1/292; that is to
say, the thickness of the protuberant equatorial ring is held to be
1/292 of the equatorial radius. From astronomical considerations, it
is true, Newcomb estimated the ratio at 1/308;[913] but for obtaining
this particular datum, geodetical methods are unquestionably to be
preferred.
The moon possesses for us a unique interest. She in all probability
shared the origin of the earth; she perhaps prefigures its
decay. She is at present its minister and companion. Her existence,
so far as we can see, serves no other purpose than to illuminate
the darkness of terrestrial nights, and to measure, by swiftly-recurring
and conspicuous changes of aspect, the long span of terrestrial
time. Inquiries stimulated by visible dependence, and aided by
relatively close vicinity, have resulted in a wonderfully minute
acquaintance with the features of the single lunar hemisphere open
to our inspection.
Selenography, in the modern sense, is little more than a hundred
years old. It originated with the publication in 1791 of Schröter’s
Selenotopographische Fragmente.[914] Not but that the lunar surface had
already been diligently studied, chiefly by Hevelius, Cassini, Riccioli,
and Tobias Mayer; the idea, however, of investigating the moon’s
physical condition, and detecting symptoms of the activity there of
natural forces through minute topographical inquiry, first obtained
effect at Lilienthal. Schröter’s delineations, accordingly, imperfect
though they were, afforded a starting-point for a comparative study
of the superficial features of our satellite.
The first of the curious objects which he named “rills” was noted
by him in 1787. Before 1801 he had found eleven; Lohrmann
added 75; Mädler 55; Schmidt published in 1866 a catalogue
of 425, of which 278 had been detected by himself;[915] and he
eventually brought the number up to nearly 1,000. They are,
then, a very persistent lunar feature, though wholly without
terrestrial analogue. There is no difference of opinion as to their
nature. They are quite obviously clefts in a rocky surface, 100 to
500 yards deep, usually a couple of miles across, and pursuing
straight, curved, or branching tracks up to 150 miles in length.
As regards their origin, the most probable view is that they are
fissures produced in cooling; but Neison inclines to consider them
rather as dried watercourses.[916]
On February 24, 1792, Schröter perceived what he took to be
distinct traces of a lunar twilight, and continued to observe them
during nine consecutive years.[917] They indicated, he thought, the
presence of a shallow atmosphere, about 29 times more tenuous than
our own. Bessel, on the other hand, considered that the only way
of “saving” a lunar atmosphere was to deny it any refractive power,
the sharpness and suddenness of star-occultations negativing the
possibility of gaseous surroundings of greater density (admitting an
extreme supposition) than 1/500 that of terrestrial air.[918] Newcomb
places the maximum at 1/400. Sir John Herschel concluded “the
non-existence of any atmosphere at the moon’s edge having 1/1980
part of the density of the earth’s atmosphere.”[919]
This decision was fully borne out by Sir William Huggins’s
spectroscopic observation of the disappearance behind the moon’s
limb of the small star ε Piscium, January 4, 1865.[920] Not the slightest[Pg 264]
sign of selective absorption or unequal refraction was discernible.
The entire spectrum went out at once, as if a slide had suddenly
dropped over it. The spectroscope has uniformly told the same
tale; for M. Thollon’s observation during the total solar eclipse at
Sohag of a supposed thickening at the moon’s rim, of certain dark
lines in the solar spectrum, is now acknowledged to have been
illusory. Moonlight, analysed with the prism, is found to be pure
reflected sunlight, diminished in quantity, owing to the low reflective
capability of the lunar surface, to less than one-fifth its incident
intensity, but wholly unmodified in quality.
Nevertheless, the diameter of the moon appeared from the
Greenwich observations discussed by Airy in 1865[921] to be 4′ smaller
than when directly measured; and the effect would be explicable
by refraction in a lunar atmosphere 2,000 times thinner than our
own at the sea-level. But the difference was probably illusory. It
resulted in part, if not wholly, from the visual enlargement by irradiation
of the bright disc of the moon. Professor Comstock, employing
the 16-inch Clark equatoreal of the Washburn Observatory,
found in 1897 the refractive displacements of occulted stars so
trifling as to preclude the existence of a permanent lunar atmosphere
of much more than 1/5000 the density of the terrestrial
envelope.[922] The possibility, however, was admitted that, on the
illuminated side of the moon, temporary exhalations of aqueous
vapour might arise from ice-strata evaporated by sun-heat. Meantime,
some renewed evidence of actual crepuscular gleams on the
moon had been gathered by MM. Paul and Prosper Henry of the
Paris Observatory, as well as by Mr. W. H. Pickering, in the pure
air of Arequipa, at an altitude of 8,000 feet above the sea.[923] An
occultation of Jupiter, too, observed by him August 12, 1892,[924] was
attended with a slight flattening of the planet’s disc through the
effect, it was supposed, of lunar refraction—but of refraction in
an atmosphere possessing, at the most, 1/4000 the density at the
sea-level of terrestrial air, and capable of holding in equilibrium no
more than 1/250 of an inch of mercury. Yet this small barometric
value corresponds, Mr. Pickering remarks, “to a pressure of hundreds
of tons per square mile of the lunar surface.” The compression
downward of gaseous strata on the moon should, in any case,
proceed very gradually, owing to the slight power of lunar gravity,[925]
and they might hence play an important part in the economy of our
satellite while evading spectroscopic and other tests. Thus—as[Pg 265]
Mr. Ranyard remarked[926]—the cliffs and pinnacles of the moon bear
witness, by their unworn condition, to the efficiency of atmospheric
protection against meteoric bombardment; and Mr. Pickering shows
that it could be afforded by such a tenuous envelope as that postulated
by him.
The first to emulate Schröter’s selenographical zeal was Wilhelm
Gotthelf Lohrmann, a land-surveyor of Dresden, who, in 1824,
published four out of twenty-five sections of the first scientifically
executed lunar chart, on a scale of 37-1/2 inches to a lunar diameter.
His sight, however, began to fail three years later, and he died in
1840, leaving materials from which the work was completed and
published in 1878 by Dr. Julius Schmidt, late director of the Athens
Observatory. Much had been done in the interim. Beer and
Mädler began at Berlin in 1830 their great trigonometrical survey
of the lunar surface, as yet neither revised nor superseded. A map,
issued in four parts, 1834-36, on nearly the same scale as Lohrmann’s,
but more detailed and authoritative, embodied the results.
It was succeeded, in 1837, by a descriptive volume bearing the
imposing title, Der Mond; oder allgemeine vergleichende Selenographie.
This summation of knowledge in that branch, though in truth
leaving many questions open, had an air of finality which tended to
discourage further inquiry.[927] It gave form to a reaction against the
sanguine views entertained by Hevelius, Schröter, Herschel and
Gruithuisen as to the possibilities of agreeable residence on the
moon, and relegated the “Selenites,” one of whose cities Schröter
thought he had discovered, and of whose festal processions Gruithuisen
had not despaired of becoming a spectator, to the shadowy
land of the Ivory Gate. All examples of change in lunar formations
were, moreover, dismissed as illusory. The light contained
in the work was, in short, a “dry light,” not stimulating to
the imagination. “A mixture of a lie,” Bacon shrewdly remarks,
“doth ever add pleasure.” For many years, accordingly, Schmidt
had the field of selenography almost to himself.
Reviving interest in the subject was at once excited and displayed
by the appointment, in 1864, of a Lunar Committee of the British
Association. The indirect were of greater value than the direct
fruits of its labours. An English school of selenography rose into
importance. Popularity was gained for the subject by the diffusion
of works conspicuous for ingenuity and research. Nasmyth’s and
Carpenter’s beautifully illustrated volume (1874) was succeeded,
after two years, by a still more weighty contribution to lunar
science in Mr. Neison’s well-known book, accompanied by a map,
based on the survey of Beer and Mädler, but adding some 500[Pg 266]
measures of positions, besides the representation of several thousand
new objects. With Schmidt’s Charte der Gebirge der Mondes,
Germany once more took the lead. This splendid delineation,
built upon Lohrmann’s foundation, embraced the detail contained
in upwards of 3,000 original drawings, representing the labour of
thirty-four years. No less than 32,856 craters are represented in it,
on a scale of seventy-five inches to a diameter. An additional help
to lunar inquiries was provided at the same time in this country by
the establishment, through the initiative of the late Mr. W. R. Birt,
of the Selenographical Society.
But the strongest incentive to diligence in studying the rugged
features of our celestial helpmate has been the idea of probable or
actual variation in them. A change always seems to the inquisitive
intellect of man like a breach in the defences of Nature’s secrets,
through which it may hope to make its way to the citadel. What
is desirable easily becomes credible; and thus statements and
rumours of lunar convulsions have successively, during the last
hundred years, obtained credence, and successively, on closer
investigation, been rejected. The subject is one as to which
illusion is peculiarly easy. Our view of the moon’s surface is a
bird’s-eye view. Its conformation reveals itself indirectly through
irregularities in the distribution of light and darkness. The forms
of its elevations and depressions can be inferred only from the
shapes of the black, unmitigated shadows cast by them. But
these shapes are in a state of perpetual and bewildering fluctuation,
partly through changes in the angle of illumination, partly through
changes in our point of view, caused by what are called the moon’s
“librations.”[928] The result is, that no single observation can be
exactly repeated by the same observer, since identical conditions
recur only after the lapse of a great number of years.
Local peculiarities of surface, besides, are liable to produce perplexing
effects. The reflection of earth-light at a particular angle
from certain bright summits completely, though temporarily,
deceived Herschel into the belief that he had witnessed, in 1783
and 1787, volcanic outbursts on the dark side of the moon. The
persistent recurrence, indeed, of similar appearances under circumstances
less amenable to explanation inclined Webb to the view[Pg 267]
that effusions of native light actually occur.[929] More cogent proofs
must, however, be adduced before a fact so intrinsically improbable
can be admitted as true.
But from the publication of Beer and Mädler’s work until 1866,
the received opinion was that no genuine sign of activity had ever
been seen, or was likely to be seen, on our satellite; that her face
was a stereotyped page, a fixed and irrevisable record of the past.
A profound sensation, accordingly, was produced by Schmidt’s
announcement, in October, 1866, that the crater “Linné,” in the
Mare Serenitatis, had disappeared,[930] effaced, as it was supposed, by
an igneous outflow. The case seemed undeniable, and is still
dubious. Linné had been known to Lohrmann and Mädler,
1822-32, as a deep crater, five or six miles in diameter, the third
largest in the dusky plain known as the “Mare Serenitatis”; and
Schmidt had observed and drawn it, 1840-43, under a practically
identical aspect. Now it appears under high light as a whitish spot,
in the centre of which, as the rays begin to fall obliquely, a pit,
scarcely two miles across, emerges into view.[931] The crateral character
of this comparatively minute depression was detected by
Father Secchi, February 11, 1867.
This is not all. Schröter’s description of Linné, as seen by
him November 5, 1788, tallies quite closely with modern observation;[932]
while its inconspicuousness in 1797 is shown by its omission
from Russell’s lunar globe and maps.[933] We are thus driven
to adopt one of two suppositions: either Lohrmann, Mädler, and
Schmidt were entirely mistaken in the size and importance of Linné,
or a real change in its outward semblance supervened during the
first half of the century, and has since passed away, perhaps again
to recur. The latter hypothesis seems the more probable: and its
probability is strengthened by much evidence of actual obscuration
or variation of tint in other parts of the lunar surface, more
especially on the floor of the great “walled plain” named “Plato.”[934]
From a re-examination with a 13-inch refractor at Arequipa in
1891-92, of this region, and of the Mare Serenitatis, Mr. W. H.
Pickering inclines to the belief that lunar volcanic action, once
apparently so potent, is not yet wholly extinct.[935]
An instance of an opposite kind of change was alleged by
Dr. Hermann J. Klein of Cologne in March, 1878.[936] In Linné the[Pg 268]
obliteration of an old crater had been assumed; in “Hyginus N.,”
the formation of a new crater was asserted. Yet, quite possibly,
the same cause may have produced the effects thought to be
apparent in both. It is, however, far from certain that any real
change has affected the neighbourhood of Hyginus. The novelty of
Klein’s observation of May 19, 1877, may have consisted simply in
the detection of a hitherto unrecognised feature. The region is one
of complex formation, consequently of more than ordinary liability
to deceptive variations in aspect under rapid and entangled fluctuations
of light and shade.[937] Moreover, it seems to be certain, from
Messrs. Pratt and Capron’s attentive study, that “Hyginus N.” is
no true crater, but a shallow, saucer-like depression, difficult of clear
discernment.[938] Under suitable illumination, nevertheless, it contains,
and is marked by, an ample shadow.[939]
In both these controverted instances of change, lunar photography
was invoked as a witness; but, notwithstanding the great advances
made in the art by De la Rue in this country, by Draper, and,
above all, by Rutherford in America, without decisive results.
Investigations of the kind began to assume a new aspect in
1890, when Professor Holden organised them at the Lick Observatory.[940]
Autographic moon-pictures were no longer taken casually,
but on system; and Dr. Weinek’s elaborate study, and skilful reproductions
of them at Prague,[941] gave them universal value. They
were designed to provide materials for an atlas on the scale of
Beer and Mädler’s, of which some beautiful specimen-plates have
been issued. At Paris, in 1894, with the aid of a large “equatoreal
coudé,” a work of similar character was set on foot by MM. Loewy
and Puiseux. Its progress has been marked by the successive
publication of five instalments of a splendid atlas, on a scale of
about eight feet to the lunar diameter, accompanied by theoretical
dissertations, designed to establish a science of “selenology.” The
moon’s formations are thus not only delineated under every variety
of light-incidence, but their meaning is sought to be elicited, and
their history and mutual relations interpreted.[942] Henceforth, at any
rate, the lunar volcanoes can scarcely, without notice taken, breathe
hard in their age-long sleep.
Melloni was the first to get undeniable heating effects from
moonlight. His experiments, made on Mount Vesuvius early in
1846,[943] were repeated with like result by Zantedeschi at Venice four
years later. A rough measure of the intensity of those effects was
arrived at by Piazzi Smyth at Guajara, on the Peak of Teneriffe, in
1856. At a distance of fifteen feet from the thermomultiplier, a
Price’s candle was found to radiate just twice as much heat as the
full moon.[944] Then, after thirteen years, in 1869-72, an exact and extensive
series of observations on the subject were made by the present
Earl of Rosse. The lunar radiations, from the first to the last
quarter, displayed, when concentrated with the Parsonstown three-foot
mirror, appreciable thermal energy, increasing with the phase,
and largely due to “dark heat,” distinguished from the quicker-vibrating
sort by inability to traverse a plate of glass. This was
supposed to indicate an actual heating of the surface, during the
long lunar day of 300 hours, to about 500° F.[945] (corrected later to
197°),[946] the moon thus acting as a direct radiator no less than as a
reflector of heat. But the conclusion was very imperfectly borne
out by Dr. Boeddicker’s observations with the same instrument and
apparatus during the total lunar eclipse of October 4, 1884.[947] This
initial opportunity of measuring the heat phases of an eclipsed moon
was used with the remarkable result of showing that the heat disappeared
almost completely, though not quite simultaneously, with
the light. Confirmatory evidence of the extraordinary promptitude
with which our satellite parts with heat already to some extent
appropriated, was afforded by Professor Langley’s bolometric observations
at Allegheny of the partial eclipse of September 23, 1885.[948]
Yet it is certain that the moon sends us a perceptible quantity of
heat on its own account, besides simply throwing back solar radiations.
For in February, 1885, Professor Langley succeeded, after many
fruitless attempts, in getting measures of a “lunar heat-spectrum.”
The incredible delicacy of the operation may be judged of from the
statement that the sum-total of the thermal energy dispersed by his
rock-salt prisms was insufficient to raise a thermometer fully exposed
to it one-thousandth of a degree Centigrade! The singular fact was,
however, elicited that this almost evanescent spectrum is made up
of two superposed spectra, one due to reflection, the other, with a
maximum far down in the infra-red, to radiation.[949] The corresponding
temperature of the moon’s sunlit surface Professor Langley[Pg 270]
considers to be about that of freezing water.[950] Repeated experiments
having failed to get any thermal effects from the dark part of
the moon, it was inferred that our satellite “has no internal heat
sensible at the surface”; so that the radiations from the lunar soil
giving the low maximum in the heat-spectrum, “must be due purely
to solar heat which has been absorbed and almost immediately re-radiated.”
Professor Langley’s explorations of the terra incognita
of immensely long wave-lengths where lie the unseen heat-emissions
from the earth into space, led him to the discovery that these,
contrary to the received opinion, are in good part transmissible by
our atmosphere, although they are completely intercepted by glass.
Another important result of the Allegheny work was the abolition
of the anomalous notion of the “temperature of space,” fixed by
Pouillet at -140° C. For space in itself can have no temperature,
and stellar radiation is a negligible quantity. Thus, it is safe to
assume “that a perfect thermometer suspended in space at the
distance of the earth or moon from the sun, but shielded from its
rays, would sensibly indicate the absolute zero,”[951] ordinarily placed
at -273° C.
A “Prize Essay on the Distribution of the Moon’s Heat” (The
Hague), 1891, by Mr. Frank W. Very, who had taken an active
part in Professor Langley’s long-sustained inquiry, embodies the
fruits of its continuation. They show the lunar disc to be tolerably
uniform in thermal power. The brighter parts are also indeed
hotter, but not much. The traces perceived of a slight retention
of heat by the substances forming the lunar surface, agreed well
with the Parsonstown observations of the total eclipse of the moon,
January 28, 1888.[952] For they brought out an unmistakable divergence
between the heat and light phases. A curious decrease of heat
previous to the first touch of the earth’s shadow upon the lunar
globe remains unexplained, unless it be admissible to suppose the
terrestrial atmosphere capable of absorbing heat at an elevation of
190 miles. The probable range of temperature on the moon was
discussed by Professor Very in 1898.[953] He concluded it to be very
wide. Hotter than boiling water under the sun’s vertical rays, the
arid surface of our dependent globe must, he found, cool in the
14-day lunar night to about the temperature of liquid air.
Although that fundamental part of astronomy known as “celestial[Pg 271]
mechanics” lies outside the scope of this work, and we therefore
pass over in silence the immense labours of Plana, Damoiseau,
Hansen, Delaunay, G. W. Hill, and Airy in reconciling the observed
and calculated motions of the moon, there is one slight but significant
discrepancy which is of such importance to the physical
history of the solar system, that some brief mention must be made
of it.
Halley discovered in 1693, by examining the records of ancient
eclipses, that the moon was going faster then than 2,000 years
previously—so much faster, as to have got ahead of the place in
the sky she would otherwise have occupied, by about two of her
own diameters. It was one of Laplace’s highest triumphs to have
found an explanation of this puzzling fact. He showed, in 1787,
that it was due to a very slow change in the ovalness of the earth’s
orbit, tending, during the present age of the world, to render it
more nearly circular. The pull of the sun upon the moon is thereby
lessened; the counter-pull of the earth gets the upper hand; and
our satellite, drawn nearer to us by something less than an inch each
year,[954] proportionately quickens her pace. Many thousands of years
hence the process will be reversed; the terrestrial orbit will close in
at the sides, the lunar orbit will open out under the growing stress
of solar gravity, and our celestial chronometer will lose instead of
gaining time.
This is all quite true as Laplace put it; but it is not enough.
Adams, the virtual discoverer of Neptune, found with surprise in
1853 that the received account of the matter was “essentially incomplete,”
and explained, when the requisite correction was introduced,
only half the observed acceleration.[955] What was to be done
with the remaining half? Here Delaunay, the eminent French
mathematical astronomer, unhappily drowned at Cherbourg in 1872
by the capsizing of a pleasure-boat, came to the rescue.[956]
It is obvious to anyone who considers the subject a little attentively,
that the tides must act to some extent as a friction-brake
upon the rotating earth. In other words, they must bring about
an almost infinitely slow lengthening of the day. For the two
masses of water piled up by lunar influence on the hither and
farther sides of our globe, strive, as it were, to detach themselves
from the unity of the terrestrial spheroid, and to follow the movements
of the moon. The moon, accordingly, holds them against the
whirling earth, which revolves like a shaft in a fixed collar, slowly[Pg 272]
losing motion and gaining heat, eventually dissipated through space.[957]
This must go on (so far as we can see) until the periods of the
earth’s rotation and of the moon’s revolution coincide. Nay, the
process will be continued—should our oceans survive so long—by
the feebler tide-raising power of the sun, ceasing only when day and
night cease to alternate, when one side of our planet is plunged in
perpetual darkness and the other seared by unchanging light.
Here, then, we have the secret of the moon’s turning always the
same face towards the earth. It is that in primeval times, when the
moon was liquid or plastic, an earth-raised tidal wave rapidly and
forcibly reduced her rotation to its present exact agreement with
her period of revolution. This was divined by Kant[958] nearly a
century before the necessity for such a mode of action presented
itself to any other thinker. In a weekly paper published at Königsberg
in 1754, the modern doctrine of “tidal friction” was clearly
outlined by him, both as regards its effects actually in progress on
the rotation of the earth, and as regards its effects already consummated
on the rotation of the moon—the whole forming a preliminary
attempt at what he called a “natural history” of the
heavens. His sagacious suggestion, however, remained entirely
unnoticed until revived—it would seem independently—by Julius
Robert Mayer in 1848;[959] while similar, and probably original, conclusions
were reached by William Ferrel of Allensville, Kentucky,
in 1858.[960]
Delaunay was not then the inventor or discoverer of tidal friction;
he merely displayed it as an effective cause of change. He showed
reason for believing that its action in checking the earth’s rotation,
far from being, as Ferrel had supposed, completely neutralised by
the contraction of the globe through cooling, was a fact to be
reckoned with in computing the movements, as well as in speculating
on the history, of the heavenly bodies. The outstanding
acceleration of the moon was thus at once explained. It was
explained as apparent only—the reflection of a real lengthening, by
one second in 100,000 years, of the day. But on this point the
last word has not yet been spoken.
Professor Newcomb undertook in 1870 the onerous task of investigating
the errors of Hansen’s Lunar Tables as compared with[Pg 273]
observations prior to 1750. The results, published in 1878,[961] proved
somewhat perplexing. They tend, in general, to reduce the amount
of acceleration left unaccounted for by Laplace’s gravitational
theory, and proportionately to diminish the importance of the part
played by tidal friction. But, in order to bring about this diminution,
and at the same time conciliate Alexandrian and Arabian
observations, it is necessary to reject as total the ancient solar
eclipses known as those of Thales and Larissa. This may be a necessary,
but it must be admitted to be a hazardous expedient. Its
upshot was to indicate a possibility that the observed and calculated
values of the moon’s acceleration might after all prove to be identical;
and the small outstanding discrepancy was still further
diminished by Tisserand’s investigation, differently conducted, of the
same Arabian eclipses discussed by Newcomb.[962] The necessity of
having recourse to a lengthening day is then less pressing than it
seemed some time ago; and the effect, if perceptible in the moon’s
motion, should, M. Tisserand remarked, be proportionately so in the
motions of all the other heavenly bodies. The presence of the
apparent general acceleration that should ensue can be tested with
most promise of success, according to the same authority, by delicate
comparisons of past and future transits of Mercury.
Newcomb further showed that small residual irregularities are
still found in the movements of our satellite, inexplicable either by
any known gravitational influence, or by any uniform value that
could be assigned to secular acceleration.[963] If set down to the
account of imperfections in the “time-keeping” of the earth, it
could only be on the arbitrary supposition of fluctuations in its rate
of going themselves needing explanation. This, it is true, might be
found in very slight changes of figure,[964] not altogether unlikely to
occur. But into this cloudy and speculative region astronomers for
the present decline to penetrate. They prefer, if possible, to deal
only with calculable causes, and thus to preserve for their “most
perfect of sciences” its special prerogative of assured prediction.
FOOTNOTES:
[796] Neueste Beyträge zur Erweiterung der Sternkunde, Bd. iii., p. 14 (1800).
[797] Ibid., p. 24.
[798] Phil. Trans., vol. xciii., p. 215.
[799] Mem. Roy. Astr. Soc., vol. vi., p. 116.
[800] Month. Not., vol. xix., pp. 11, 25.
[801] Ibid., vol. xxxviii., p. 398.
[802] Am. Jour. of Sc., vol. xvi., p. 124.
[803] Wash. Obs. for 1876, Part ii., p. 34.
[804] Pop. Astr., vol. ii., p. 168; Astr. Jour., No. 335.
[805] Astr. and Astrophysics, vol. xiii., p. 866.
[806] Ibid., p. 867.
[807] Month. Not., vol. xxiv., p. 18.
[808] Ibid., vol. xxiii., p. 234 (Challis).
[809] Untersuchungen über die Spectra der Planeten, p. 9.
[810] Sirius, vol. vii., p. 131.
[811] Potsdam Publ., No. 30; Astr. Nach., No. 3,171; Frost, Astr. and Astrophysics,
vol. xii., p. 619.
[812] Zöllner and Winnecke made it=O·13, Astr. Nach., No. 2,245.
[813] Neueste Beyträge, Bd. iii., p. 50.
[814] Astr. Jahrbuch, 1804, pp. 97-102.
[815] Webb, Celestial Objects, p. 46 (4th ed.).
[816] L’Astronomie, t. ii., p. 141.
[817] Observations sur les Planètes Vénus et Mercure, p. 87.
[818] Observatory, vol. vi., p. 40.
[819] Atti dell’ Accad. dei Lincei, t. v. ii., p. 283, 1889; Astr. Nach., No. 2,944.
[820] Astr. Nach. No. 2,479.
[821] Memoirs Amer. Acad., vol. xii., No. 4, p. 464.
[822] Hist. de l’Astr., p. 682.
[823] Comptes Rendus, t. xlix., p. 379.
[824] Comptes Rendus, t. l., p. 40.
[825] Ibid., p. 46.
[826] Astr. Nach., Nos. 1,248 and 1,281.
[827] Comptes Rendus, t. lxxxiii., pp. 510, 561.
[828] Handbuch der Mathematik, Bd. ii., p. 327.
[829] Comptes Rendus, t. lxxxiii., p. 721.
[830] Nature, vol. xviii., pp. 461, 495, 539.
[831] Oppolzer, Astr. Nach., No. 2,239.
[832] Ibid., Nos. 2,253-4 (C. H. F. Peters).
[833] Ibid., Nos. 2,263 and 2,277. See also Tisserand in Ann. Bur. des Long.,
1882, p. 729.
[834] See J. Bauschinger’s Untersuchungen (1884), summarised in Bull. Astr., t. i.,
p. 506, and Astr. Nach., No. 2,594. Newcomb finds the anomalous motion of the
perihelion to be even larger (43′ instead of 38′) than Leverrier made it. Month.
Not., February, 1884, p. 187. Harzer’s attempt to account for it in Astr. Nach.,
No. 3,030, is more ingenious than successful.
[835] Jour. des Sçavans, December, 1667, p. 122.
[836] Élémens d’Astr., p. 525. Cf. Chandler, Pop. Astr., February, 1897, p. 393.
[837] Beobachtungen über die sehr beträchtlichen Gebirge und Rotation der Venus,
1792, p. 35. Schröter’s final result in 1811 was 23h. 21m. 7·977s. Monat.
Corr., Bd. xxv., p. 367.
[838] Astr. Nach., No. 404.
[839] Rendiconti del R. Istituto Lombardo, t. xxiii., serie ii.
[840] Astr. Nach., No. 3,304.
[841] Bothkamp Beobachtungen, Heft ii., p. 120.
[842] Comptes Rendus, t. cxi., p. 542; t. cxxii., p. 395.
[843] Month. Not., vol. lvii., p. 402; Astr. Nach., No. 3,406.
[844] Mem. Spettroscopisti Italiani, t. xxv., p. 93; Nature, vol. liii., p. 306.
[845] Astr. Nach., No. 3,329.
[846] Ibid.
[847] Bull. de l’Acad. de Belgique, t. xxi., p. 452, 1891.
[848] Observations sur les Planètes Vénus et Mercure, 1892.
[849] Astr. Nach., No. 3,300.
[850] Ibid., No. 3,332.
[851] Ibid., No. 3,314.
[852] Ibid., No. 3,170.
[853] Ibid., No. 3,641. The velocity of a point on the equator of Venus, if
Brenner’s period of 23h. 57m. were exact, would be 0·28 miles per second; but
the displacements due to this rate would be doubled by reflection.
[854] Novæ Observationes, p. 92.
[855] Mém. de l’Ac., 1700, p. 296.
[856] Phil. Trans., vol. lxxxiii., p. 201.
[857] Webb, Cel. Objects, p. 58.
[858] Month. Not., vol. xlii., p. 111.
[859] Bull. Ac. de Bruxelles, t. xliii., p. 22.
[860] Phil. Trans., vol. lxxxii., p. 309; Aphroditographische Fragmente, p. 85
(1796).
[861] Astr. Nach., No. 679.
[862] Month. Not., vol. xiv., p. 169.
[863] Ibid., vol. xxiv., p. 25.
[864] Am. Jour. of Sc., vol. xliii., p. 129 (2d ser.); vol. ix., p. 47 (3d ser.).
[865] Astroph. Jour., vol. ix., p. 284.
[866] Month. Not., vol. xxxvi., p. 347.
[867] Old and New Astronomy, p. 448.
[868] Hist. Phys. Astr., p. 431.
[869] Mem. Roy. Astr. Soc., vol. xlvii., pp. 77, 84.
[870] Astr. Reg., vol. xiii., p. 132.
[871] L’Astronomie, t. ii., p. 27; Astr. Nach., No. 2,021; Am. Jour. of Sc.,
vol. xxv., p. 430.
[872] Mem. Spettr. Ital., Dicembre, 1882; Am. Jour. of Sc., vol. xxv., p. 328.
[873] Comptes Rendus, t. cxvi., p. 288.
[874] Vogel, Spectra der Planeten, p. 15.
[875] Nature, vol. xix., p. 23.
[876] Nova Acta Acad. Naturæ Curiosorum, Bd. x., 239.
[877] Astr. Jahrbuch, 1809, p. 164.
[878] Month Not., vol. xliii., p. 331.
[879] Report Brit. Ass., 1873, p. 407. The paper contains a valuable record of
observations of the phenomenon.
[880] Photom. Untersuchungen, p. 301.
[881] Bothkamp Beobachtungen, Heft ii., p. 126.
[882] Astr. Nach., No. 2,818.
[883] Mémoires de l’Acad. de Bruxelles, t. xlix., No. 5, 4to; Astr. Nach., No. 2,809;
f. Schorr, Der Venusmond, 1875.
[884] Phil. Trans., 1839, 1841, 1842.
[885] Delaunay objected (Comptes Rendus, t. lxvii., p. 65) that the viscosity of the
contained liquid (of which Hopkins took no account) would, where the movements
were so excessively slow as those of the earth’s axis, almost certainly cause
it to behave like a solid. Lord Kelvin, however (Report Brit. Ass., 1876, ii., p. 1),
considered Hopkins’s argument valid as regards the comparatively quick solar
semi-annual and lunar fortnightly nutations.
[886] Phil. Trans., cliii., p. 573.
[887] Report Brit. Ass., 1868, p. 494.
[888] Ibid., 1882, p. 474.
[889] Albrecht, Astr. Nach., No. 3,131.
[890] Astr. Jour., Nos. 248, 249.
[891] Ibid., No. 258.
[892] Month. Not., vol. lii., p. 336.
[893] Astr. Nach., No. 3,097; Phil. Trans., vol. clxxxvi., A., p. 469; Proc. Roy.
Soc., vol. lix.
[894] See Chandler’s searching investigations, Astr. Jour., Nos. 329, 344, 351, 392,
402, 406, 412, 446, 489, 490, 494, 495.
[895] Rees, Pop. Astr., No. 74, 1900.
[896] Nature, vol. lxi., p. 447; see also A. V. Bäcklund, Astr. Nach., No. 3,787.
[897] Trans. Geol. Soc., vol. iii. (2d ser.), p. 293.
[898] See his Treatise on Astronomy, p. 199 (1833).
[899] Phil. Mag., vol. xxviii. (4th ser.), p. 121.
[900] Climate and Time, 1875; Discussions on Climate and Cosmology, 1885.
[901] See for a popular account of the theory, Sir R. Ball’s The Cause of an Ice
Age, 1892.
[902] See A. Woeikof, Phil. Mag., vol. xxi., p. 223.
[903] The Ice Age in North America, London, 1890.
[904] Phil. Trans., vol. lxviii., p. 783.
[905] Comptes Rendus, t. lxxvi., p. 954.
[906] Potsdam Publ., Nos. 22, 23.
[907] Phil. Trans., vol. clxxxii., p. 565; Adams Prize Essay for 1893.
[908] Denkschriften Akad. der Wiss. Wien, Bd. lxiv.; quoted by Poynting.
Nature, vol. lxii., p. 404.
[909] Report on the Geodetic Survey of S. Africa, 1894.
[910] Nature, vol. lxii., p. 622; Hollis, Observatory, vol. xxiii., p. 337; Poincaré,
Comptes Rendus, July 23, 1900.
[911] Astr. Nach., No. 2,228.
[912] Young’s Gen. Astr., p. 601.
[913] Astr. Constants, p. 195.
[914] The second volume was published at Göttingen in 1802.
[915] Ueber Rillen auf dem Monde, p. 13. Cf. The Moon, by T. Gwyn Elger, p. 20.
W. H. Pickering, Harvard Annals, vol. xxxii., p. 249.
[916] The Moon, p. 73.
[917] Selen. Fragm., Th. ii., p. 399.
[918] Astr. Nach., No. 263 (1834); Pop. Vorl., pp. 615-620 (1838).
[919] Outlines of Astr., par. 431.
[920] Month. Not., vol. xxv., p. 61.
[921] Month. Not., vol. xxv., p. 264.
[922] Astroph. Jour., vol. vi., p. 422.
[923] Harvard Annals, vol. xxxii., p. 81.
[924] Astr. and Astrophysics, vol. xi., p. 778.
[925] Neison, The Moon, p. 25.
[926] Knowledge, vol. xvii., p. 85.
[927] Neison, The Moon, p. 104.
[928] The combination of a uniform rotational with an unequal orbital movement
causes a slight swaying of the moon’s globe, now east, now west, by which we
are able to see round the edges of the averted hemisphere. There is also a
“parallactic” libration, depending on the earth’s rotation; and a species of
nodding movement—the “libration in latitude”—is produced by the inclination
of the moon’s axis to her orbit, and by her changes of position with regard to the
terrestrial equator. Altogether, about 2/11 of the invisible side come into view.
[929] Cel. Objects, p. 58 (4th ed.).
[930] Astr. Nach., No. 1,631.
[931] Cf. Leo Brenner, Naturwiss. Wochenschrift, January 13, 1895; Jour. Brit.
Astr. Ass., vol. v., pp. 29, 222.
[932] Respighi, Les Mondes, t. xiv., p. 294; Huggins, Month. Not., vol. xxvii., p. 298.
[933] Birt, Ibid., p. 95.
[934] Report Brit. Ass., 1872, p. 245.
[935] Observatory, vol. xv., p. 250.
[936] Astr. Reg., vol. xvi., p. 265; Astr. Nach., No. 2,275.
[937] Lindsay and Copeland, Month. Not., vol. xxxix., p. 195.
[938] Observatory, vols. ii., p. 296; iv., p. 373. N. E. Green (Astr. Reg., vol. xvii.,
p. 144) concluded the object a mere “spot of colour,” dark under oblique light.
[939] Webb, Cel. Objects, p. 101.
[940] Publ. Lick Observatory, vol. iii., p. 7.
[941] Ibid., p. 21; Mee, Knowledge, vol. xviii., p. 135.
[942] Comptes Rendus, t. cxxii., p. 967; Bull. Astr., August, 1899; Ann. Bureau
des Long., 1898; Nature, vols. lii., p. 439; lvi., p. 280; lix., p. 304; lx., p. 491;
Astroph. Jour. vol. vi., p. 51.
[943] Comptes Rendus, t. xxii., p. 541.
[944] Phil. Trans., vol. cxlviii., p. 502.
[945] Proc. Roy. Soc., vol. xvii., p. 443.
[946] Phil. Trans., vol. clxiii., p. 623.
[947] Trans. R. Dublin Soc., vol. iii., p. 321.
[948] Science, vol. vii., p. 9.
[949] Amer. Jour. of Science, vol. xxxviii., p. 428.
[950] “The Temperature of the Moon,” Memoirs National Acad. of Sciences,
vol. iv., p. 193, 1889.
[951] Temperature of the Moon, p. iii.; see also App. ii., p. 206.
[952] Trans. R. Dublin Soc., vol. iv., p. 481, 1891; Rosse, Proc. Roy. Institution,
May 31, 1895.
[953] Astroph. Jour., vol. viii., pp. 199, 265.
[954] Airy, Observatory, vol. iii., p. 420.
[955] Phil. Trans., vol. cxliii., p. 397; Proc. Roy. Soc., vol. vi., p. 321.
[956] Comptes Rendus, t. lxi., p. 1023.
[957] Professor Darwin calculated that the heat generated by tidal friction in the
course of lengthening the earth’s period of rotation from 23 to 24 hours, equalled
23 million times the amount of its present annual loss by cooling. Nature,
vol. xxxiv., p. 422.
[958] Sämmtl. Werke (ed. 1839), Th. vi., pp. 5-12. See also C. J. Monro’s useful
indications in Nature, vol. vii., p. 241.
[959] Dynamik des Himmels, p. 40.
[960] Gould’s Astr. Jour., vol. iii., p. 138.
[961] Wash. Obs. for 1875, vol. xxii., App. ii.
[962] Comptes Rendus, t. cxiii., p. 669; Annuaire, Paris, 1892.
[963] Newcomb, Pop. Astr. (4th ed.), p. 101.
[964] Sir W. Thomson, Report Brit. Ass., 1876, p. 12.
CHAPTER VIII
PLANETS AND SATELLITES—(continued)
“The analogy between Mars and the earth is perhaps by far the
greatest in the whole solar system.” So Herschel wrote in 1783,[965]
and so we may safely say to-day, after six score further years of
scrutiny. The circumstance lends a particular interest to inquiries
into the physical habitudes of our exterior planetary neighbour.
Fontana first caught glimpses, at Naples in 1636 and 1638,[966] of
dusky stains on the ruddy disc of Mars. They were next seen by
Hooke and Cassini in 1666, and this time with sufficient distinctness
to serve as indexes to the planet’s rotation, determined by the
latter as taking place in a period of twenty-four hours forty
minutes.[967] Increased confidence was given to this result through
Maraldi’s precise verification of it in 1719.[968] Among the spots
observed by him, he distinguished two as stable in position, though
variable in size. They were of a peculiar character, showing as bright
patches round the poles, and had already been noticed during sixty
years back. A current conjecture of their snowy nature obtained
validity when Herschel connected their fluctuations in extent with
the progress of the Martian seasons. The inference of frozen precipitations
could scarcely be resisted when once it was clearly
perceived that the shining polar zones did actually by turns
diminish and grow with the alternations of summer and winter in
the corresponding hemisphere.
This, it may be said, was the opening of our acquaintance
with the state of things prevailing on the surface of Mars. It
was accompanied by a steady assertion, on Herschel’s part, of
permanence in the dark markings, notwithstanding partial obscurations
by clouds and vapours floating in a “considerable but moderate
atmosphere.” Hence the presumed inhabitants of the planet were[Pg 275]
inferred to “probably enjoy a situation in many respects similar to
ours.”[969]
Schröter, on the other hand, went altogether wide of the truth as
regards Mars. He held that the surface visible to us is a mere
shell of drifting cloud, deriving a certain amount of apparent
stability from the influence on evaporation and condensation of
subjacent but unseen areographical features;[970] and his opinion
prevailed with his contemporaries. It was, however, rejected by
Kunowsky in 1822, and finally overthrown by Beer and Mädler’s
careful studies during five consecutive oppositions, 1830-39. They
identified at each the same dark spots, frequently blurred with
mists, especially when the local winter prevailed, but fundamentally
unchanged.[971] In 1862 Lockyer established a “marvellous agreement”
with Beer and Mädler’s results of 1830, leaving no doubt as to the
complete fixity of the main features, amid “daily, nay, hourly,”
variations of detail through transits of clouds.[972] On seventeen nights
of the same opposition, F. Kaiser of Leyden obtained drawings in
which nearly all the markings noted in 1830 at Berlin reappeared,
besides spots frequently seen respectively by Arago in 1813, by
Herschel in 1783, and one sketched by Huygens in 1672 with a
writing-pen in his diary.[973] From these data the Leyden observer
arrived at a period of rotation of 24h. 37m. 22·62s., being just one
second shorter than that deduced, exclusively from their own
observations, by Beer and Mädler. The exactness of this result was
practically confirmed by the inquiries of Professor Bakhuyzen of
Leyden.[974] Using for a middle term of comparison the disinterred
observations of Schröter, with those of Huygens at one, and of
Schiaparelli at the other end of an interval of 220 years, he was
enabled to show, with something like certainty, that the time of
rotation (24h. 37m. 22·735s.) ascribed to Mars by Mr. Proctor[975] in
reliance on a drawing executed by Hooke in 1666, was too long by
nearly one-tenth of a second. The minuteness of the correction
indicates the nicety of care employed. Nor employed vainly; for,
owing to the comparative antiquity of the records available in this
case, an almost infinitesimal error becomes so multiplied by frequent
repetition as to produce palpable discrepancies in the positions
of the markings at distant dates. Hence Bakhuyzen’s period[Pg 276]
of 24h. 37m. 22·66s. is undoubtedly of a precision unapproached
as regards any other heavenly body save the earth itself.
Two facts bearing on the state of things at the surface of Mars
were, then, fully acquired to science in or before the year 1862.
The first was that of the seasonal fluctuations of the polar spots; the
second, that of the general permanence of certain dark gray or
greenish patches, perceived with the telescope as standing out from
the deep yellow ground of the disc. That these varieties of tint
correspond to the real diversities of a terraqueous globe, the “ripe
cornfield”[976] sections representing land, the dusky spots and streaks,
oceans and straits, has long been the prevalent opinion. Sir J. Herschel
in 1830 led the way in ascribing the redness of the planet’s light
to an inherent peculiarity of soil.[977] Previously it had been assimilated
to our sunset glows rather than to our red sandstone
formations—set down, that is, to an atmospheric stoppage of blue
rays. But the extensive Martian atmosphere, implicitly believed
in on the strength of some erroneous observations by Cassini and
Römer in the seventeenth century, vanished before the sharp
occultation of a small star in Leo, witnessed by Sir James South
in 1822;[978] and Dawes’s observation in 1865,[979] that the ruddy tinge
is deepest near the central parts of the disc, certified its non-atmospheric
origin. The absolute whiteness of the polar snow-caps
was alleged in support of the same inference by Sir William
Huggins in 1867.[980]
All recent operations tend to show that the atmosphere of Mars
is much thinner than our own. This was to have been expected
à priori, since the same proportionate mass of air would on his
smaller globe form a relatively sparse covering.[981] Besides, gravity
there possesses less than four-tenths its force here, so that this
sparser covering would weigh less, and be less condensed, than if
it enveloped the earth. Atmospheric pressure would accordingly
be of about two and a quarter, instead of fifteen terrestrial pounds
per square inch. This corresponds with what the telescope shows
us. It is extremely doubtful whether any features of the earth’s
actual surface could be distinguished by a planetary spectator,
however well provided with optical assistance. Professor Langley’s
inquiries[982] led him to conclude that fully twice as much light[Pg 277]
is absorbed by our air as had previously been supposed—say 40
per cent. of vertical rays in a clear sky. Of the sixty reaching the
earth, less than a quarter would be reflected even from white sandstone;
and this quarter would again pay heavy toll in escaping back
to space. Thus not more than perhaps ten or twelve out of the
original hundred sent by the sun would, under the most favourable
circumstances, and from the very centre of the earth’s disc, reach the
eye of a Martian or lunar observer. The light by which he views
our world is, there is little doubt, light reflected from the various
strata of our atmosphere, cloud or mist-laden or serene, as the case
may be, with an occasional snow-mountain figuring as a permanent
white spot.
This consideration at once shows us how much more tenuous the
Martian air must be, since it admits of topographical delineations
of the Martian globe. The clouds, too, that form in it seem in
general to be rather of the nature of ground-mists than of heavy
cumulus.[983] Occasionally, indeed, durable and extensive strata
become visible. During the latter half of October, 1894, for instance,
a region as large as Europe remained apparently cloud-covered. Yet
most recent observers are unable to detect the traces of aqueous
absorption in the Martian spectrum noted by Huggins in 1867[984] and
by Vogel in 1873.[985] Campbell vainly looked for them,[986] visually in
1894, spectrographically in 1896; Keeler was equally unsuccessful;[987]
Jewell[988] holds that they could, with present appliances, only be
perceived if the atmosphere of Mars were much richer in water-vapour
than that of the earth. There can be little doubt, however,
that its supply is about the minimum adequate to the needs of a
living, and perhaps a life-nuturing planet.
The climate of Mars seems to be unexpectedly mild. Its
theoretical mean temperature, taking into account both distance
from the sun and albedo, is 34° C. below freezing.[989] Yet its polar
snows are both less extensive and less permanent than those on
the earth. The southern white hood, noticed by Schiaparelli in 1877
to have survived the summer only as a small lateral patch, melted
completely in 1894. Moreover, Mr. W. H. Pickering observed
with astonishment the disappearance, in the course of thirty-three
days of June and July, 1892, of 1,600,000 square miles of southern[Pg 278]
snow.[990] Curiously enough, the initial stage of shrinkage in the
white calotte was marked by its division into two unequal parts, as
if in obedience to the mysterious principle of duplication governing
so many Martian phenomena.[991] Changes of the hues associated
respectively with land and water accompanied in lower latitudes,
and were thought to be occasioned by floods ensuing upon this rapid
antarctic thaw. It is true that scarcity of moisture would account
for the scantiness and transitoriness of snowy deposits easily liquefied
because thinly spread. But we might expect to see the whole
wintry hemisphere, at any rate, frost-bound, since the sun radiates
less than half as much heat on Mars as on the earth. Water seems,
nevertheless, to remain, as a rule, uncongealed everywhere outside
the polar regions. We are at a loss to imagine by what beneficent
arrangement the rigorous conditions naturally to be looked for can
be modified into a climate which might be found tolerable by
creatures constituted like ourselves.
Martian topography may be said to form nowadays a separate
sub-department of descriptive astronomy. The amount of detail
become legible by close scrutiny on a little disc which, once in
fifteen years, attains a maximum of about 1/5000 the area of the full
moon, must excite surprise and might provoke incredulity. Spurious
discoveries, however, have little chance of holding their own where
there are so many competitors quite as ready to dispute as to confirm.
The first really good map of Mars was constructed in 1869 by
Proctor from drawings by Dawes. Kaiser of Leyden followed in
1872 with a representation founded upon data of his own providing
in 1862-64; and Terby, in his valuable Aréographie, presented to the
Brussels Academy in 1873[992] a careful discussion of all important observations
from the time of Fontana downwards, thus virtually
adding to knowledge by summarising and digesting it. The
memorable opposition of September 5, 1877, marked a fresh epoch
in the study of Mars. While executing a trigonometrical survey
(the first attempted) of the disc, then of the unusual size of 25′
across, G. V. Schiaparelli, director of the Milan Observatory,
detected a novel and curious feature. What had been taken for
Martian continents were found to be, in point of fact, agglomerations
of islands, separated from each other by a network of so-called
“canals” (more properly channels).[993] These are obviously extensions
of the “seas,” originating and terminating in them, and sharing
their gray-green hue, but running sometimes to a length of three or[Pg 279]
four thousand miles in a straight line, and preserving throughout
a nearly uniform breadth of about sixty miles. Further inquiries
have fully substantiated the discovery made at the Brera Observatory.
The “canals” of Mars are an actually existent and permanent
phenomenon. An examination of the drawings in his possession
showed M. Terby that they had been seen, though not distinctively
recognised, by Dawes, Secchi, and Holden; several were independently
traced out by Burton at the opposition of 1879; all
were recovered by Schiaparelli himself in 1879 and 1881-82; and
their indefinite multiplication resulted from Lovell’s observations in
1894 and 1896.
When the planet culminated at midnight, and was therefore in
opposition, December 26, 1881, its distance was greater, and its
apparent diameter less than in 1877, in the proportion of sixteen to
twenty-five. Its atmosphere was, however, more transparent, and
ours of less impediment to northern observers, the object of scrutiny
standing considerably higher in northern skies. Never before, at
any rate, had the true aspect of Mars come out so clearly as at
Milan, with the 8-3/4-inch Merz refractor of the observatory, between
December, 1881, and February, 1882. The canals were all again
there, but this time they were—in as many as twenty cases—seen
in duplicate. That is to say, a twin-canal ran parallel to the
original one at an interval of 200 to 400 miles.[994]
We are here brought face to face with an apparently insoluble
enigma. Schiaparelli regards the “germination” of his canals as a
periodical phenomenon depending on the Martian seasons. It is,
assuredly, not an illusory one, since it was plainly apparent, during
the opposition of 1886, to MM. Perrotin and Thollon at Nice,[995] and
to the former, using the new 30-inch refractor of that observatory,
in 1888; Mr. A. Stanley Williams, with the help of only a 6-1/2-inch
reflector, distinctly perceived in 1890 seven of the duplicate objects
noted at Milan,[996] and the Lick observations, both of 1890 and of
1892, together with the drawings made at Flagstaff and Mexico
during the last favourable oppositions of the nineteenth century,
brought unequivocal confirmation to the accuracy of Schiaparelli’s
impressions.[997] Various conjectures have been hazarded in explanation
of this bizarre appearance. The difficulty of conceiving a physical
reality corresponding to it has suggested recourse to an optical
rationale. Proctor regarded it as an effect of diffraction;[998] Stanislas[Pg 280]
Meunier, of oblique reflection from overlying mist-banks;[999] Flammarion
considers it possible that companion-canals might, under
special circumstances, be evoked by refraction as a kind of mirage.[1000]
But none of these speculations are really admissible, when all the
facts are taken into account. The view that the canals of Mars are
vast rifts due to the cooling of the globe, is recommended by the
circumstance that they tend to follow great circles; nevertheless,
it would break down if, as Schiaparelli holds, the fluctuations in
their visibility depend upon actual obliterations and re-emergencies.
Fantastic though the theory of their artificial origin appear, it is
held by serious astronomers. Its vogue is largely due to Mr. Lowell’s
ingenious advocacy. He considers the Martian globe to be everywhere
intersected by an elaborate system of irrigation-works, rendered
necessary by a perennial water-famine, relieved periodically by the
melting of the polar snows. Nor does he admit the existence of
oceans, or lakes. What have been taken for such are really tracts
covered with vegetation, the bright areas intermixed with them representing
sandy deserts. And it is noteworthy in this connection
that Professor Barnard obtained in 1894,[1001] with the great Lick
refractor, “suggestive and impressive views” disclosing details of
light and shade on the gray-green patches so intricate and minute
as almost to preclude the supposition of their aqueous nature.
The closeness of the terrestrial analogy has thus of late been much
impaired. Even if the surface of Mars be composed of land and
water, their distribution must be of a completely original type. The
interlacing everywhere of continents with arms of the sea (if that be
the correct interpretation of the visual effects) implies that their levels
scarcely differ;[1002] and Schiaparelli carries most observers with him in
holding that their outlines are not absolutely constant, encroachments
of dusky upon bright tints suggesting extensive inundations.[1003]
The late N. E. Green’s observations at Madeira in 1877 indicated,
on the other hand, a rugged south polar region. The contour of
the snow-cap not only appeared indented, as if by valleys and
promontories, but brilliant points were discerned outside the white
area, attributed to isolated snow-peaks.[1004] Still more elevated, if
similarly explained, must be the “ice island” first seen in a comparatively
low latitude by Dawes in January, 1865.
On August 4, 1892, Mars stood opposite to the sun at a distance
of only 34,865,000 miles from the earth. In point of vicinity, then,
its situation was scarcely less favourable than in 1877. The low[Pg 281]
altitude of the planet, however, practically neutralised this advantage
for northern observers, and public expectation, which had been
raised to the highest pitch by the announcements of sensation-mongers,
was somewhat disappointed at the “meagreness” of the
news authentically received from Mars. Valuable series of observations
were, nevertheless, made at Lick and Arequipa; and they
unite in testifying to the genuine prevalence of surface-variability,
especially in certain regions of intermediate tint, and perhaps of the
“crude consistence” of “boggy Syrtes, neither sea, nor good dry
land.” Professor Holden insisted on the “enormous difficulties in
the way of completely explaining the recorded phenomena by
terrestrial analogies”;[1005] Mr. W. H. Pickering spoke of “conspicuous
and startling changes.” They, however, merely overlaid, and
partially disguised, a general stability. Among the novelties
detected by Mr. Pickering were a number of “lakes,” or “oases”
(in Lowell’s phraseology), under the aspect of black dots at the
junctions of two or more canals;[1006] and he, no less than the Lick
astronomers and M. Perrotin at Nice,[1007] observed brilliant clouds
projecting beyond the terminator, or above the limb, while carried
round by the planet’s rotation. They seemed to float at an altitude
of at least twenty miles, or about four times the height of terrestrial
cirrus; but this was not wonderful, considering the low power of
gravity acting upon them. Great capital was made in the journalistic
interest out of these imaginary signals from intelligent
Martians, desirous of opening communications with (to them)
problematical terrestrial beings. Similar effects had, however, been
seen before by Mr. Knobel in 1873, by M. Terby in 1888, and at the
Lick Observatory in 1890; and they were discerned again with
particular distinctness by Professor Hussey at Lick, August 27,
1896.[1008]
The first photograph of Mars was taken by Gould at Cordoba in
1879. Little real service in planetary delineation has, it is true,
been so far rendered by the art, yet one achievement must be
recorded to its credit. A set of photographs obtained by Mr.
W. H. Pickering on Wilson’s Peak, California, April 9, 1890,
showed the southern polar cap of Mars as of moderate dimensions,
but with a large dim adjacent area. Twenty-four hours later, on
a corresponding set, the dim area was brilliantly white. The polar
cap had become enlarged in the interim, apparently through a
wide-spreading snow-fall, by the annexation of a territory equal to[Pg 282]
that of the United States. The season was towards the close of
winter in Mars. Never until then had the process of glacial
extension been actually (it might be said) superintended in that
distant globe.
Mars was gratuitously supplied with a pair of satellites long
before he was found actually to possess them. Kepler interpreted
Galileo’s anagram of the “triple” Saturn in this sense; they
were perceived by Micromégas on his long voyage through space;
and the Laputan astronomers had even arrived at a knowledge,
curiously accurate under the circumstances, of their distances
and periods. But terrestrial observers could see nothing of them
until the night of August 11, 1877. The planet was then within
one month of its second nearest approach to the earth during
the last century; and in 1845 the Washington 26-inch refractor
was not in existence.[1009] Professor Asaph Hall, accordingly, determined
to turn the conjecture to account for an exhaustive inquiry
into the surroundings of Mars. Keeping his glaring disc just
outside the field of view, a minute attendant speck of light was
“glimpsed” August 11. Bad weather, however, intervened, and it
was not until the 16th that it was ascertained to be what it
appeared—a satellite. On the following evening a second, still
nearer to the primary, was discovered, which, by the bewildering
rapidity of its passages hither and thither, produced at first the
effect of quite a crowd of little moons.[1010]
Both these delicate objects have since been repeatedly observed,
both in Europe and America, even with comparatively small instruments.
At the opposition of 1884, indeed, the distance of the
planet was too great to permit of the detection of both elsewhere
than at Washington. But the Lick equatoreal showed them,
July 18, 1888, when their brightness was only 0·12 its amount at
the time of their discovery; so that they can now be followed for
a considerable time before and after the least favourable oppositions.
The names chosen for them were taken from the Iliad, where
“Deimos” and “Phobos” (Fear and Panic) are represented as the
companions in battle of Ares. In several respects, they are interesting
and remarkable bodies. As to size, they may be said to stand
midway between meteorites and satellites. From careful photometric
measures executed at Harvard in 1877 and 1879, Professor
Pickering concluded their diameters to be respectively six and
seven miles.[1011] This is on the assumption that they reflect the same[Pg 283]
proportion of the light incident upon them that their primary does.
But it may very well be that they are less reflective, in which case
they would be more extensive. The albedo of Mars is put by
Müller at 0·27; his surface, in other words, returns 27 per cent. of
the rays striking it. If we put the albedo of his satellites equal to
that of our moon, 0·17, their diameters will be increased from 6 and
7 to 7-1/2 and 9 miles, Phobos, the inner one, being the larger.
Mr. Lowell, however, formed a considerably larger estimate of their
dimensions.[1012] It is interesting to note that Deimos, according to
Professor Pickering’s very distinct perception, does not share the
reddish tint of Mars.
Deimos completes its nearly circular revolutions in thirty hours
eighteen minutes, at a distance from the surface of its ruling body
of 12,500 miles; Phobos traverses an elliptical orbit[1013] in seven hours
thirty-nine minutes twenty-two seconds, at a distance of only
3,760 miles. This is the only known instance of a satellite
circulating faster than its primary rotates, and is a circumstance
of some importance as regards theories of planetary development.
To a Martian spectator the curious effect would ensue of a celestial
object, seemingly exempt from the general motion of the sphere,
rising in the west, setting in the east, and culminating twice, or even
thrice a day; which, moreover, in latitudes above 69° north or south,
would be permanently and altogether hidden by the intervening
curvature of the globe.
The detection of new members of the solar system has come to be
one of the most ordinary of astronomical events. Since 1846 no
single year has passed without bringing its tribute of asteroidal
discovery. In the last of the seventies alone, a full score of
miniature planets were distinguished from the thronging stars amid
which they seem to move; 1875 brought seventeen such recognitions;
their number touched a minimum of one in 1881; it rose in
1882, and again in 1886, to eleven; dropped to six in 1889, and
sprang up with the aid of photography to twenty-seven in 1892.
That high level has since, on an average, been maintained; and on
January 1, 1902, nearly 500 asteroids were recognised as revolving
between the orbits of Mars and Jupiter. Of these, considerably
more than one hundred are claimed by one investigator alone—Dr.
Max Wolf of Heidelburg; M. Charlois of Nice comes second
with 102; while among the earlier observers Palisa of Vienna contributed
86, and C. H. F. Peters of Clinton (N. Y.), whose varied
and useful career terminated July 19, 1890, 52 to the grand total.
The construction by Chacornac and his successors at Paris, and
more recently by Peters at Clinton, of ecliptical charts showing all
stars down to the thirteenth and fourteenth magnitudes respectively,
rendered the picking out of moving objects above that brightness
a mere question of time and diligence. Both, however, are vastly
economised by the photographic method. Tedious comparisons of
the sky with charts are no longer needed for the identification of
unrecorded, because simulated stars. Planetary bodies declare
themselves by appearing upon the plate, not in circular, but in linear
form. Their motion converts their images into trails, long or short
according to the time of exposure. The first asteroid (No. 323) thus
detected was by Max Wolf, December 22, 1891.[1014] Eighteen others
were similarly discovered in 1892, by the same skilful operator; and
ten more through Charlois’s adoption at Nice of the novel plan now
in exclusive use for picking up errant light-specks. Far more
onerous than the task of their discovery is that of keeping them in
view once discovered—of tracking out their paths, fixing their places,
and calculating the disturbing effects upon them of the mighty
Jovian mass. These complex operations have come to be centralised
at Berlin under the superintendence of Professor Tietjen, and their
results are given to the public through the medium of the Berliner
Astronomisches Jahrbuch.
The cui bono? however, began to be agitated. Was it worth
while to maintain a staff of astronomers for the sole purpose of
keeping hold over the identity of the innumerable component
particles of a cosmical ring? The prospect, indeed, of all but a select
few of the asteroids being thrown back by their contemptuous
captors into the sea of space seemed so imminent that Professor
Watson provided by will against the dereliction of the twenty-two
discovered by himself. But the fortunes of the whole family improved
through the distinction obtained by one of them. On
August 14, 1898, the trail of a rapidly-moving, star-like object of
the eleventh magnitude imprinted itself on a plate exposed by Herr
Witt at the Urania Observatory, Berlin. Its originator proved to
be unique among asteroids. “Eros” is, in sober fact,
‘one of those mysterious stars
Which hide themselves between the Earth and Mars,’
divined or imagined by Shelley.[1015] True, several of its congeners
invade the Martian sphere at intervals; but the proper habitat of
Eros is within that limit, although its excursions transcend it. In
other words, its mean distance from the sun is about 135, as[Pg 285]
compared with the Martian distance of 141 million miles. Further,
its orbit being so fortunately circumstanced as to bring it once in
sixty-seven years within some 15 millions of miles of the earth, it is of
extraordinary value to celestial surveyors. The calculation of its
movements was much facilitated by detections, through a retrospective
search,[1016] of many of its linear images among the star-dots on the
Harvard plates.[1017] The little body—which can scarcely be more than
twenty miles in diameter—shows peculiarities of behaviour as well
as of position. Dr. von Oppolzer, in February, 1901,[1018] announced
it to be extensively and rapidly variable. Once in 2 hours 38 minutes
it lost about three-fourths of its light,[1019] but these fluctuations
quickly diminished in range, and in the beginning of May ceased
altogether.[1020] Evidently, then, they depend upon the situation of the
asteroid relatively to ourselves; and, so far, events lent countenance
to M. André’s eclipse hypothesis, since mutual occultations of the
supposed planetary twins could only take place when the plane of
their revolutions passed through the earth, and this condition would
be transitory. Yet the recognition in Eros of an “Algol asteroid”
seems on other grounds inadmissible;[1021] nor until the phenomenon
is conspicuously renewed—as it probably will be at the opposition
of 1903—can there be much hope of finding its appropriate
rationale.
The crowd of orbits disclosed by asteroidal detections invites attentive
study. D’Arrest remarked in 1851,[1022] when only thirteen minor
planets were known, that supposing their paths to be represented by
solid hoops, not one of the thirteen could be lifted from its place
without bringing the others with it. The complexity of interwoven
tracks thus illustrated has grown almost in the numerical proportion
of discovery. Yet no two actually intersect, because no two lie
exactly in the same plane, so that the chances of collision are at
present nil. There is only one case, indeed, in which it seems to be
eventually possible. M. Lespiault has pointed out that the curves
traversed by “Fidés” and “Maïa” approach so closely that a time
may arrive when the bodies in question will either coalesce or unite
to form a binary system.[1023]
The maze threaded by the 500 asteroids contrasts singularly with
the harmoniously ordered and rhythmically separated orbits of the
larger planets. Yet the seeming confusion is not without a plan.
The established rules of our system are far from being totally disregarded
by its minor members. The orbit of Pallas, with its
inclination of 34° 42′, touches the limit of departure from the
ecliptic level; the average obliquity of the asteroidal paths is somewhat
less than that of the sun’s equator;[1024] their mean eccentricity is
below that of the curve traced out by Mercury, and all without
exception are pursued in the planetary direction—from west to
east.
The zone in which these small bodies travel is about three times
as wide as the interval separating the earth from the sun. It
extends perilously near to Jupiter, and dovetails into the sphere of
Mars.
Their distribution is very unequal. They are most densely
congregated about the place where a single planet ought, by Bode’s
Law, to revolve; it may indeed be said that only stragglers from
the main body are found more than fifty million miles within or
without a mean distance from the sun 2·8 times that of the earth.
Significant gaps, too, occur where some force prohibitive of their
presence would seem to be at work. The probable nature of that
force was suggested by the late Professor Kirkwood, first in 1866,
when the number of known asteroids was only eighty-eight, and
again with more confidence in 1876, from the study of a list then
run up to 172.[1025] It appears that these bare spaces are found just
where a revolving body would have a period connected by a simple
relation with that of Jupiter. It would perform two or three
circuits to his one, five to his two, nine to his five, and so on.
Kirkwood’s inference was that the gaps in question were cleared of
asteroids by the attractive influence of Jupiter. For disturbances
recurring time after time—owing to commensurability of periods—nearly
at the same part of the orbit, would have accumulated until
the shape of that orbit was notably changed. The body thus
displaced would have come in contact with other cosmical particles
of the same family with itself—then, it may be assumed, more
evenly scattered than now—would have coalesced with them, and
permanently left its original track. In this way the regions of
maximum perturbation would gradually have become denuded of
their occupants.
We can scarcely doubt that this law of commensurability has
largely influenced the present distribution of the asteroids. But its
effects must have been produced while they were still in an unformed,
perhaps a nebular condition. In a system giving room for[Pg 287]
considerable modification through disturbance, the recurrence of
conjunctions with a dominating mass at the same orbital point need
not involve instability.[1026] On the whole, the correspondence of facts
with Kirkwood’s hypothesis has not been impaired by their more
copious collection.[1027] Some chasms of secondary importance have
indeed been bridged; but the principal stand out more conspicuously
through the denser scattering of orbits near their margins. Nor is
it doubtful that the influence of Jupiter in some way produced
them. M. de Freycinet’s study of the problem they present[1028] has,
however, led him to the conclusion that they existed ab origine, thus
testifying rather to the preventive than to the perturbing power of
the giant planet.
The existence, too, of numerous asteroidal pairs travelling in
approximately coincident tracks, must date from a remote antiquity.
They result, Professor Kirkwood[1029] believed, from the divellent action
of Jupiter upon embryo pigmy planets, just as comets moving in
pursuit of one another are a consequence of the sundering influence
of the sun.
Leverrier fixed, in 1853,[1030] one-fourth of the earth’s mass as the
outside limit for the combined masses of all the bodies circulating
between Mars and Jupiter; but it is far from probable that this
maximum is at all nearly approached. M. Berberich[1031] held that the
moon would more than outweigh the whole of them, a million of
the lesser bodies shining like stars of the twelfth magnitude being
needed, according to his judgment, to constitute her mass. And
M. Niesten estimated that the whole of the 216 asteroids discovered
up to August, 1880, amounted in volume to only 1/4000th of our globe,[1032]
and we may safely add—since they are tolerably certain to be
lighter, bulk for bulk, than the earth—that their proportionate mass
is smaller still. A fairly concordant result was published in 1895
by Mr. B. M. Roszel.[1033] He found that the lunar globe probably
contains forty times, the terrestrial globe 3,240 times the quantity
of matter parcelled out among the first 311 minor planets. The
actual size of a few of them may now be said to be known.
Professor Pickering, from determinations of light-intensity, assigned
to Vesta a diameter of 319 miles, to Pallas 167, to Juno 94, down[Pg 288]
to twelve and fourteen for the smaller members of the group.[1034] An
albedo equal to that of Mars was assumed as the basis of the calculation.
Moreover, Professor G. Müller[1035] of Potsdam examined photometrically
the phases of seven among them, of which four—namely,
Vesta, Iris, Massalia, and Amphitrite—were found to conform
precisely to the behaviour of Mars as regards light-change from
position, while Ceres, Pallas, and Irene varied after the manner of
the moon and Mercury. The first group were hence inferred
to resemble Mars in physical constitution, nature of atmosphere,
and reflective capacity; the second to be moon-like bodies.
Finally, Professor Barnard, directly measuring with the Yerkes
refractor the minute discs presented by the original quartette,
obtained the following authentic data concerning them:[1036] Diameter
of Ceres, 477 miles, albedo = 0·18; diameter of Pallas, 304 miles,
albedo = 0·23; diameter of Vesta, 239 miles, albedo = 0·74; diameter
of Juno, 120 miles, albedo = 0·45. Thus, the rank of premier
asteroid proves to belong to Ceres, and to have been erroneously
assigned to Vesta in consequence of its deceptive brilliancy. What
kind of surface this indicates, it is hard to say. The dazzling
whiteness of snow can hardly be attributed to bare rock; yet the
dynamical theory of gases—as Dr. Johnstone Stoney pointed out
in 1867[1037]—prohibits the supposition that bodies of insignificant
gravitative power can possess aerial envelopes. Even our moon, it
is calculated, could not permanently hold back the particles of
oxygen, nitrogen, or water-gas from escaping into infinite space;
still less, a globe one thousand times smaller. Vogel’s suspicion of
an air-line in the spectrum of Vesta[1038] has, accordingly, not been
confirmed.
Crossing the zone of asteroids on our journey outward from the
sun, we meet with a group of bodies widely different from the
“inferior” or terrestrial planets. Their gigantic size, low specific
gravity, and rapid rotation, obviously from the first threw the
“superior” planets into a class apart; and modern research has
added qualities still more significant of a dissimilar physical constitution.
Jupiter, a huge globe 86,000 miles in diameter, stands pre-eminent
among them. He is, however, only primus inter pares; all
the wider inferences regarding his condition may be extended, with
little risk of error, to his fellows; and inferences in his case rest on
surer grounds than in the case of the others, from the advantages
offered for telescopic scrutiny by his comparative nearness.
Now the characteristic modern discovery concerning Jupiter is
that he is a body midway between the solar and terrestrial stages
of cosmical existence—a decaying sun or a developing earth, as we
choose to put it—whose vast unexpended stores of internal heat are
mainly, if not solely, efficient in producing the interior agitations
betrayed by the changing features of his visible disc. This view,
impressed upon modern readers by Mr. Proctor’s popular works, was
anticipated in the last century. Buffon wrote in his Époques de la
Nature (1778):[1039]—”La surface de Jupiter est, comme l’on sait,
sujette à des changemens sensibles, qui semblent indiquer que
cette grosse planète est encore dans un état d’inconstance et de
bouillonnement.”
Primitive incandescence, attendant, in his fantastic view, on
planetary origin by cometary impacts with the sun, combined, he
concluded, with vast bulk to bring about this result. Jupiter has
not yet had time to cool. Kant thought similarly in 1785;[1040] but
the idea did not commend itself to the astronomers of the time, and
dropped out of sight until Mr. Nasmyth arrived at it afresh in 1853.[1041]
Even still, however, terrestrial analogies held their ground. The
dark belts running parallel to the equator, first seen at Naples in
1630, continued to be associated—as Herschel had associated them
in 1781—with Jovian trade-winds, in raising which the deficient
power of the sun was supposed to be compensated by added swiftness
of rotation. But opinion was not permitted to halt here.
In 1860 G. P. Bond of Cambridge (U.S.) derived some remarkable
indications from experiments on the light of Jupiter.[1042] They showed
that fourteen times more of the photographic rays striking it are
reflected by the planet than by our moon, and that, unlike the
moon, which sends its densest rays from the margin, Jupiter is
brightest near the centre. But the most perplexing part of his
results was that Jupiter actually seemed to give out more light than
he received. Bond, however, rightly considered his data too uncertain
for the support of so bold an assumption as that of original
luminosity, and, even if the presence of native light were proved,
thought that it might emanate from auroral clouds of the terrestrial
kind. The conception of a sun-like planet was still a remote, and
seemed an extravagant one.
Only since it was adopted and enforced by Zöllner in 1865,[1043] can
it be regarded as permanently acquired to science. The rapid
changes in the cloud-belts both of Jupiter and Saturn, he remarked,
attest a high internal temperature. For we know that all atmospheric[Pg 290]
movements on the earth are sun-heat transformed into
motion. But sun-heat at the distance of Jupiter possesses but 1/27,
at that of Saturn 1/100 of its force here. The large amount of energy,
then, obviously exerted in those remote firmaments must have some
other source, to be found nowhere else than in their own active
and all-pervading fires, not yet banked in with a thick solid crust.
The same acute investigator dwelt, in 1871,[1044] on the similarity
between the modes of rotation of the great planets and of the sun,
applying the same principles of explanation to each case. The fact
of this similarity is undoubted. Cassini[1045] and Schröter both noticed
that markings on Jupiter travelled quicker the nearer they were to
his equator; and Cassini even hinted at their possible assimilation
to sun-spots.[1046] It is now well ascertained that, as a rule (not without
exceptions), equatorial spots give a period some 5-1/2 minutes shorter
than those in latitudes of about 30°. But, as Mr. Denning has
pointed out,[1047] no single period will satisfy the observations either of
different markings at the same epoch, or of the same markings at
different epochs. Accelerations and retardations, depending upon
processes of growth or change, take place in very much the same
kind of way as in solar maculæ, inevitably suggesting similarity of
origin.
The interesting query as to Jupiter’s surface incandescence has
been studied since Bond’s time with the aid of all the appliances
furnished to physical inquirers by modern inventiveness, yet without
bringing to it a categorical reply. Zöllner in 1865, Müller in
1893, estimated his albedo at 0·62 and 0·75 respectively, that of
fresh-fallen snow being 0·78, and of white paper 0·70.[1048] But the disc
of Jupiter is by no means purely white. The general ground is
tinged with ochre; the polar zones are leaden or fawn coloured; large
spaces are at times stained or suffused with chocolate-browns and
rosy hues. It is occasionally seen ruled from pole to pole with
dusky bars, and is never wholly free from obscure markings. The
reflection, then, by it, as a whole, of about 70 per cent. of the
rays impinging upon it, might well suggest some original reinforcement.
Nevertheless, the spectroscope gives little countenance to the
supposition of any considerable permanent light-emission. The
spectrum of Jupiter, as examined by Huggins, 1862-64, and by
Vogel, 1871-73, shows the familiar Fraunhofer rays belonging to
reflected sunlight. But it also shows lines of native absorption.
[Pg 291]
Some of these are identical with those produced by the action of our
own atmosphere, especially one or more groups due to aqueous
vapours; others are of unknown origin; and it is remarkable that
one among the latter—a strong band in the red—agrees in position
with a dark line in the spectra of some ruddy stars.[1049] There is,
besides, a general absorption of blue rays, intensified—as Le Sueur
observed at Melbourne in 1869[1050]—in the dusky markings, evidently
through an increase of depth in the atmospheric strata traversed by
the light proceeding from them.
All these observations, however (setting aside the stellar line as of
doubtful significance), point to a cool planetary atmosphere. One
spectrograph, it is true, taken by Dr. Henry Draper, September 27,
1879,[1051] seemed to attest the action of intrinsic light; but the peculiarity
was referred by Dr. Vogel, with convincing clearness, to a flaw
in the film.[1052] So far, then, native emissions from any part of
Jupiter’s diversified surface have not been detected; and, indeed,
the blackness of the shadows cast by his satellites on his disc sufficiently
proves that he sends out virtually none but reflected light.[1053]
This conclusion, however, by no means invalidates that of his high
internal temperature.
The curious phenomena attending Jovian satellite-transits may be
explained, partly as effects of contrast, partly as due to temporary
obscurations of the small discs projected on the large disc of Jupiter.
At their first entry upon its marginal parts, which are several times
less luminous than those near the centre, they invariably show as
bright spots, then usually vanish as the background gains lustre, to
reappear, after crossing the disc, thrown into relief, as before, against
the dusky limb. But instances are not rare, more especially of the
third and fourth satellites standing out, during the entire middle
part of their course, in such inky darkness as to be mistaken for
their own shadows. The earliest witness of a “black transit” was
Cassini, September 2, 1665; Römer in 1677, and Maraldi in 1707
and 1713, made similar observations, which have been multiplied
in recent years. In some cases the process of darkening has
been visibly attended by the formation, or emergence into view,
of spots on the transiting body, as noted by the two Bonds at
Harvard, March 18, 1848.[1054] The third satellite was seen by Dawes,
half dark, half bright, when crossing Jupiter’s disc, August 21,[Pg 292]
1867;[1055] one-third dark by Davidson of California, January 15, 1884,
under the same circumstances;[1056] and unmistakably spotted, both
on and off the planet, by Schröter, Secchi, Dawes, and Lassell.
The first satellite sometimes looks dusky, but never absolutely
black, in travelling over the disc of Jupiter. The second appears
uniformly white—a circumstance attributed by Dr. Spitta[1057] to its
high albedo. The singularly different aspects, even during successive
transits, of the third and fourth satellites, are connected by Professor
Holden[1058] with the varied luminosity of the segments of the planetary
surface they are projected upon, and W. H. Pickering inclines to
the same opinion; but fluctuations in their own brightness[1059] may be
a concurrent cause. Herschel concluded in 1797 that, like our moon,
they always turn the same face towards their primary, and as
regards the outer satellite, Engelmann’s researches in 1871, and
C. E. Burton’s in 1873, made this almost certain; while both for
the third and fourth Jovian moons it was completely assured by
W. H. Pickering’s and A. E. Douglass’s observations at Arequipa in
1892,[1060] and at Flagstaff in 1894-95.[1061] Strangely enough, however, the
interior members of the system have preserved a relatively swift
rotation, notwithstanding the enormous checking influence upon it
of Jove-raised tides.
All the satellites are stated, on good authority, to be more or less
egg-shaped. On September 8, 1890, Barnard saw the first elongated
and bisected by a bright equatorial belt, during one of its dark
transits;[1062] and his observation, repeated August 3, 1891, was completely
verified by Schaeberle and Campbell, who ascertained, moreover,
that the longer axis of the prolate body was directed towards
Jupiter’s centre.[1063] The ellipticity of its companions was determined
by Pickering and Douglass; indeed, that of No. 3 had long previously
been noticed by Secchi.[1064] No. 3 also shows equatorial stripes, perceived
in 1891 by Schaeberle and Campbell,[1065] and evident later to
Pickering and Douglass;[1066] nor need we hesitate to admit as authentic
their records of similar, though less conspicuous markings on the[Pg 293]
other satellites. A constitution analogous to that of Jupiter himself
was thus unexpectedly suggested; and Vogel’s detection of lines—or
traces of lines—in their spectra, agreeing with absorption-rays derived
from their primary, lends support to the conjecture that they possess
gaseous envelopes similar to his.
The system of Jupiter, as it was discovered by Galileo, and investigated
by Laplace, appeared in its outward aspect so symmetrical,
and displayed in its inner mechanism such harmonious dynamical
relations, that it might well have been deemed complete. Nevertheless,
a new member has been added to it. Near midnight on
September 9, 1892, Professor Barnard discerned with the Lick
36-inch “a tiny speck of light,” closely following the planet.[1067] He
instantly divined its nature, watched its hurried disappearance in
the adjacent glare, and made sure of the reality of his discovery on
the ensuing night. It was a delicate business throughout, the Liliputian
luminary subsiding into invisibility before the slightest glint
of Jovian light, and tarrying, only for brief intervals, far enough
from the disc to admit of its exclusion by means of an occulting
plate. The new satellite is estimated to be of the thirteenth stellar
magnitude, and, if equally reflective of light with its next neighbour,
Io (satellite No. 1), its diameter must be about one hundred miles.
It revolves at a distance of 112,500 miles from Jupiter’s centre, and
of 68,000 from his bulging equatorial surface. Its period of 11h.
57m. 23s. is just two hours longer than Jupiter’s period of rotation,
so that Phobos still remains a unique example of a secondary body
revolving faster than its primary rotates. Jupiter’s innermost moon
conforms in its motions strictly, indeed inevitably, to the plane of
his equatorial protuberance, following, however, a sensibly elliptical
path the major axis of which is in rapid revolution.[1068] Its very insignificance
raises the suspicion that it may not prove solitary. Possibly
it belongs to a zone peopled by asteroidal satellites. More than
fifteen thousand such small bodies could be furnished out of the
materials of a single full-sized satellite spoiled in the making. But
we must be content for the present to register the fact without
seeking to penetrate the meaning of its existence. Very high and
very fine telescopic power is needed for its perception. Outside the
United States, it has been very little observed. The only instruments
in this country successfully employed for its detection are, we
believe, Dr. Common’s 5-foot reflector and Mr. Newall’s 25-inch
refractor.
In the course of his observations on Jupiter at Brussels in 1878,
M. Niesten was struck with a rosy cloud attached to a whitish zone[Pg 294]
beneath the dark southern equatorial band.[1069] Its size was enormous.
At the distance of Jupiter, its measured dimensions of 13′ by 3′
implied a real extension in longitude of 30,000, in latitude of something
short of 7,000 miles. The earliest record of its appearance
seems to be by Professor Pritchett, director of the Morrison Observatory
(U.S.), who figured and described it July 9, 1878.[1070] It was
again delineated August 9, by Tempel at Florence.[1071] In the following
year it attracted the wonder and attention of almost every
possessor of a telescope. Its colour had by that time deepened into
a full brick-red, and was set off by contrast with a white equatorial
spot of unusual brilliancy. During three ensuing years these remarkable
objects continued to offer a visible and striking illustration
of the compound nature of the planet’s rotation. The red spot completed
a circuit in nine hours fifty-five minutes thirty-six seconds;
the white spot in about five and a half minutes less. Their relative
motion was thus no less than 260 miles an hour, bringing them
together in the same meridian at intervals of forty-four days ten
hours forty-two minutes. Neither, however, preserved continuously
the same uniform rate of travel. The period of each had lengthened
by some seconds in 1883, while sudden displacements, associated
with the recovery of lustre after recurrent fadings, were observed in
the position of the white spot,[1072] recalling the leap forward of a reviving
sun-spot. Just the opposite effect attended the rekindling of
the companion object. While semi-extinct, in 1882-84, it lost little
motion; but a fresh access of retardation was observed by Professor
Young[1073] in connection with its brightening in 1886. This suggests
very strongly that the red spot is fed from below. A shining aureola
of “faculæ,” described by Bredichin at Moscow, and by Lohse at
Potsdam, as encircling it in September, 1879,[1074] was held to strengthen
the solar analogy.
The conspicuous visibility of this astonishing object lasted three
years. When the planet returned to opposition in 1882-83, it had
faded so considerably that Riccò’s uncertain glimpse of it at Palermo,
May 31, 1883, was expected to be the last. It had, nevertheless,
begun to recover in December, and presented to Mr. Denning in the
beginning of 1886 much the same aspect as in October, 1882.[1075]
Observed by him in an intermediate stage, February 25, 1885, when
“a mere skeleton of its former self,” it bore a striking likeness to
an “elliptical ring” descried in the same latitude by Mr. Gledhill in[Pg 295]
1869-70. This, indeed, might be called the preliminary sketch for
the famous object brought to perfection ten years later, but which
Mr. H. C. Russell of Sydney saw and drew still unfinished in June,
1876,[1076] before it had separated from its matrix, the dusky south
tropical belt. In earlier times, too, a marking “at once fixed and
transient” had been repeatedly perceived attached to the southernmost
of the central belts. It gave Cassini in 1665 a rotation-period
of nine hours fifty-six minutes,[1077] reappeared and vanished eight times
during the next forty-three years, and was last seen by Maraldi in
1713. It was, however, very much smaller than the recent object,
and showed no unusual colour.[1078]
The assiduous observations made on the “Great Red Spot” by
Mr. Denning at Bristol and by Professor Hough at Chicago afforded
grounds only for negative conclusions as to its nature. It certainly
did not represent the outpourings of a Jovian volcano; it was
in no sense attached to the Jovian soil—if the phrase have any
application to that planet; it was not a mere disclosure of a
glowing mass elsewhere seethed over by rolling vapours. It was,
indeed, certainly not self-luminous, a satellite projected upon it in
transit having been seen to show as bright as upon the dusky
equatorial bands. A fundamental objection to all three hypotheses
is that the rotation of the spot was variable. It did not then ride
at anchor, but floated free. Some held that its surface was depressed
below the average cloud-level, and that the cavity was filled with
vapours. Professor Wilson, on the other hand, observing with the
16-inch equatorial of the Goodsell Observatory in Minnesota,
received a persistent impression of the object “being at a higher
level than the other markings.”[1079] A crucial experiment on this
point was proposed by Mr. Stanley Williams in 1890.[1080] A dark spot
moving faster along the same parallel was timed to overtake the red
spot towards the end of July. A unique opportunity hence appeared
to be at hand of determining the relative vertical depths of the
two formations, one of which must inevitably, it was thought, pass
above the other. No forecast included a third alternative, which
was nevertheless adopted by the dark spot. It evaded the obstacle
in its path by skirting round its southern edge.[1081] Nothing, then,
was gained by the conjunction, beyond an additional proof of the
singular repellent influence exerted by the red spot over the[Pg 296]
markings in its vicinity. It has, for example, gradually carved out
a deep bay for its accommodation in the gray belt just north of it.
The effect was not at first steadily present. A premonitory excavation
was drawn by Schwabe at Dessau, September 5, 1831, and again by
Trouvelot, Barnard, and Elvins in 1879; yet there was no sign of
it in the following year. Its development can be traced in Dr.
Boeddicker’s beautiful delineations of Jupiter, made with the Parsonstown
3-foot reflector, from 1881 to 1886.[1082] They record the
belt as straight in 1881, but as strongly indented from January, 1883;
and the cavity now promises to outlast the spot. So long as it
survives, however, the forces at work in the spot can have lost little
of their activity. For it must be remembered that the belt has a
shorter rotation-period than the red spot, which, accordingly (as
Mr. Elvins of Toronto has pointed out), breasts and diverts, by its
interior energy, a current of flowing matter, ever ready to fill up its
natural bed, and override the barrier of obstruction.
The famous spot was described by Keeler in 1889, as “of a pale
pink colour, slightly lighter in the middle. Its outline was a fairly
true ellipse, framed in by bright white clouds.”[1083] The fading continuously
in progress from 1887 was temporarily interrupted in
1891. The revival, indeed, was brief. Professor Barnard wrote
in August, 1892: “The great red spot is still visible, but it has just
passed through a crisis that seemingly threatened its very existence.
For the past month it has been all but impossible to catch the
feeblest trace of the spot, though the ever-persistent bay in the
equatorial belt close north of it, and which has been so intimately
connected with the history of the red spot, has been as conspicuous
as ever. It is now, however, possible to detect traces of the entire
spot. An obscuring medium seems to have been passing over it,
and has now drifted somewhat preceding the spot.”[1084]
The object is now always inconspicuous, and often practically
invisible, and may be said to float passively in the environing
medium.[1085] Yet there are sparks beneath the ashes. A rosy tinge
faintly suffused it in April, 1900,[1086] and its absolute end may still be
remote.
The extreme complexity of the planet’s surface-movements has
been strikingly evinced by Mr. Stanley Williams’s detailed investigations.
He enumerated in 1896[1087] nine principal currents, all flowing[Pg 297]
parallel to the equator, but unsymmetrically placed north and south
of it, and showing scant signs of conformity to the solar rule of
retardation with increase of latitude. The linear rate of the planet’s
equatorial rotation was spectroscopically determined by Bélopolsky
and Deslandres in 1895. Both found it to fall short of the calculated
speed, whence an enlargement, by self-refraction, of the
apparent disc was inferred.[1088]
Jupiter was systematically photographed with the Lick 36-inch
telescope during the oppositions of 1890, 1891, and 1892, the
image thrown on the plates (after eightfold direct enlargement)
being one inch in diameter. Mr. Stanley Williams’s measurements
and discussion of the set for 1891 showed the high value of the
materials thus collected, although much more minute details can be
seen than can at present be photographed. The red spot shows as
“very distinctly annular” in several of these pictures.[1089] Recently,
the planet has been portrayed by Deslandres with the 62-foot
Meudon refractor.[1090] The extreme actinic feebleness of the
equatorial bands was strikingly apparent on his plates.
In 1870, Mr. Ranyard[1091]—whose death, December 14, 1894, was a
serious loss to astronomy—acting upon an earlier suggestion of
Sir William Huggins, collected records of unusual appearances on
the disc of Jupiter, with a view to investigate the question of their
recurrence at regular intervals. He concluded that the development
of the deeper tinges of colour, and of the equatorial “port-hole”
markings girdling the globe in regular alternations of bright and
dusky, agreed, so far as could be ascertained, with epochs of sun-spot
maximum. The further inquiries of Dr. Lohse at Bothkamp
in 1873[1092] went to strengthen the coincidence, which had been
anticipated à priori by Zöllner in 1871.[1093] Moreover, separate and
distinct evidence was alleged by Mr. Denning in 1899 of decennial
outbreaks of disturbance in north temperate regions.[1094] It may,
indeed, be taken for granted that what Hahn terms the universal
pulse of the solar system[1095] affects the vicissitudes of Jupiter; but
the law of those vicissitudes is far from being so obviously subordinate
to the rhythmical flow of central disturbance as are certain
terrestrial phenomena. The great planet, being in fact himself a
“semi-sun,” may be regarded as an originator, no less than a
recipient, of agitating influences, the combined effects of which may
well appear insubordinate to any obvious law.
It is likely that Saturn is in a still earlier stage of planetary
development than Jupiter. He is the lightest for his size of all the
planets. In fact, he would float in water. And since his density is
shown, by the amount of his equatorial bulging, to increase centrally,[1096]
it follows that his superficial materials must be of a specific gravity
so low as to be inconsistent, on any probable supposition, with the
solid or liquid states. Moreover, the chief arguments in favour of
the high temperature of Jupiter, apply, with increased force, to
Saturn; so that it may be concluded, without much risk of error,
that a large proportion of his bulky globe, 73,000 miles in diameter,
is composed of heated vapours, kept in active and agitated circulation
by the process of cooling.
His unique set of appendages has, since the middle of the last
century, formed the subject of searching and fruitful inquiries, both
theoretical and telescopic. The mechanical problem of the stability of
Saturn’s rings was left by Laplace in a very unsatisfactory condition.
Considering them as rotating solid bodies, he pointed out that they
could not maintain their position unless their weight were in some
way unsymmetrically distributed; but made no attempt to determine
the kind or amount of irregularity needed to secure this end. Some
observations by Herschel gave astronomers an excuse for taking for
granted the fulfilment of the condition thus vaguely postulated;
and the question remained in abeyance until once more brought
prominently forward by the discovery of the dusky ring in 1850.
The younger Bond led the way, among modern observers, in
denying the solidity of the structure. The fluctuations in its
aspect were, he asserted in 1851,[1097] inconsistent with such a hypothesis.
The fine dark lines of division, frequently detected in both
bright rings, and as frequently relapsing into imperceptibility, were
due, in his opinion, to the real nobility of their particles, and
indicated a fluid formation. Professor Benjamin Peirce of Harvard
University immediately followed with a demonstration, on abstract
grounds, of their non-solidity.[1098] Streams of some fluid denser than
water were, he maintained, the physical reality giving rise to the
anomalous appearance first disclosed by Galileo’s telescope.
The mechanism of Saturn’s rings, proposed as the subject of the
Adams Prize, was dealt with by James Clerk Maxwell in 1857.
His investigation forms the groundwork of all that is at present
known in the matter. Its upshot was to show that neither solid
nor fluid rings could continue to exist, and that the only possible
composition of the system was by an aggregated multitude of
unconnected particles, each revolving independently in a period[Pg 299]
corresponding to its distance from the planet.[1099] This idea of a
satellite-formation had been, remarkably enough, several times
entertained and lost sight of. It was first put forward by Roberval
in the seventeenth century, again by Jacques Cassini in 1715, and
with perfect definiteness by Wright of Durham in 1750.[1100] Little
heed, however, was taken of these casual anticipations of a truth
which reappeared, a virtual novelty, as the legitimate outcome of
the most refined modern methods.
The details of telescopic observation accord, on the whole,
admirably with this hypothesis. The displacements or disappearance
of secondary dividing-lines—the singular striated appearance,
first remarked by Short in the eighteenth century, last by Perrotin
and Lockyer at Nice, March 18, 1884[1101]—show the effects of waves
of disturbance traversing a moving mass of gravitating particles;[1102]
the broken and changing line of the planet’s shadow on the ring
gives evidence of variety in the planes of the orbits described by
those particles. The whole ring-system, too, appears to be somewhat
elliptical.[1103]
The satellite-theory has derived unlooked-for support from
photometric inquiries. Professor Seeliger pointed out in 1888[1104]
that the unvarying brilliancy of the outer rings under all angles of
illumination, from 0° to 30°, can be explained from no other point
of view. Nor does the constitution of the obscure inner ring offer
any difficulty. For it is doubtless formed of similar small bodies to
those aggregated in the lucid members of the system, only much
more thinly strewn, and reflecting, consequently, much less light.
It is not, indeed, at first easy to see why these sparser flights
should show as a dense dark shading on the body of Saturn. Yet
this is invariably the case. The objection has been urged by
Professor Hastings of Baltimore. The brightest parts of these
appendages, he remarked,[1105] are more lustrous than the globe they
encircle; but if the inner ring consists of identical materials,
possessing presumably an equal reflective capacity, the mere fact
of their scanty distribution would not cause them to show as dark
against the same globe. Professor Seeliger, however, replied[1106] that
the darkening is due to the never-ending swarms of their separate[Pg 300]
shadows transiting the planet’s disc. Sunlight is not, indeed,
wholly excluded. Many rays come and go between the open ranks
of the meteorites. For the dusky ring is transparent. The planet
it encloses shows through it, as if veiled with a strip of crape. A
beautiful illustration of its quality in this respect was derived by
Professor Barnard from an eclipse of Japetus, November 1, 1889.[1107]
The eighth moon remained steadily visible during its passage through
the shadow of the inner ring, but with a progressive loss of lustre in
approaching its bright neighbour. There was no breach of continuity.
The satellite met no gap, corresponding to that between the dusky
ring and the body of Saturn, through which it could shine with
undiminished light, but was slowly lost sight of as it plunged into
deeper and deeper gloom. The important facts were thus established,
that the brilliant and obscure rings merge into each other, and that
the latter thins out towards the Saturnian globe.
The meteoric constitution of these appendages was beautifully
demonstrated in 1895 by Professor Keeler,[1108] then director of the
Alleghany Observatory, Pittsburgh. From spectrographs taken
with the slit adjusted to coincidence with the equatorial plane of
the system, he determined the comparative radial velocities of its
different parts. And these supply a crucial test of Clerk Maxwell’s
theory. For if the rings were solid, the swiftest rates of rotation
should be at their outer edges, corresponding to wider circles
described in the same period; while, if they are pulverulent, the
inverse relation must hold good. This proved to be actually the
case. The motion slowed off outward, in agreement with the
diminishing speed of particles travelling freely, each in its own
orbit. Keeler’s result was promptly confirmed by Campbell,[1109] as well
as by Deslandres and Bélopolsky.
A question of singular interest, and one which we cannot refrain
from putting to ourselves, is—whether we see in the rings of Saturn
a finished structure, destined to play a permanent part in the
economy of the system; or whether they represent merely a stage
in the process of development out of the chaotic state in which it
is impossible to doubt that the materials of all planets were
originally merged. M. Otto Struve attempted to give a definite
answer to this important query.
A study of early and later records of observations disclosed to
him, in 1851, an apparent progressive approach of the inner edge of
the bright ring to the planet. The rate of approach he estimated at
about fifty-seven English miles a year, or 11,000 miles during the[Pg 301]
194 years elapsed since the time of Huygens.[1110] Were it to continue,
a collapse of the system must be far advanced within three centuries.
But was the change real or illusory—a plausible, but deceptive
inference from insecure data? M. Struve resolved to put it to the
test. A set of elaborately careful micrometrical measures of the
dimensions of Saturn’s rings, executed by himself at Pulkowa in the
autumn of 1851, was provided as a standard of future comparison;
and he was enabled to renew them, under closely similar circumstances,
in 1882.[1111] But the expected diminution of the space
between Saturn’s globe and his rings had not taken place. A slight
extension in the width of the system, both outward and inward,
was indeed, hinted at; and it is worth notice that just such a
separation of the rings was indicated by Clerk Maxwell’s theory, so
that there is an à priori likelihood of its being in progress. Yet
Hall’s measures in 1884-87[1112] failed to supply evidence of alteration
with time; and Barnard’s, executed at Lick in 1894-95,[1113] showed no
sensible divergence from them. Hence, much weight cannot be laid
upon Huygens’s drawings and descriptions, which had been held to
prove conclusively a partial filling up, since 1657, of the interval
between the ring and the planet.[1114]
The rings of Saturn replace, in Professor G. H. Darwin’s view,[1115]
an abortive satellite, scattered by tidal action into annular form.
For they lie closer to the planet than is consistent with the
integrity of a revolving body of reasonable bulk. The limit of
possible existence for such a mass was fixed by Roche of Montpellier,
in 1848,[1116] at 2·44 mean radii of its primary; while the outer
edge of the ring-system is distant 2·38 radii of Saturn from his
centre. The virtual discovery of its pulverulent condition dates,
then, according to Professor Darwin, from 1848. He conjectures
that the appendage will eventually disappear, partly through the
dispersal of its constituent particles inward, and their subsidence
upon the planet’s surface, partly by their dispersal outward, to a
region beyond “Roche’s limit,” where coalescence might proceed
unhindered by the strain of unequal attractions. One modest
satellite, revolving inside Mimas, would then be all that was left of
the singular appurtenances we now contemplate with admiration.
There seems reason to admit that Kirkwood’s law of commensurability
has had some effect in bringing about the present
distribution of the matter composing them. Here the influential[Pg 302]
bodies are Saturn’s moons, while the divisions and boundaries of
the rings represent the spaces where their disturbing action
conspires to eliminate revolving particles. Kirkwood, in fact,
showed, in 1867,[1117] that a body circulating in the chasm between the
bright rings known as “Cassini’s division,” would have a period
nearly commensurable with those of four out of the eight moons;
and Meyer of Geneva subsequently calculated all such combinations,
with the result of bringing out coincidences between regions of
maximum perturbation and the limiting and dividing lines of the
system.[1118] This is in itself a strong confirmation of the view that the
rings are made up of independently revolving small bodies.
On December 7, 1876, Professor Asaph Hall discovered at
Washington a bright equatorial spot on Saturn, which he followed
and measured through above sixty rotations, each performed in ten
hours fourteen minutes twenty-four seconds.[1119] This, he was careful
to add, represented the period, not necessarily of the planet, but only
of the individual spot. The only previous determination of Saturn’s
axial movement (setting aside some insecure estimates by Schröter)
was Herschel’s in 1794, giving a period of ten hours sixteen minutes.
The substantial accuracy of Hall’s result was verified by Mr. Denning
in 1891.[1120] In May and June of that year, ten vague bright
markings near the equator were watched by Mr. Stanley Williams,
who derived from them a rotation period only two seconds shorter
than that determined at Washington. Nevertheless, similarly
placed spots gave in 1892 and 1893 notably quicker rates;[1121] so that
the task of timing the general drift of the Saturnian surface by the
displacements of such objects is hampered, to an indefinite extent,
by their individual proper motions.
Saturn’s outermost satellite, Japetus, is markedly variable—so
variable that it sends us, when brightest, just 4-1/2 times as much
light as when faintest. Moreover, its fluctuations depend upon its
orbital position in such a way as to make it a conspicuous telescopic
object when west, a scarcely discernible one when east of the
planet. Herschel’s inference[1122] of a partially obscured globe turning
always the same face towards its primary seems the only admissible
one, and is confirmed by Pickering’s measurements of the varying
intensity of its light. He remarked further that the dusky and
brilliant hemispheres must be so posited as to divide the disc, viewed
from Saturn, into nearly equal parts; so that this Saturnian moon,[Pg 303]
even when “full,” appears very imperfectly illuminated over one-half
of its surface.[1123]
Zöllner estimated the albedo of Saturn at 0·51, Müller at 0·88, a
value impossibly high, considering that the spectrum includes no
vestige of original emissions. Closely similar to that of Jupiter, it
shows the distinctive dark line in the red (wave-length 618), which
we may call the “red-star line”; and Janssen, from the summit of
Etna in 1867[1124] found traces in it of aqueous absorption. The light
from the ring appears to be pure reflected sunshine unmodified by
original atmospheric action.[1125]
Uranus, when favourably situated, can easily be seen with the
naked eye as a star between the fifth and sixth magnitudes. There
is indeed, some reason to suppose that he had been detected as a
wandering orb by savage “watchers of the skies” in the Pacific long
before he swam into Herschel’s ken. Nevertheless, inquiries into
his physical habitudes are still in an early stage. They are exceedingly
difficult of execution, even with the best and largest modern
telescopes; and their results remain clouded with uncertainty.
It will be remembered that Uranus presents the unusual spectacle
of a system of satellites travelling nearly at right angles to the plane
of the ecliptic. The existence of this anomaly gives a special
interest to investigations of his axial movement, which might be
presumed, from the analogy of the other planets, to be executed in
the same tilted plane. Yet this is far from being certainly the case.
Mr. Buffham in 1870-72 caught traces of bright markings on the
Uranian disc, doubtfully suggesting a rotation in about twelve hours
in a plane not coincident with that in which his satellites circulate.[1126]
Dusky bands resembling those of Jupiter, but very faint, were
barely perceptible to Professor Young at Princeton in 1883. Yet,
though almost necessarily inferred to be equatorial, they made a
considerable angle with the trend of the satellites’ orbits.[1127] More
distinctly by the brothers Henry, with the aid of their fine refractor,
two gray parallel rulings, separated by a brilliant zone, were discerned
every clear night at Paris from January to June, 1884.[1128] What were
taken to be the polar regions appeared comparatively dusky. The
direction of the equatorial rulings (for so we may safely call them)
made an angle of 40° with the satellites’ line of travel. Similar observations
were made at Nice by MM. Perrotin and Thollon, March
to June, 1884, a lucid spot near the equator, in addition, indicating[Pg 304]
rotation in a period of about ten hours.[1129] The discrepancy was,
however, considerably reduced by Perrotin’s study of the planet in
1889 with the new 30-inch equatoreal.[1130] The dark bands, thus
viewed to better advantage than in 1884, appeared to deviate no
more than 10° from the satellites’ orbit-plane. No definitive results,
on the other hand, were derived by Professors Holden, Schaeberle,
and Keeler from their observations of Uranus in 1889-90 with the
potent instrument on Mount Hamilton. Shadings, it is true, were
almost always, though faintly, seen; but they appeared under an
anomalous, possibly an illusory aspect. They consisted, not of parallel,
but of forked bands.[1131]
Measurements of the little sea-green disc which represents to
us the massive bulk of Uranus, by Young, Schiaparelli,[1132] Safarik,
H. C. Wilson[1133] and Perrotin, prove it to be quite distinctly bulged.
The compression at once caught Barnard’s trained eye in 1894,[1134]
when he undertook at Lick a micrometrical investigation of the
system; and he was surprised to perceive that the major axis
of the elliptical surface made an angle of about 28° with the line
of travel pursued by the satellites. Nothing more can be learned
on this curious subject for some years, since the pole of the planet
is just now turned nearly towards the earth; but Barnard’s
conclusion is unlikely to be seriously modified. He fixed the mean
diameter of Uranus at 34,900 miles. But this estimate was
materially reduced through Dr. See’s elimination of irradiative
effects by means of daylight measures, executed at Washington in
1901.[1135]
The visual spectrum of this planet was first examined by Father
Secchi in 1869, and later, with more advantages for accuracy, by
Huggins, Vogel,[1136] and Keeler.[1137] It is a very remarkable one. In lieu
of the reflected Fraunhofer lines, imperceptible perhaps through
feebleness of light, six broad bands of original absorption appear,
one corresponding to the blue-green ray of hydrogen (F), another
to the “red-star line” of Jupiter and Saturn, the rest as yet
unidentified. The hydrogen band seems much too strong and diffuse
to be the mere echo of a solar line, and might accordingly be held
to imply the presence of free hydrogen in the Uranian atmosphere.
This, however, would be difficult of reconcilement with Keeler’s identification
of an absorption-group in the yellow with a telluric waterband.
Notwithstanding its high albedo—0·62, according to Zöllner—proof
is wanting that any of the light of Uranus is inherent. Mr.
Albert Taylor announced, indeed, in 1889, his detection, with Common’s
giant reflector, of bright flutings in its spectrum;[1138] but Professor
Keeler’s examination proved them to be merely contrast effects.[1139]
Sir William and Lady Huggins, moreover, obtained about the same
time a photograph purely solar in character. The spectrum it represented
was crossed by numerous Fraunhofer lines, and by no others.
It was, then, presumably composed entirely of reflected light.
Judging from the indications of an almost evanescent spectrum,
Neptune, as regards physical condition, is the twin of Uranus, as
Saturn of Jupiter. Of the circumstances of his rotation we are as
good as completely ignorant. Mr. Maxwell Hall, indeed, noticed at
Jamaica, in November and December, 1883, certain rhythmical
fluctuations of brightness, suggesting revolution on an axis in slightly
less than eight hours;[1140] but Professor Pickering reduces the supposed
variability to an amount altogether too small for certain perception,
and Dr. G. Müller denies its existence in toto. It is true their observations
were not precisely contemporaneous with those of
Mr. Hall[1141] who believes the partial obscurations recorded by himself
to have been of a passing kind, and to have suddenly ceased after a
fortnight of prevalence. Their less conspicuous renewal was visible
to him in November, 1884, confirming a rotation period of 7·92
hours.
It was ascertained at first by indirect means that the orbit of Neptune’s
satellite is inclined about 20° to his equator. Mr. Marth[1142]
having drawn attention to the rapid shifting of its plane of motion,
M. Tisserand and Professor Newcomb[1143] independently published the
conclusion that such shifting necessarily results from Neptune’s
ellipsoidal shape. The movement is of the kind exemplified—although
with inverted relations—in the precession of the equinoxes.
The pole of the satellite, owing to the pull of Neptune’s equatorial
protuberance, describes a circle round the pole of his equator in a
retrograde direction, and in a period of over five hundred years.
The amount of compression indicated for the primary body is, at the
outside, 1/85; whence it can be inferred that Neptune possesses a
lower rotatory velocity than the other giant planets. Direct[Pg 306]
verification of the trend theoretically inferred for the satellite’s movement
was obtained by Dr. See in 1899. The Washington 26-inch
refractor disclosed to him, under exceptionally favourable conditions,
a set of equatorial belts on the disc of Neptune, and they took just
the direction prescribed by theory. Their objective reality cannot
be doubted, although Barnard was unable, either with the Lick or
the Yerkes telescope,[1144] to detect any definite markings on this planet.
Its diameter was found by him to be 32,900 miles.
The possibility that Neptune may not be the most remote body
circling round the sun has been contemplated ever since he has been
known to exist. Within the last few years the position at a given
epoch of a planet far beyond his orbital verge has been approximately
fixed by two separate investigators.
Professor George Forbes of Edinburgh adopted in 1880 a novel
plan of search for unknown members of the solar system, the first
idea of which was thrown out by M. Flammarion in November,
1879.[1145] It depends upon the movements of comets. It is well known
that those of moderately short periods are, for a reason already
explained, connected with the larger planets in such a way that the
cometary aphelia fall near some planetary orbit. Jupiter claims a
large retinue of such partial dependents, Neptune owns six, and there
are two considerable groups, the farthest distances of which from
the sun lie respectively near 100 and 300 times that of the earth.
At each of these vast intervals, one involving a period of 1,000,
the other of 5,000 years, Professor Forbes maintains that an unseen
planet circulates. He even computed elements for the nearer
of the two, and fixed its place on the celestial sphere;[1146] but the
photographic searches made for it by Dr. Roberts at Crowborough
and by Mr. Wilson at Daramona proved unavailing. Undeterred
by Deichmüller’s discouraging opinion that cometary orbits extending
beyond the recognised bounds of the solar system are too
imperfectly known to serve as the basis of trustworthy conclusions,[1147]
the Edinburgh Professor returned to the attack in 1901.[1148] He now
sought to prove that the lost comet of 1556 actually returned in
1844, but with elements so transformed by ultra-Neptunian perturbations
as to have escaped immediate identification. If so, the
“wanted” planet has just entered the sign Libra, and, being larger
than Jupiter, should be possible to find.
Almost simultaneously with Forbes, Professor Todd set about[Pg 307]
groping for the same object by the help of a totally different set of
indications. Adams’s approved method commended itself to him;
but the hypothetical divagations of Neptune having scarcely yet
had time to develop, he was thrown back upon the “residual
errors” of Uranus. They gave him a virtually identical situation
for the new planet with that derived from the clustering of cometary
aphelia.[1149] Yet its assigned distance was little more than half that of
the nearer of Professor Forbes’s remote pair, and it completed a
revolution in 375 instead of 1,000 years. The agreement in them
between the positions determined, on separate grounds, for the
ultra-Neptunian traveller was merely an odd coincidence; nor can
we be certain, until it is seen, that we have really got into touch
with it.
FOOTNOTES:
[965] Phil. Trans., vol. lxxiv., p. 260.
[966] Novæ Observationes, p. 105.
[967] Phil. Trans., vol. i., p. 243.
[968] Mém. de l’Ac., 1720, p. 146.
[969] Phil. Trans., vol. lxxiv., p. 273.
[970] A large work, entitled Areographische Fragmente, in which Schröter embodied
the results of his labours on Mars, 1785-1803, narrowly escaped the conflagration
of 1813, and was published at Leyden in 1881.
[971] Beiträge, p. 124.
[972] Mem. R. A. Soc., vol. xxxii., p. 183.
[973] Astr. Nach., No. 1,468.
[974] Observatory, vol. viii., p. 437.
[975] Month. Not., vols. xxviii., p. 37; xxix., p. 232; xxxiii., p. 552.
[976] Flammarion, L’Astronomie, t. i., p. 266.
[977] Smyth, Cel. Cycle, vol. i., p. 148 (1st ed.).
[978] Phil. Trans., vol. cxxi., p. 417.
[979] Month. Not., vol. xxv., p. 227.
[980] Phil. Mag., vol. xxxiv., p. 75.
[981] Proctor, Quart. Jour. of Science, vol. x., p. 185; Maunder, Sunday Mag.,
January, February, March, 1882; Campbell, Publ. Astr. Pac. Soc., vol. vi., p. 273.
[982] Am. Jour. of Sc., vol. xxviii., p. 163.
[983] Burton, Trans. Roy. Dublin Soc., vol. i., 1880, p. 169.
[984] Month. Not., vol. xxvii., p. 179; Astroph. Journ., vol. i., p. 193.
[985] Untersuchungen über die Spectra der Planeten, p. 20; Astroph. Journ., vol. i.,
p. 203.
[986] Publ. Astr. Pac. Soc., vols. vi., p. 228; ix., p. 109; Astr. and Astroph.,
vol. xiii., p. 752; Astroph. Jour., vol. ii., p. 28.
[987] Ibid., vol. v., p. 328.
[988] Ibid., vols. i., p. 311; iii., p. 254.
[989] C. Christiansen, Beiblätter, 1886, p. 532.
[990] Astr. and Astrophysics, vol. xi., p. 671.
[991] Flammarion, La Planète Mars, p. 574.
[992] Mémoires Couronnés, t. xxxix.
[993] Lockyer, Nature, vol. xlvi., p. 447.
[994] Mem. Spettr. Italiani, t. xi., p. 28.
[995] Bull. Astr., t. iii., p. 324.
[996] Journ. Brit. Astr. Ass., vol. i., p. 88.
[997] Publ. Pac. Astr. Soc., vol. ii., p. 299; Percival Lowell, Mars, 1896; Annals
of the Lowell Observatory, vol. ii., 1900.
[998] Old and New Astr., p. 545.
[999] L’Astronomie, t. xi., p. 445.
[1000] La Planète Mars, p. 588.
[1001] Month. Notices, vol. lvi., p. 166.
[1002] L’Astronomie, t. viii.
[1003] Astr. Nach., No. 3,271; Astr. and Astrophysics, vol. xiii., p. 716.
[1004] Month. Not., vol. xxxviii., p. 41; Mem. Roy. Astr. Soc., vol. xliv., p. 123.
[1005] Astr. and Astrophysics, vol. xi., p. 668.
[1006] Ibid., p. 850.
[1007] Comptes Rendus, t. cxv., p. 379.
[1008] Astr. Jour., No. 384; Publ. Astr. Pac. Soc., vol. vi., p. 109. Cf. Observatory
vol. xvii., pp. 295-336.
[1009] See Mr. Wentworth Erck’s remarks in Trans. Roy. Dublin Soc., vol. i., p. 29.
[1010] Month. Not., vol. xxxviii., p. 206.
[1011] Annals Harvard Coll. Obs., vol. xi., pt. ii., p. 217.
[1012] Young, Gen. Astr., p. 366.
[1013] Campbell, Publ. Pac. Astr. Soc., vol. vi., p. 270.
[1014] Astr. Nach., No. 3,319.
[1015] Witch of Atlas, stanza iii. I am indebted to Dr. Garnett for the reference.
[1016] Recommended by Chandler, Astr. Jour., No. 452.
[1017] Harvard Circulars, Nos. 36, 37, 51.
[1018] Astr. Nach., No. 3,687.
[1019] Montangerand, Comptes Rendus, March 11, 1901.
[1020] Pickering, Astroph. Jour., vol. xiii., p. 277.
[1021] Harvard Circular, No. 58.
[1022] Astr. Nach., No. 752.
[1023] L. Niesten, Annuaire, Bruxelles, 1881, p. 269.
[1024] According to Svedstrup (Astr. Nach., Nos. 2,240-41), the inclination to the
ecliptic of the “mean asteroid’s” orbit is = 6°.
[1025] Smiths. Report, 1876, p. 358; The Asteroids (Kirkwood), p. 42, 1888.
[1026] Tisserand, Annuaire, Paris, 1891, p. B. 15; Newcomb, Astr. Jour., No. 477;
Backlund, Bull. Astr., t. xvii., p. 81; Parmentier, Bull. Soc. Astr. de France,
March, 1896; Observatory, vol. xviii., p. 207.
[1027] Berberich, Astr. Nach., No. 3,088.
[1028] Bull. Astr., t. xviii., p. 39.
[1029] The Asteroids, p. 48; Publ. Astr. Pac. Soc., vols. ii., p. 48; iii., p. 95.
[1030] Comptes Rendus, t. xxxvii., p. 797.
[1031] Bull. Astr., t. v., p. 180.
[1032] Annuaire, Bruxelles, 1881, p. 243.
[1033] Johns Hopkins Un. Circular, January, 1895; Observatory, vol. xviii., p. 127.
[1034] Harvard Annals, vol. xi., part ii., p. 294.
[1035] Astr. Nach., Nos. 2,724-5.
[1036] Month. Not., vol. lxi., p. 69.
[1037] Astroph. Jour., vol. vii., p. 25.
[1038] Spectra der Planeten, p. 24.
[1039] Tome i., p. 93.
[1040] Berlinische Monatsschrift, 1785, p. 211.
[1041] Month. Not., vol. xiii., p. 40.
[1042] Mem. Am. Ac., vol. viii., p. 221.
[1043] Photom. Unters., p. 303.
[1044] Astr. Nach., No. 1,851.
[1045] Mém. de l’Ac., t. x., p. 514.
[1046] Ibid., 1692, p. 7.
[1047] Month. Not., vol. xliv., p. 63.
[1048] Photom. Unters., pp. 165, 273; Potsdam Publ., No. 30.
[1049] Vogel, Sp. der Planeten, p. 33, note.
[1050] Proc. Roy. Soc., vol. xviii., p. 250.
[1051] Month. Not., vol. xl., p. 433.
[1052] Sitzungsberichte, Berlin, 1895, ii., p. 15.
[1053] The anomalous shadow-effects recorded by Webb (Cel. Objects, p. 170,
4th ed.) are obviously of atmospheric and optical origin.
[1054] Engelmann, Ueber die Helligkeitsverhältnisse der Jupiterstrabanten, p. 59.
[1055] Month. Not., vol. xxviii., p. 11.
[1056] Observatory, vol. vii., p. 175.
[1057] Month. Not., vol. xlviii., p. 43.
[1058] Publ. Astr. Pac. Soc., vol. ii., p. 296.
[1059] Pickering failed to obtain any photometric evidence of their variability.
Harvard Annals, vol. xi., p. 245.
[1060] Astr. and Astroph., vol. xii., pp. 194, 481.
[1061] Annals Lowell Obs., vol. ii., pt. i.
[1062] Astr. Nach., Nos. 2,995, 3,206; Month. Not., vols. li., p. 556; liv., p. 134.
Barnard remains convinced that the oval forms attributed to Jupiter’s satellites
are illusory effects of their markings. Astr. Nach., Nos. 3,206, 3,453; Astr. and
Astroph., vol. xiii., p. 272.
[1063] Publ. Astr. Pac. Soc., vol. iii., p. 355.
[1064] Astr. Nach., No. 1,017.
[1065] Publ. Astr. Pac. Soc., vol. iii., p. 359.
[1066] Astr. Nach., No. 3,432.
[1067] Astr. Jour., Nos. 275, 325, 367, 472; Observatory, vol. xv., p. 425.
[1068] Tisserand, Comptes Rendus, October 8, 1894; Cohn, Astr. Nach., No. 3,404.
[1069] Bull. Ac. R. Bruxelles, t. xlviii., p. 607.
[1070] Astr. Nach., No. 2,294.
[1071] Ibid., No. 2,284.
[1072] Denning, Month. Not., vol. xliv., pp. 64, 66; Nature, vol. xxv., p. 226.
[1073] Sidereal Mess., December, 1886, p. 289.
[1074] Astr. Nach., Nos. 2,280, 2,282.
[1075] Month. Not., vol. xlvi., p. 117.
[1076] Proc. Roy. Soc. N. S. Wales, vol. xiv., p. 68.
[1077] Phil. Trans., vol. i., p. 143.
[1078] For indications relative to the early history of the red spot, see Holden,
Publ. Astr. Pac. Soc., vol. ii., p. 77; Noble, Month. Not., vol. xlvii., p. 515;
A. S. Williams, Observatory, vol. xiii., p. 338.
[1079] Astr. and Astrophysics, vol. xi., p. 192.
[1080] Month. Not., vol. l., p. 520.
[1081] Observatory, vol. xiii., pp. 297, 326.
[1082] Trans. R. Dublin Soc., vol. iv., p. 271, 1889.
[1083] Publ. Astr. Pac. Soc., vol. ii., p. 289.
[1084] Astr. and Astrophysics, vol. xi., p. 686.
[1085] Denning, Knowledge, vol. xxiii., p. 200; Observatory, vol. xxiv., p. 312;
Pop. Astr., vol. ix., p. 448; Nature, vol. lv., p. 89.
[1086] Williams, Observatory, vol. xxiii., p. 282.
[1087] Month. Not., vol. lvi., p. 143.
[1088] Bélopolsky, Astr. Nach., No. 3,326.
[1089] Publ. Astr. Pac. Soc., vol. iv., p. 176.
[1090] Bull. Astr., 1900, p. 70.
[1091] Month. Not., vol. xxxi., p. 34.
[1092] Beobachtungen, Heft ii., p. 99.
[1093] Ber. Sächs. Ges. der Wiss., 1871, p. 553.
[1094] Month. Not., vol. lix., p. 76.
[1095] Beziehungen der Sonnenfleckenperiode, p. 175.
[1096] A. Hall, Astr. Nach., No. 2,269.
[1097] Astr. Jour. (Gould’s), vol. ii., p. 17.
[1098] Ibid., p. 5.
[1099] On the Stability of the Motion of Saturn’s Rings, p. 67.
[1100] Mém. de l’Ac., 1715, p. 47; Montucla, Hist. des Math., t. iv., p. 19; An
Original Theory of the Universe, p. 115.
[1101] Comptes Rendus, t. xcviii., p. 718.
[1102] Proctor, Saturn and its System (1865), p. 125.
[1103] Perrotin, Comptes Rendus, t. cvi., p. 1716.
[1104] Abhandl. Akad. der Wiss., Munich, Bd. xvi., p. 407.
[1105] Smiths. Report, 1880 (Holden).
[1106] Quoted by Dr. E. Anding, Astr. Nach., No. 2,881.
[1107] Astr. and Astrophysics, vol. xi., p. 119; Month. Not., vol. l., p. 108.
[1108] Astroph. Jour., vol. i., p. 416.
[1109] Ibid., vol. ii., p. 127.
[1110] Mém. de l’Ac. Imp. (St. Petersb.), t. vii., 1853, p. 464.
[1111] Astr. Nach., No. 2,498.
[1112] Washington Observations, App. ii., p. 22
[1113] Month. Not., vol. lvi., p. 163.
[1114] T. Lewis, Observatory, vol. xviii., p. 379.
[1115] Harper’s Magazine, June, 1889.
[1116] Mém. de l’Acad. de Montpellier, t. viii., p. 296, 1873.
[1117] Meteoric Astronomy, chap. xii. He carried the subject somewhat farther in
1871. See Observatory, vol. vi., p. 335.
[1118] Astr. Nach., No. 2,527.
[1119] Amer. Jour. of Sc., vol. xiv., p. 325.
[1120] Observatory, vol. xiv., p. 369.
[1121] Month. Not., vol. liv., p. 297.
[1122] Phil. Trans., vol. lxxxii., p. 14.
[1123] Smiths. Report, 1880.
[1124] Comptes Rendus, t. lxiv., p. 1304.
[1125] Huggins, Proc. R. Soc., vol. xlvi., p. 231; Keeler, Astr. Nach., No. 2,927;
Vogel, Astroph. Jour., vol. i., p. 278.
[1126] Month. Not., vol. xxxiii., p. 164.
[1127] Astr. Nach., No 2,545.
[1128] Comptes Rendus, t. xcviii., p. 1419.
[1129] Comptes Rendus, t. xcviii., pp. 718, 967.
[1130] V. J. S. Astr. Ges., Jahrg. xxiv., p. 267.
[1131] Publ. Astr. Pac. Soc., vol. iii., p. 287.
[1132] Astr. Nach., No. 2,526.
[1133] Ibid., No. 2,730.
[1134] Astr. Jour., Nos. 370, 374.
[1135] Astr. Nach., No. 3,768.
[1136] Ann. der Phys., Bd. clviii., p. 470; Astroph. Jour., vol. i., p. 280.
[1137] Astr. Nach., No. 2,927.
[1138] Month. Not., vol. xlix., p. 405.
[1139] Astr. Nach., No. 2,927; Scheiner’s Spectralanalyse, p. 221.
[1140] Month. Not., vol. xliv., p. 257.
[1141] Observatory, vol. vii., pp. 134, 221, 264.
[1142] Month. Not., vol. xlvi., p. 507.
[1143] Comptes Rendus, t. cvii., p. 804; Astr. and Astroph., vol. xiii., p. 291; Astr.
Jour., No. 186.
[1144] Astr. Jour., Nos. 342, 436, 508.
[1145] Astr. Pop., p. 661; La Nature, January 3, 1880.
[1146] Proc. Roy. Soc. Edinb., vols. x., p. 429; xi., p. 89.
[1147] Vierteljahrsschrift. Astr. Ges., Jahrg. xxi., p. 206.
[1148] Proc. Roy. Soc. Edinb., vol. xxiii., p. 370; Nature, vol. lxiv., p. 524.
[1149] Amer. Jour. of Science, vol. xx., p. 225.
CHAPTER IX
THEORIES OF PLANETARY EVOLUTION
We cannot doubt that the solar system, as we see it, is the result of
some process of growth—that, during innumerable ages, the forces
of Nature were at work upon its materials, blindly modelling them
into the shape appointed for them from the beginning by Omnipotent
Wisdom. To set ourselves to inquire what that process was
may be an audacity, but it is a legitimate, nay, an inevitable one.
For man’s implanted instinct to “look before and after” does not
apply to his own little life alone, but regards the whole history of
creation, from the highest to the lowest—from the microscopic germ
of an alga or a fungus to the visible frame and furniture of the
heavens.
Kant considered that the inquiry into the mode of origin of the
world was one of the easiest problems set by Nature; but it cannot
be said that his own solution of it was satisfactory. He, however,
struck out in 1755 a track which thought still pursues. In his
Allgemeine Naturgeschichte the growth of sun and planets was traced
from the cradle of a vast and formless mass of evenly diffused
particles, and the uniformity of their movements was sought to be
accounted for by the unvarying action of attractive and repulsive
forces, under the dominion of which their development was carried
forward.
In its modern form, the “Nebular Hypothesis” made its appearance
in 1796.[1150] It was presented by Laplace with diffidence, as a
speculation unfortified by numerical buttresses of any kind, yet
with visible exultation at having, as he thought, penetrated the
birth-secret of our system. He demanded, indeed, more in the way
of postulates than Kant had done. He started with a sun ready
made,[1151] and surrounded with a vast glowing atmosphere, extending[Pg 309]
into space out beyond the orbit of the farthest planet, and endowed
with a slow rotatory motion. As this atmosphere or nebula cooled,
it contracted; and as it contracted, its rotation, by a well-known
mechanical law, became accelerated. At last a point arrived when
tangential velocity at the equator increased beyond the power of
gravity to control, and equilibrium was restored by the separation
of a nebulous ring revolving in the same period as the generating
mass. After a time, the ring broke up into fragments, all eventually
reunited in a single revolving and rotating body. This was the first
and farthest planet.
Meanwhile the parent nebula continued to shrink and whirl
quicker and quicker, passing, as it did so, through successive crises
of instability, each resulting in, and terminated by, the formation of
a planet, at a smaller distance from the centre, and with a shorter
period of revolution than its predecessor. In these secondary
bodies the same process was repeated on a reduced scale, the birth
of satellites ensuing upon their contraction, or not, according to
circumstances. Saturn’s ring, it was added, afforded a striking confirmation
of the theory of annular separation,[1152] and appeared to have
survived in its original form in order to throw light on the genesis
of the whole solar system; while the four first discovered asteroids
offered an example in which the débris of a shattered ring had failed
to coalesce into a single globe.
This scene of cosmical evolution was a characteristic bequest
from the eighteenth century to the nineteenth. It possessed the
self-sufficing symmetry and entireness appropriate to the ideas of
a time of renovation, when the complexity of nature was little
accounted of in comparison with the imperious orderliness of the
thoughts of man. Since its promulgation, however, knowledge has
transgressed many boundaries, and set at naught much ingenious
theorising. How has it fared with Laplace’s sketch of the origin
of the world? It has at least not been discarded as effete. The
groundwork of speculation on the subject is still furnished by it.
It is, nevertheless, admittedly inadequate. Of much that exists
it gives no account, or an erroneous one. The march of events
certainly did not everywhere—even if it did anywhere—follow the
exact path prescribed for it. Yet modern science attempts to
supplement, but scarcely ventures to supersede it.
Thought has, in many directions, been profoundly modified
by Mayer’s and Joule’s discovery, in 1842, of the equivalence
between heat and motion. Its corollary was the grand idea of
the “conservation of energy,” now one of the cardinal principles
of science. This means that, under the ordinary circumstances[Pg 310]
of observation, the old maxim ex nihilo nihil fit applies to force
as well as to matter. The supplies of heat, light, electricity,
must be kept up, or the stream will cease to flow. The question
of the maintenance of the sun’s heat was thus inevitably raised;
and with the question of maintenance that of origin is indissolubly
connected.
Dr. Julius Robert Mayer, a physician residing at Heilbronn,
was the first to apply the new light to the investigation of what
Sir John Herschel had termed the “great secret.” He showed
that if the sun were a body either simply cooling or in a state
of combustion, it must long since have “gone out.” Had an
equal mass of coal been set alight four or five centuries after
the building of the Pyramid of Cheops, and kept burning at
such a rate as to supply solar light and heat during the interim,
only a few cinders would now remain in lieu of our undiminished
glorious orb. Mayer looked round for an alternative. He found
it in the “meteoric hypothesis” of solar conservation.[1153] The
importance in the economy of our system of the bodies known
as falling stars was then (in 1848) beginning to be recognised.
It was known that they revolved in countless swarms round the
sun; that the earth daily encountered millions of them; and it
was surmised that the cone of the zodiacal light represented their
visible condensation towards the attractive centre. From the
zodiacal light, then, Mayer derived the store needed for supporting
the sun’s radiations. He proved that, by the stoppage of their
motion through falling into the sun, bodies would evolve from
4,600 to 9,200 times as much heat (according to their ultimate
velocity) as would result from the burning of equal masses of coal,
their precipitation upon the sun’s surface being brought about by
the resisting medium observed to affect the revolutions of Encke’s
comet. There was, however, a difficulty. The quantity of matter
needed to keep, by the sacrifice of its movement, the hearth of our
system warm and bright would be very considerable. Mayer’s
lowest estimate put it at 94,000 billion kilogrammes per second, or
a mass equal to that of our moon bi-annually. But so large an
addition to the gravitating power of the sun would quickly become
sensible in the movement of the bodies dependent upon him.
Their revolutions would be notably accelerated. Mayer admitted
that each year would be shorter than the previous one by a not
insignificant fraction of a second, and postulated an unceasing waste
of substance, such as Newton had supposed must accompany
emission of the material corpuscles of light, to neutralise continual
reinforcement.
Mayer’s views obtained a very small share of publicity, and owned
Mr. Waterston as their independent author in this country. The
meteoric, or “dynamical,” theory of solar sustentation was expounded
by him before the British Association in 1853. It was developed
with his usual ability by Lord Kelvin, in the following year.
The inflow of meteorites, he remarked, “is the only one of all
conceivable causes of solar heat which we know to exist from
independent evidence.”[1154] We know it to exist, but we now also
know it to be entirely insufficient. The supplies presumed to
be contained in the zodiacal light would be quickly exhausted; a
constant inflow from space would be needed to meet the demand.
But if moving bodies were drawn into the sun at anything like the
required rate, the air, even out here at ninety-three millions of miles
distance, would be thick with them; the earth would be red-hot
from their impacts;[1155] geological deposits would be largely meteoric;[1156]
to say nothing of the effects on the mechanism of the heavens.
Lord Kelvin himself urged the inadmissibility of the “extra-planetary”
theory of meteoric supply on the very tangible ground
that, if it were true, the year would be shorter now, actually by six
weeks, than at the opening of the Christian era. The “intra-planetary”
supply, however, is too scanty to be anything more than
a temporary makeshift.
The meteoric hypothesis was naturally extended from the maintenance
of the sun’s heat to the formation of the bodies circling round
him. The earth—no less doubtless than the other planets—is still
growing. Cosmical matter in the shape of falling stars and aërolites,
to the amount, adopting Professor Newton’s estimate, of 100 tons
daily, is swept up by it as it pursues its orbital round. Inevitably
the idea suggested itself that this process of appropriation gives the
key to the life-history of our globe, and that the momentary streak of
fire in the summer sky represents a feeble survival of the glowing
hailstorm by which in old times it was fashioned and warmed. Mr.
E. W. Brayley supported this view of planetary production in 1864,[1157]
and it has recommended itself to Haidinger, Helmholtz, Proctor, and
Faye. But the negative evidence of geological deposits appears fatal
to it.
The theory of solar energy now generally regarded as the true
one was enounced by Helmholtz in a popular lecture in 1854. It
depends upon the same principle of the equivalence of heat and
motion which had suggested the meteoric hypothesis. But here the
movement surrendered and transformed belongs to the particles, not[Pg 312]
of any foreign bodies, but of the sun itself. Drawn together from a
wide ambit by the force of their own gravity, their fall towards the
sun’s centre must have engendered a vast thermal store, of which 453/454
are computed to be already spent. Presumably, however, this
stream of reinforcement is still flowing. In the very act of parting
with heat, the sun develops a fresh stock. His radiations, in short,
are the direct result of shrinkage through cooling. A diminution of
the solar diameter by 380 feet yearly would just suffice to cover the
present rate of emission, and would for ages remain imperceptible
with our means of observation, since, after the lapse of 6,000 years,
the lessening of angular size would scarcely amount to one second.[1158]
But the process, though not terminated, is strictly a terminable one.
In less than five million years, the sun will have contracted to half
its present bulk. In seven million more, it will be as dense as the
earth. It is difficult to believe that it will then be a luminous body.[1159]
Nor can an unlimited past duration be admitted. Helmholtz considered
that radiation might have gone on with its actual intensity
for twenty-two, Langley allows only eighteen million years.
The period can scarcely be stretched, by the most generous allowances,
to double the latter figure. But this is far from meeting the
demands of geologists and biologists.
An attempt was made in 1881 to supply the sun with machinery
analogous to that of a regenerative furnace, enabling it to consume
the same fuel over and over again, and so to prolong indefinitely its
beneficent existence. The inordinate “waste” of energy, which
shocks our thrifty ideas, was simultaneously abolished. The earth
stops and turns variously to account one 2,250-millionth part of the
solar radiations; each of the other planets and satellites takes a
proportionate share; the rest, being all but an infinitesmal fraction
of the whole, is dissipated through endless space, to serve what
purpose we know not. Now, on the late Sir William Siemens’s
plan, this reckless expenditure would cease; the solar incomings
and outgoings would be regulated on approved economic principles,
and the inevitable final bankruptcy would be staved off to remote
ages.
But there was a fatal flaw in its construction. He imagined a
perpetual circulation of combustible materials, alternately surrendering
and regaining chemical energy, the round being kept going by
the motive force of the sun’s rotation.[1160] This, however, was merely to
perch the globe upon a tortoise, while leaving the tortoise in the air.
The sun’s rotation contains a certain definite amount of mechanical
power—enough, according to Lord Kelvin, if directly converted into[Pg 313]
heat, to keep up the sun’s emission during 116 years and six days—a
mere moment in cosmical time. More economically applied, it
would no doubt go farther. Its exhaustion would, nevertheless,
under the most favourable circumstances, ensue in a comparatively
short period.[1161] Many other objections equally unanswerable have
been urged to the “regenerative” hypothesis, but this one suffices.
Dr. Croll’s collision hypothesis[1162] is less demonstrably unsound, but
scarcely less unsatisfactory. By the mutual impact of two dark
masses rushing together with tremendous speed, he sought to
provide the solar nebula with an immense original stock of heat for
the reinforcement of that subsequently evolved in the course of its
progressive contraction. The sun, while still living on its capital,
would thus have a larger capital to live on, and the time-demands of
the less exacting geologists and biologists might be successfully met.
But the primitive event, assumed for the purpose of dispensing them
from the inconvenience of “hurrying up their phenomena,” is not
one that a sane judgment can readily admit to have ever, in point of
actual fact, happened.
There remains, then, as the only intelligible rationale of solar
sustentation, Helmholtz’s shrinkage theory. And this has a very
important bearing upon the nebular view of planetary formation; it
may, in fact, be termed its complement. For it involves the idea
that the sun’s materials, once enormously diffused, gradually condensed
to their present volume with development of heat and light,
and, it may plausibly be added, with the separation of dependent
globes. The data furnished by spectrum analysis, too, favour the
supposition of a common origin for sun and planets by showing their
community of substance; while gaseous nebulæ present examples of
vast masses of tenuous vapour, such as our system may plausibly be
conjectured to have primitively sprung from.
But recent science raises many objections to the details, if it
supplies some degree of confirmation to the fundamental idea of
Laplace’s cosmogony. The detection of the retrograde movement of
Neptune’s satellite made it plain that the anomalous conditions of
the Uranian world were due to no extraordinary disturbance, but to
a systematic variety of arrangement at the outskirts of the solar
domain. So that, were a trans-Neptunian planet discovered, we
should be fully prepared to find it rotating, and surrounded by
satellites circulating from east to west. The uniformity of movement,[Pg 314]
upon the probabilities connected with which the French
geometer mainly based his scheme, thus at once vanishes.
The excessively rapid revolution of the inner Martian moon is a
further stumbling-block. On Laplace’s view, no satellite can revolve
in a shorter time than its primary rotates; for in its period of circulation
survives the period of rotation of the parent mass which
filled the sphere of its orbit at the time of giving it birth. And
rotation quickens as contraction goes on; therefore, the older time
of axial rotation should invariably be the longer. This obstacle
can, however, as we shall presently see, be turned.
More serious is one connected with the planetary periods, pointed
out by Babinet in 1861.[1163] In order to make them fit in with the
hypothesis of successive separation from a rotating and contracting
body, certain arbitrary assumptions have to be made of fluctuations
in the distribution of the matter forming that body at the various
epochs of separation.[1164] Such expedients usually merit the distrust
which they inspire. Primitive and permanent irregularities of
density in the solar nebula, such as Miss Young’s calculations
suggest,[1165] do not, on the other hand, appear intrinsically improbable.
Again, it was objected by Professor Kirkwood in 1869[1166] that there
could be no sufficient cohesion in such an enormously diffused mass
as the planets are supposed to have sprung from to account for the
wide intervals between them. The matter separated through the
growing excess of centrifugal speed would have been cast off, not by
rarely recurring efforts, but continually, fragmentarily, pari passu
with condensation and acceleration. Each wisp of nebula, as it
found itself unduly hurried, would have declared its independence,
and set about revolving and condensing on its own account. The
result would have been a meteoric, not a planetary system.
Moreover, it is a question whether the relative ages of the planets
do not follow an order just the reverse of that concluded by Laplace.
Professor Newcomb holds the opinion that the rings which eventually
constituted the planets divided from the main body of the
nebula almost simultaneously, priority, if there were any, being on
the side of the inner and smaller ones;[1167] while in M. Faye’s cosmogony,[1168]
the retrograde motion of the systems formed by the two
outer planets is ascribed—on grounds, it is true, of dubious validity—to
their comparatively late origin.
This ingenious scheme was designed, not merely to complete, but[Pg 315]
to supersede that of Laplace, which, undoubtedly, through the
inclusion by our system of oppositely directed rotations, forfeits its
claim simply and singly to account for the fundamental peculiarities
of its structure.
M. Faye’s leading contention is that, under the circumstances
assumed by Laplace, not the two outer planets alone, but the whole
company must have been possessed of retrograde rotation. For they
were formed—ex hypothesi—after the sun; central condensation had
reached an advanced stage when the rings they were derived from
separated; the principle of inverse squares consequently held good,
and Kepler’s Laws were in full operation. Now, particles circulating
in obedience to these laws can only—since their velocity decreases
outward from the centre of attraction—coalesce into a globe with
a backward axial movement. Nor was Laplace blind to this flaw in
his theory; but his effort to remove it, though it passed muster for
the best part of a century,[1169] was scarcely successful. His planet-forming
rings were made to rotate all in one piece, their outer parts
thus necessarily travelling at a swifter linear rate than their inner
parts, and eventually uniting, equally of necessity, into a forward-spinning
body. The strength of cohesion involved may, however,
safely be called impossible, especially when it is considered that
nebulous materials were in question.
The reform proposed by M. Faye consists in admitting that all
the planets inside Uranus are of pre-solar origin—that they took
globular form in the bosom of a nearly homogeneous nebula,
revolving in a single period, with motion accelerated from centre
to circumference, and hence agglomerating into masses with a direct
rotation. Uranus and Neptune owe their exceptional characteristics
to their later birth. When they came into existence, the development
of the sun was already far advanced, central force had acquired
virtually its present strength, unity of period had been abolished by
its predominance, and motion was retarded outward.
Thus, what we may call the relative chronology of the solar
system is thrown once more into confusion. The order of seniority
of the planets is now no easier to determine than the “Who first,
who last?” among the victims of Hector’s spear. For M. Faye’s
arrangements, notwithstanding the skill with which he has presented
them, cannot be unreservedly accepted. The objections to them,
thoughtfully urged by M. C. Wolf[1170] and Professor Darwin,[1171] are
grave. Not the least so is his omission to take account of an
agency of change presently to be noticed.
A further valuable discussion of the matter was published by[Pg 316]
M. du Ligondès in 1897.[1172] His views are those of Faye, modified
to disarm the criticisms they had encountered; and special attention
may be claimed for his weighty remark that each planet has
a life-history of its own, essentially distinct from those of the
others, and, despite original unity, not to be confounded with them.
The drift of recent investigations seems, indeed, to be to find the
embryonic solar system already potentially complete in the parent
nebula, like the oak in an acorn, and to relegate detailed explanations
of its peculiarities to the dim pre-nebular fore-time.
We now come to a most remarkable investigation—one, indeed,
unique in its profession to lead us back with mathematical certainty
towards the origin of a heavenly body. We refer to Professor
Darwin’s inquiries into the former relations of the earth and
moon.[1173]
They deal exclusively with the effects of tidal friction, and
primarily with those resulting, not from oceanic, but from “bodily”
tides, such as the sun and moon must have raised in past ages on a
liquid or viscous earth. The immediate effect of either is, as already
explained, to destroy the rotation of the body on which the tide is
raised, as regards the tide-raising body, bringing it to turn always
the same face towards its disturber. This, we can see, has been
completely brought about in the case of the moon. There is, however,
a secondary or reactive effect. Action is always mutual.
Precisely as much as the moon pulls the terrestrial tidal wave
backward, the tidal wave pulls the moon forward. But pulling a
body forward in its orbit implies the enlargement of that orbit; in
other words, the moon is, as a consequence of tidal friction, very
slowly receding from the earth. This will go on (other circumstances
remaining unchanged) until the lengthening day overtakes
the more tardily lengthening month, when each will be of about
1,400 hours.[1174] A position of what we may call tidal equilibrium
between earth and moon will (apart from disturbance by other
bodies) then be attained.
If, however, it be true that, in the time to come, the moon will
be much farther from us, it follows that in the time past she was
much nearer to us than she now is. Tracing back her history by
the aid of Professor Darwin’s clue, we at length find her revolving
in a period of somewhere between two and four hours, almost in[Pg 317]
contact with an earth rotating just at the same rate. This was
before tidal friction had begun its work of grinding down axial
velocity and expanding orbital range. But the position was not one
of stable equilibrium. The slightest inequality must have set on
foot a series of uncompensated changes. If the moon had whirled
the least iota faster than the earth spun she must have been
precipitated upon it. Her actual existence shows that the trembling
balance inclined the other way. By a second or two to begin with,
the month exceeded the day; the tidal wave crept ahead of the
moon; tidal friction came into play, and our satellite started on
its long spiral journey outward from the parent globe. This must
have occurred, it is computed, at least fifty-four million years ago.
That this kind of tidal reactive effect played its part in bringing
the moon into its present position, and is still, to some slight extent,
at work in changing it, there can be no doubt whatever. An
irresistible conjecture carried the explorer of its rigidly deducible
consequences one step beyond them. The moon’s time of revolution,
when so near the earth as barely to escape contact with it, must
have been, by Kepler’s Law, more than two and less than two and
a half hours. Now it happens that the most rapid rate of rotation
of a fluid mass of the earth’s average density, consistent with
spheroidal equilibrium, is two hours and twenty minutes. Quicken
the movement but by one second and the globe must fly asunder.
Hence the inference that the earth actually did fly asunder through
over-fast spinning, the ensuing disruption representing the birth-throes
of the moon. It is likely that the event was hastened or
helped by solar tidal disturbance.
To recapitulate. Analysis tracks backward the two bodies until
it leaves them in very close contiguity, one rotating and the other
revolving in approximately the same time, and that time certainly
not far different from, and quite possibly identical with, the critical
period of instability for the terrestrial spheroid. “Is this,” Professor
Darwin asks, “a mere coincidence, or does it not rather point to the
break-up of the primeval planet into two masses in consequence of a
too rapid rotation?”[1175]
We are tempted, but are not allowed to give an unqualified assent.
Mr. James Nolan of Victoria has made it clear that the moon could
not have subsisted as a continuous mass under the powerful disruptive
strain which would have acted upon it when revolving almost in
contact with the present surface of the earth; and Professor Darwin,
admitting the objection, concedes to our satellite, in its initial stage,
the alternative form of a flock of meteorites.[1176] But such a congregation[Pg 318]
must have been quickly dispersed, by tidal action, into a
meteoric ring. The same investigator subsequently fixed 6,500 miles
from centre to centre as the minimum distance at which the moon
could have revolved in its entirety; and he concluded it “necessary
to suppose that, after the birth of a satellite, if it takes place at all
in this way, a series of changes occur which are quite unknown.”[1177]
The evidence, however, for the efficiency of tidal friction in bringing
about the actual configuration of the lunar-terrestrial system is not
invalidated by this failure to penetrate its natal mystery. Under
its influence the principal elements of that system fall into interdependent
mutual relations. It connects, casually and quantitatively,
the periods of the moon’s revolution and of the earth’s rotation, the
obliquity of the ecliptic, the inclination and eccentricity of the lunar
orbit. All this can scarcely be accidental.
Professor Darwin’s first researches on this subject were communicated
to the Royal Society, December 18, 1879. They were
followed, January 20, 1881,[1178] by an inquiry on the same principles
into the earlier condition of the entire solar system. The results
were a warning against hasty generalisation. They showed that the
lunar-terrestrial system, far from being a pattern for their development,
was a singular exception among the bodies swayed by the sun.
Its peculiarity resides in the fact that the moon is proportionately by
far the most massive attendant upon any known planet. Its disturbing
power over its primary is thus abnormally great, and tidal
friction has, in consequence, played a predominant part in bringing
their mutual relations into their present state.
The comparatively late birth of the moon tends to ratify this
inference. The dimensions of the earth did not differ (according
to our present authority) very greatly from what they now are
when her solitary offspring came, somehow, into existence. This is
found not to have been the case with any other of the planets. It
is unlikely that the satellites of Jupiter, Saturn, or Mars (we may
safely add, of Uranus or Neptune) ever revolved in much narrower
orbits than those they now traverse; it is practically certain that
they did not, like our moon, originate very near the present surfaces
of their primaries.[1179] What follows? The tide-raising power of a
body grows with vicinity in a rapidly accelerated ratio. Lunar
tides must then have been on an enormous scale when the moon
swung round at a fraction of its actual distance from the earth. But
no other satellite with which we are acquainted occupied at any
time a corresponding position. Hence no other satellite ever
possessed tide-raising capabilities in the least comparable to those of[Pg 319]
the moon. We conclude once more that tidal friction had an
influence here very different from its influence elsewhere. Quite
possibly, however, that influence may be more nearly spent than in
less advanced combinations of revolving globes. Mr. Nolan concluded
in 1895[1180] that it still retains appreciable efficacy in the several
domains of the outer planets. The moons of Jupiter and Saturn are,
by his calculations, in course of sensible retreat, under compulsion of
the perennial ripples raised by them on the surfaces of their gigantic
primaries. He thus connects the interior position of the fifth Jovian
satellite with its small mass. The feebleness of its tide-raising power
obliged it to remain behind its companions; for there is no sign of
its being more juvenile than the Galilean quartette.
The yielding of plastic bodies to the strain of unequal attractions
is a phenomenon of far-reaching consequence. We know that the
sun as well as the moon causes tides in our oceans. There must,
then, be solar, no less than lunar, tidal friction. The question at
once arises: What part has it played in the development of the
solar system? Has it ever been one of leading importance, or has
its influence always been, as it now is, subordinate, almost negligible?
To this, too, Professor Darwin supplies an answer.
It can be stated without hesitation that the sun did not give birth
to the planets, as the earth has been supposed to have given birth to
the moon, by the disruption of its already condensed, though viscous
and glowing mass, pushing them then gradually backward from its
surface into their present places. For the utmost possible increase
in the length of the year through tidal friction is one hour; and five
minutes is a more probable estimate.[1181] So far as the pull of tide-waves
raised on the sun by the planets is concerned, then, the
distances of the latter have never been notably different from what
they now are; though that cause may have converted the paths
traversed by them from circles into ellipses.
Over their physical history, however, it was probably in a large
measure influential. The first vital issue for each of them was—satellites
or no satellites? Were they to be governors as well as
governed, or should they revolve in sterile isolation throughout the
æons of their future existence? Here there is strong reason to
believe that solar tidal friction was the overruling power. It is
remarkable that planetary fecundity increases—at least so far outward
as Saturn—with distance from the sun. Can these two facts
be in any way related? In other words, is there any conceivable
way by which tidal influence could prevent or impede the throwingoff[Pg 320]
of secondary bodies? We have only to think for a moment in
order to see that this is precisely one of its direct results.[1182]
Tidal friction, whether solar or lunar, tends to reduce the axial
movement of the body it acts upon. But the separation of satellites
depends—according to the received view—upon the attainment of a
disruptive rate of rotation. Hence, if solar tidal friction were strong
enough to keep down the pace below this critical point, the contracting
mass would remain intact—there would be no satellite-production.
This, in all probability, actually occurred in the case
both of Mercury and Venus. They cooled without dividing,
because the solar friction-brake applied to them was too strong to
permit acceleration to pass the limit of equilibrium. The complete
destruction of their relative axial movement has been rendered
probable by recent observations; and that the process went on
rapidly is a reasonable further inference. The earth barely escaped
the fate of loneliness incurred by her neighbours. Her first and
only epoch of instability was retarded until she had nearly reached
maturity. The late appearance of the moon accounts for its large
relative size—through the increased cohesion of an already strongly
condensed parent mass—and for the distinctive peculiarities of its
history and influence on the producing globe.
Solar tidal friction, although it did not hinder the formation of
two minute dependents of Mars, has been invoked to explain the
anomalously rapid revolution of one of them. Phobos, we have
seen, completes more than three revolutions while Mars rotates once.
But this was probably not always so. The two periods were originally
nearly equal. The difference, it is alleged, was brought about
by tidal waves raised by the sun on the semi-fluid spheroid of Mars.
Rotatory velocity was thereby destroyed, the Martian day slowly
lengthened, and, as a secondary consequence, the period of the inner
satellite, become shorter than the augmented day, began progressively
to diminish. So that Phobos, unlike our moon, was in the
beginning farther from its primary than now.
But here again Mr. Nolan entered a caveat. Applying the simple
test of numerical evaluation, he showed that before solar tidal friction
could lengthen the rotation-period of Mars by so much as one
minute, Phobos should have been precipitated upon its surface.[1183]
For the enormous disparity of mass between it and the sun is so far
neutralised by the enormous disparity in their respective distances
from Mars that solar tidal force there is only fifty times that of the[Pg 321]
little satellite. But the tidal effects of a satellite circulating quicker
than its primary rotates exactly reverse those of one moving, like
our moon, comparatively slowly, so that the tides raised by Phobos
tend to shorten both periods. Its orbital momentum, however, is so
extremely small in proportion to the rotational momentum of Mars,
that any perceptible inroad upon the latter is attended by a lavish
and ruinous expenditure of the former. It is as if a man owning a
single five-pound note were to play for equal stakes with a man
possessing a million. The bankruptcy sure to ensue is typified by
the coming fate of the Martian inner satellite. The catastrophe of
its fall needs to bring it about only a very feeble reactive pull compared
with the friction which the sun should apply in order to
protract the Martian day by one minute. And from the proportionate
strength of the forces at work, it is quite certain that one
result cannot take place without the other. Nor can things have
been materially different in the past; hence the idea must be
abandoned that the primitive time of rotation of Mars survives in
the period of its inner satellite.
The anomalous shortness of the latter may, however, in M.
Wolf’s opinion,[1184] be explained by the “traînées elliptiques” with
which Roche supplemented nebular annulation.[1185] These are traced
back to the descent of separating strata from the shoulders of the
great nebulous spheroid towards its equatorial plane. Their rotational
velocity being thus relatively small, they formed “inner rings,”
very much nearer to the centre of condensation than would have
been possible on the unmodified theory of Laplace. Phobos might,
in this view, be called a polar offset of Mars; and the rings of
Saturn are thought to own a similar origin.
Outside the orbit of Mars, solar tidal friction can scarcely be said
to possess at present any sensible power. But it is far from certain
that this was always so. It seems not unlikely that its influence
was the overruling one in determining the direction of planetary
rotation. M. Faye, as we have seen, objected to Laplace’s scheme
that only retrograde secondary systems could be produced by it.
In this he was anticipated by Kirkwood, who, however, supplied an
answer to his own objection.[1186]
Sun-raised tides must have acted with great power on the
diffused masses of the embryo planets. By their means they
doubtless very soon came to turn (in lunar fashion) the same
hemisphere always towards their centre of motion. This amounts
to saying that even if they started with retrograde rotation, it was,[Pg 322]
by solar tidal friction, quickly rendered direct.[1187] For it is scarcely
necessary to point out that a planet turning an invariable face to
the sun rotates in the same direction in which it revolves, and in
the same period. As, with the progress of condensation, tides
became feebler and rotation more rapid, the accelerated spinning
necessarily proceeded in the sense thus prescribed for it. Hence
the backward axial movements of Uranus and Neptune may very
well be a survival, due to the inefficiency of solar tides at their great
distance, of a state of things originally prevailing universally
throughout the system.
The general outcome of Mr. Darwin’s researches has been to leave
Laplace’s cosmogony untouched. He concludes nothing against
it, and, what perhaps tells with more weight in the long run, has
nothing to substitute for it. In one form or the other, if we
speculate at all on the development of the planetary system, our
speculations are driven into conformity with the broad lines of the
Nebular Hypothesis—to the extent, at least, of admitting an original
material unity and motive uniformity. But we can see now, better
than formerly, that these supply a bare and imperfect sketch of the
truth. We should err gravely were we to suppose it possible to
reconstruct, with the help of any knowledge our race is ever likely
to possess, the real and complete history of our admirable system.
“The subtlety of nature,” Bacon says, “transcends in many ways
the subtlety of the intellect and senses of man.” By no mere
barren formula of evolution, indiscriminately applied all round, the
results we marvel at, and by a fragment of which our life is
conditioned, were brought forth; but by the manifold play of
interacting forces, variously modified and variously prevailing,
according to the local requirements of the design they were
appointed to execute.
FOOTNOTES:
[1150] Exposition du Système du Monde, t. ii., p. 295.
[1151] In later editions a retrospective clause was added admitting a prior condition
of all but evanescent nebulosity.
[1152] Méc. Cél., lib. xiv., ch. iii.
[1153] Beiträge zur Dynamik des Himmels, p. 12.
[1154] Trans. Roy. Soc. of Edinburgh, vol. xxi., p. 66.
[1155] Newcomb, Pop. Astr., p. 521 (2nd ed.).
[1156] M. Williams, Nature, vol. iii., p. 26.
[1157] Comp. Brit. Almanac, p. 94.
[1158] Radau, Bull. Astr., t. ii., p. 316.
[1159] Newcomb, Pop. Astr., pp. 521-525.
[1160] Proc. Roy. Soc., vol. xxxiii., p. 393.
[1161] To this hostile argument, as urged by Mr. E. Douglas Archibald, Sir W.
Siemens opposed the increase of rotative velocity through contraction (Nature,
vol. xxv., p. 505). But contraction cannot restore lost momentum.
[1162] Stellar Evolution, and its Relations to Geological Time, 1889.
[1163] Comptes Rendus, t. lii., p. 481. See also Kirkwood, Observatory, vol. iii.,
p. 409.
[1164] Fouché, Comptes Rendus, t. xcix., p. 903.
[1165] Astroph. Jour., vol. xiii., p. 338.
[1166] Month. Not., vol. xxix., p. 96.
[1167] Pop. Astr., p. 257.
[1168] Sur l’Origine du Monde, 1884.
[1169] Kirkwood adverted to it in 1864, Am. Jour., vol. xxxviii., p. 1.
[1170] Bull. Astr., t. ii.
[1171] Nature, vol. xxxi., p. 506.
[1172] Formation Mécanique du Système du Monde; Bull. Astr., t. xiv., p. 313
(O. Callandreau). See also, Le Problème Solaire, by l’Abbé Th. Moreux, 1900.
[1173] Phil. Trans., vol. clxxi., p. 713.
[1174] Mr. J. Nolan has pointed out (Nature, vol. xxxiv., p. 287) that the length of
the equal day and month will be reduced to about 1,240 hours by the effects of
solar tidal friction.
[1175] Phil. Trans., vol. clxxi., p. 835.
[1176] Nature, vol. xxxiii., p. 368; see also Nolan, Ibid., vol. xxxiv., p. 286.
[1177] Phil. Trans., vol. clxxviii., p. 422.
[1178] Ibid., vol. clxxii., p. 491.
[1179] Ibid., p. 530.
[1180] Satellite Evolution, Melbourne, 1895; Knowledge, vol. xviii., p. 205.
[1181] Phil. Trans., vol. clxxii., p. 533.
[1182] This was perceived by M. Ed. Roche in 1872. Mém. de l’Acad. des Sciences
de Montpellier, t. viii., p. 247.
[1183] Nature, vol. xxxiv., p. 287.
[1184] Bull. Astr., t. ii., p. 223.
[1185] Montpellier Méms., t. viii., p. 242.
[1186] Amer. Jour., vol. xxxviii. (1864), p. 1.
[1187] Wolf, Bull. Astr., t. ii., p. 76.
CHAPTER X
RECENT COMETS
On the 2nd of June, 1858, Giambattista Donati discovered at
Florence a feeble round nebulosity in the constellation Leo, about
one-tenth the diameter of the full moon. It proved to be a comet
approaching the sun. But it changed little in apparent place or
brightness for some weeks. The gradual development of a central
condensation of light was the first symptom of coming splendour.
At Harvard, in the middle of July, a strong stellar nucleus was
seen; on August 14 a tail began to be thrown out. As the comet
wanted still over six weeks of the time of its perihelion-passage, it
was obvious that great things might be expected of it. They did
not fail of realisation.
Not before the early days of September was it generally
recognised with the naked eye, though it had been detected without
a glass at Pulkowa, August 19. But its growth was thenceforward
surprisingly rapid, as it swept with accelerated motion under
the hindmost foot of the Great Bear, and past the starry locks of
Berenice. A sudden leap upward in lustre was noticed on September
12, when the nucleus shone with about the brightness of the
pole-star, and the tail, notwithstanding large foreshortening, could
be traced with the lowest telescopic power over six degrees of the
sphere. The appendage, however, attained its full development
only after perihelion, September 30, by which time, too, it lay
nearly square to the line of sight from the earth. On October 10 it
stretched in a magnificent scimitar-like curve over a third and
upwards of the visible hemisphere, representing a real extension in
space of fifty-four million miles. But the most striking view was
presented on October 5, when the brilliant star Arcturus became
involved in the brightest part of the tail, and during many hours
contributed, its lustre undiminished by the interposed nebulous
screen, to heighten the grandeur of the most majestic celestial object
of which living memories retain the impress. Donati’s comet was,[Pg 324]
according to Admiral Smyth’s testimony,[1188] outdone “as a mere sight-object”
by the great comet of 1811; but what it lacked in splendour,
it surely made up in grace, and variety of what we may call “scenic”
effects.
Some of these were no less interesting to the student than
impressive to the spectator. At Pulkowa, on the 16th September,
Winnecke,[1189] the first director of the Strasburg Observatory,
observed a faint outer envelope resembling a veil of almost evanescent
texture flung somewhat widely over the head. Next
evening, the first of the “secondary” tails appeared, possibly as part
of the same phenomenon. This was a narrow straight ray, forming
a tangent to the strong curve of the primary tail, and reaching to a
still greater distance from the nucleus. It continued faintly visible
for about three weeks, during part of which time it was seen in
duplicate. For from the chief train itself, at a point where its
curvature abruptly changed, issued, as if through the rejection of
some of its materials, a second beam nearly parallel to the first, the
rigid line of which contrasted singularly with the softly diffused and
waving aspect of the plume of light from which it sprang. Olbers’s
theory of unequal repulsive forces was never more beautifully
illustrated. The triple tail seemed a visible solar analysis of
cometary matter.
The processes of luminous emanation going on in this body
forcibly recalled the observations made on the comets of 1744 and
1835. From the middle of September, the nucleus, estimated by
Bond to be under five hundred miles in diameter, was the centre
of action of the most energetic kind. Seven distinct “envelopes”
were detached in succession from the nebulosity surrounding the
head, and after rising towards the sun during periods of from four
to seven days, finally cast their material backward to form the right
and left branches of the great train. The separation of these by an
obscure axis—apparently as black, quite close up to the nucleus, as
the sky—indicated for the tail a hollow, cone-like structure;[1190] while
the repetition of certain spots and rays in the same corresponding
situation on one envelope after another served to show that the
nucleus—to some local peculiarity of which they were doubtless due—had
no proper rotation, but merely shifted sufficiently on an axis
to preserve the same aspect towards the sun as it moved round it.[1191]
This observation of Bond’s was strongly confirmatory of Bessel’s
hypothesis of opposite polarities in such bodies’ opposite sides.
The protrusion towards the sun, on September 25, of a brilliant
luminous fan-shaped sector completed the resemblance to Halley’s[Pg 325]
comet. The appearance of the head was now somewhat that of
a “bat’s-wing” gaslight. There were, however, no oscillations to
and fro, such as Bessel had seen and speculated upon in 1835. As
the size of the nucleus contracted with approach to perihelion, its
intensity augmented. On October 2, it outshone Arcturus, and for
a week or ten days was a conspicuous object half an hour after
sunset. Its lustre—setting aside the light derived from the tail—was,
at that date, 6,300 times what it had been on June 15, though
theoretically—taking into account, that is, only the differences of
distance from sun and earth—it should have been only 1/33 of that
amount. Here, it might be thought, was convincing evidence of
the comet itself becoming ignited under the growing intensity of
the solar radiations. Yet experiments with the polariscope were
interpreted in an adverse sense, and Bond’s conclusion that the
comet sent us virtually unmixed reflected sunshine was generally
acquiesced in. It was, nevertheless, negatived by the first application
of the spectroscope to these bodies.
Very few comets have been so well or so long observed as
Donati’s. It was visible to the naked eye during 112 days; it was
telescopically discernible for 275, the last observation having been
made by Mr. William Mann at the Cape of Good Hope, March 4,
1859. Its course through the heavens combined singularly with
the orbital place of the earth to favour curious inspection. The
tail, when near its greatest development, lost next to nothing by
the effects of perspective, and at the same time lay in a plane
sufficiently inclined to the line of sight to enable it to display its
exquisite curves to the greatest advantage. Even the weather was,
on both sides of the Atlantic, propitious during the period of
greatest interest, and the moon as little troublesome as possible.
The volume compiled by the younger Bond is a monument to
the care and skill with which these advantages were turned to
account. Yet this stately apparition marked no turning-point in
the history of cometary science. By its study knowledge was
indeed materially advanced, but along the old lines. No quick and
vivid illumination broke upon its path. Quite insignificant objects—as
we have already partly seen—have often proved more vitally
instructive.
Donati’s comet has been identified with no other. Its path is an
immensely elongated ellipse, lying in a plane far apart from that of the
planetary movements, carrying it at perihelion considerably within
the orbit of Venus, and at aphelion out into space to 5-1/2 times the
distance from the sun of Neptune. The entire circuit occupies over
2,000 years, and is performed in a retrograde direction, or against the
order of the Signs. Before its next return, about the year 4000 A.D.,[Pg 326]
the enigma of its presence and its purpose may have been to some
extent—though we may be sure not completely—penetrated.
On June 30, 1861, the earth passed, for the second time in the
century, through the tail of a great comet. Some of our readers
may remember the unexpected disclosure, on the withdrawal of the
sun below the horizon on that evening, of an object so remarkable
as to challenge universal attention. A golden-yellow planetary disc,
wrapt in dense nebulosity, shone out while the June twilight of these
latitudes was still in its first strength. The number and complexity
of the envelopes surrounding the head produced, according to the
late Mr. Webb,[1192] a magnificent effect. Portions of six distinct emanations
were traceable. “It was as though a number of light, hazy
clouds were floating round a miniature full moon.” As the sky
darkened the tail emerged to view.[1193] Although in brightness and
sharpness of definition it could not compete with the display of 1858,
its dimensions proved to be extraordinary. It reached upwards
beyond the zenith when the head had already set. By some
authorities its extreme length was stated at 118°, and it showed no
trace of curvature. Most remarkable, however, was the appearance
of two widely divergent rays, each pointing towards the head, though
cut off from it by sky-illumination, of which one was seen by Mr.
Webb, and both by Mr. Williams at Liverpool, a quarter of an hour
before midnight. There seems no doubt that Webb’s interpretation
was the true one, and that these beams were, in fact, “the perspective
representation of a conical or cylindrical tail, hanging closely
above our heads, and probably just being lifted up out of our atmosphere.”[1194]
The cometary train was then rapidly receding from the
earth, so that the sides of the “outspread fan” of light shown by it
when we were right in the line of its axis must have appeared (as
they did) to close up in departure. The swiftness with which the
visually opened fan shut proved its vicinity; and, indeed, Mr. Hind’s
calculations showed that we were not so much near as actually within
its folds at that very time.
Already M. Liais, from his observations at Rio de Janeiro, June
11 to 14, and Mr. Tebbutt, by whom the comet was discovered in
New South Wales on May 13, had anticipated such an encounter,
while the former subsequently proved that it must have occurred in
such a way as to cause an immersion of the earth in cometary matter
to a depth of 300,000 miles.[1195] The comet then lay between the earth
and the sun at a distance of about fourteen million miles from the
former; its tail stretched outward just along the line of intersection[Pg 327]
of its own with the terrestrial orbit to an extent of fifteen
million miles; so that our globe, happening to pass at the time,
found itself during some hours involved in the flimsy appendage.
No perceptible effects were produced by the meeting; it was
known to have occurred by theory alone. A peculiar glare in the
sky, thought by some to have distinguished the evening of June 30,
was, at best, inconspicuous. Nor were there any symptoms of
unusual electric excitement. The Greenwich instruments were,
indeed, disturbed on the following night, but it would be rash to
infer that the comet had art or part in their agitation.
The perihelion-passage of this body occurred June 11, 1861; and
its orbit has been shown by M. Kreutz of Bonn, from a very complete
investigation founded on observations extending over nearly
a year, to be an ellipse traversed in a period of 409 years.[1196]
Towards the end of August, 1862, a comet became visible to the
naked eye high up in the northern hemisphere, with a nucleus
equalling in brightness the lesser stars of the Plough and a feeble
tail 20° in length. It thus occupied quite a secondary position
among the members of its class. It was, nevertheless, a splendid
object in comparison with a telescopic nebulosity discovered by
Tempel at Marseilles, December 19, 1865. This, the sole comet of
1866, slipped past perihelion, January 11, without pomp of train or
other appendages, and might have seemed hardly worth the trouble
of pursuing. Fortunately, this was not the view entertained by
observers and computers; since upon the knowledge acquired of
the movements of these two bodies has been founded one of the
most significant discoveries of modern times. The first of them is
now styled the comet (1862 iii.) of the August meteors, the second
(1866 i.) that of the November meteors. The steps by which this
curious connection came to be ascertained were many, and were
taken in succession by a number of individuals. But the final result
was reached by Schiaparelli of Milan, and remains deservedly
associated with his name.
The idea prevalent in the eighteenth century as to the nature of
shooting stars was that they were mere aerial ignes fatui—inflammable
vapours accidentally kindled in our atmosphere. But Halley had
already entertained the opinion of their cosmical origin; and Chladni
in 1794 formally broached the theory that space is filled with minute
circulating atoms, which, drawn by the earth’s attraction, and
ignited by friction in its gaseous envelope, produce the luminous
effects so frequently witnessed.[1197] Acting on his suggestion, Brandes[Pg 328]
and Benzenberg, two students at the University of Göttingen,
began in 1798 to determine the heights of falling stars by simultaneous
observations at a distance. They soon found that they
move with planetary velocities in the most elevated regions of our
atmosphere, and by the ascertainment of this fact laid a foundation
of distinct knowledge regarding them. Some of the data collected,
however, served only to perplex opinion, and even caused Chladni
temporarily to renounce his. Many high authorities, headed by
Laplace in 1802, declared for the lunar-volcanic origin of meteorites;
but thought on the subject was turbid, and inquiry seemed only to
stir up the mud of ignorance. It needed one of those amazing
spectacles, at which man assists, no longer in abject terror for his
own frail fortunes, but with keen curiosity and the vivid expectation
of new knowledge, to bring about a clarification.
On the night of November 12-13, 1833, a tempest of falling stars
broke over the earth. North America bore the brunt of its pelting.
From the Gulf of Mexico to Halifax, until daylight with some
difficulty put an end to the display, the sky was scored in every
direction with shining tracks and illuminated with majestic fireballs.
At Boston the frequency of meteors was estimated to be about half
that of flakes of snow in an average snowstorm. Their numbers,
while the first fury of their coming lasted, were quite beyond counting;
but as it waned, a reckoning was attempted, from which it
was computed, on the basis of that much diminished rate, that
240,000 must have been visible during the nine hours they continued
to fall.[1198]
Now there was one very remarkable feature common to the
innumerable small bodies which traversed, or were consumed in our
atmosphere that night. They all seemed to come from the same part
of the sky. Traced backward, their paths were invariably found to
converge to a point in the constellation Leo. Moreover, that point
travelled with the stars in their nightly round. In other words, it
was entirely independent of the earth and its rotation. It was a
point in inter-planetary space.
The effective perception of this fact[1199] amounted to a discovery, as
Olmsted and Twining, who had “simultaneous ideas” on the subject,
were the first to realize. Denison Olmsted was then Professor of
Mathematics in Yale College. He showed early in 1834[1200] that the
emanation of the showering meteors from a fixed “radiant” proved[Pg 329]
their approach to the earth along nearly parallel lines, appearing to
diverge by an effect of perspective; and that those parallel lines
must be sections of orbits described by them round the sun and
intersecting that of the earth. For the November phenomenon was
now seen to be a periodical one. On the same night of the year
1832, although with less dazzling and universal splendour than in
America in 1833, it had been witnessed over great part of Europe
and in Arabia. Olmsted accordingly assigned to the cloud of
cosmical particles (or “comet,” as he chose to call it), by terrestrial
encounters with which he supposed the appearances in question to
be produced, a period of about 182 days; its path a narrow ellipse,
meeting, near its farthest end from the sun, the place occupied by
the earth on November 12.
Once for all, then, as the result of the star-fall of 1833, the study
of luminous meteors became an integral part of astronomy. Their
membership of the solar system was no longer a theory or a conjecture—it
was an established fact. The discovery might be compared
to, if it did not transcend in importance, that of the asteroidal
group. “C’est un nouveau monde planétaire,” Arago wrote,[1201] “qui
commence à se révéler à nous.”
Evidences of periodicity continued to accumulate. It was remembered
that Humboldt and Bonpland had been the spectators at
Cumana, after midnight on November 12, 1799, of a fiery shower
little inferior to that of 1833, and reported to have been visible from
the equator to Greenland. Moreover, in 1834 and some subsequent
years, there were waning repetitions of the display, as if through
the gradual thinning-out of the meteoric supply. The extreme
irregularity of its distribution was noted by Olbers in 1837, who
conjectured that we might have to wait until 1867 to see the phenomenon
renewed on its former scale of magnificence.[1202] This was the
first hint of a thirty-three or thirty-four year period.
The falling stars of November did not alone attract the attention
of the learned. Similar appearances were traditionally associated
with August 10 by the popular phrase in which they figured as “the
tears of St. Lawrence.” But the association could not be taken on trust
from mediæval authority. It had to be proved scientifically, and
this Quetelet of Brussels succeeded in doing in December, 1836.[1203]
A second meteoric revolving system was thus shown to exist.
But its establishment was at once perceived to be fatal to the
“cosmical cloud” hypothesis of Olmsted. For if it be a violation
of probability to attribute to one such agglomeration a period of an
exact year, or sub-multiple of a year, it would be plainly absurd to[Pg 330]
suppose the movements of two or more regulated by such highly
artificial conditions. An alternative was proposed by Adolf Erman
of Berlin in 1839.[1204] No longer in clouds, but in closed rings, he
supposed meteoric matter to revolve round the sun. Thus the mere
circumstance of intersection by a meteoric of the terrestrial orbit,
without any coincidence of period, would account for the earth
meeting some members of the system at each annual passage through
the “node” or point of intersection. This was an important step
in advance, yet it decided nothing as to the forms of the orbits of
such annular assemblages; nor was it followed up in any direction
for a quarter of a century.
Hubert A. Newton took up, in 1864,[1205] the dropped thread of
inquiry. The son of a mathematical mother, he attained, at the
age of twenty-five, to the dignity of Professor of Mathematics in
Yale University, and occupied the post until his death in 1896.
The diversion of his powers, however, from purely abstract studies
stimulated their effective exercise, and constituted him one of the
founders of meteoric astronomy.
A search through old records carried the November phenomenon
back to the year 902 A.D., long distinguished as “the year of the
stars.” For in the same night in which Taormina was captured by
the Saracens, and the cruel Aghlabite tyrant Ibrahim ibn Ahmed
died “by the judgment of God” before Cosenza, stars fell from
heaven in such abundance as to amaze and terrify beholders far
and near. This was on October 13, and recurrences were traced
down through the subsequent centuries, always with a day’s delay
in about seventy years. It was easy, too, to derive from the dates
a cycle of 33-1/4 years, so that Professor Newton did not hesitate to
predict the exhibition of an unusually striking meteoric spectacle on
November 13-14, 1866.[1206]
For the astronomical explanation of the phenomena, recourse
was had to a method introduced by Erman of computing meteoric
orbits. It was found, however, that conspicuous recurrences every
thirty-three or thirty-four years could be explained on the supposition
of five widely different periods, combined with varying degrees
of extension in the revolving group. Professor Newton himself
gave the preference to the shortest—of 354-1/2 days, but indicated
the means of deciding with certainty upon the true one. It was
furnished by the advancing motion of the node, or that day’s
delay of the November shower every seventy years, which the old
chronicles had supplied data for detecting. For this is a strictly[Pg 331]
measurable effect of gravitational disturbance by the various planets,
the amount of which naturally depends upon the course pursued by
the disturbed bodies. Here the great mathematical resources of
Professor Adams were brought to bear. By laborious processes of
calculation, he ascertained that four out of Newton’s five possible
periods were entirely incompatible with the observed nodal displacement,
while for the fifth—that of 33-1/4 years—a perfectly harmonious
result was obtained.[1207] This was the last link in the chain of evidence
proving that the November meteors—or “Leonids,” as they had by
that time come to be called—revolve round the sun in a period of
33·27 years, in an ellipse spanning the vast gulf between the orbits
of the earth and Uranus, the group being so extended as to occupy
nearly three years in defiling past the scene of terrestrial encounters.
But before it was completed in March, 1867, the subject had assumed
a new aspect and importance.
Professor Newton’s prediction of a remarkable star-shower in
November, 1866, was punctually fulfilled. This time, Europe served
as the main target of the celestial projectiles, and observers were
numerous and forewarned. The display, although, according to
Mr. Baxendell’s memory,[1208] inferior to that of 1833, was of extraordinary
impressiveness. Dense crowds of meteors, equal in lustre
to the brightest stars, and some rivalling Venus at her best,[1209] darted
from east to west across the sky with enormous apparent velocities,
and with a certain determinateness of aim, as if let fly with a
purpose, and at some definite object.[1210] Nearly all left behind them
trains of emerald green or clear blue light, which occasionally lasted
many minutes, before they shrivelled and curled up out of sight.
The maximum rush occurred a little after one o’clock on the morning
of November 14, when attempts to count were overpowered by
frequency. But during a previous interval of seven minutes five
seconds, four observers at Mr. Bishop’s observatory at Twickenham
reckoned 514, and during an hour 1,120.[1211] Before daylight the earth
had fairly cut her way through the star-bearing stratum; the
“ethereal rockets” had ceased to fly.
This event brought the subject of shooting stars once more vividly
to the notice of astronomers. Schiaparelli had, indeed, been already
attracted by it. The results of his studies were made known in
four remarkable letters, addressed, before the close of the year
1866, to Father Secchi, and published in the Bulletino of the Roman
Observatory.[1212] Their upshot was to show, in the first place, that[Pg 332]
meteors possess a real velocity considerably greater than that of the
earth, and travel, accordingly, to enormously greater distances from
the sun along tracks resembling those of comets in being very
eccentric, in lying at all levels indifferently, and in being pursued in
either direction. It was next inferred that comets and meteors
equally have an origin foreign to the solar system, but are drawn
into it temporarily by the sun’s attraction, and occasionally fixed
in it by the backward pull of some planet. But the crowning fact
was reserved for the last. It was the astonishing one that the
August meteors move in the same orbit with the bright comet of
1862—that the comet, in fact, is but a larger member of the family
named “Perseids” because their radiant point is situated in the
constellation Perseus.
This discovery was quickly capped by others of the same kind.
Leverrier published, January 21, 1867,[1213] elements for the November
swarm, founded on the most recent and authentic observations; at
once identified by Dr. C. F. W. Peters of Altona with Oppolzer’s
elements for Tempel’s comet of 1866.[1214] A few days later, Schiaparelli,
having recalculated the orbit of the meteors from improved data,
arrived at the same conclusion; while Professor Weiss of Vienna
pointed to the agreement between the orbits of a comet which had
appeared in 1861 and of a star-shower found to recur on April 20
(Lyraïds), as well as between those of Biela’s comet and certain
conspicuous meteors of November 28.[1215]
These instances do not seem to be exceptional. The number of
known or suspected accordances of cometary tracks with meteor
streams contained in a list drawn up in 1878[1216] by Professor Alexander
S. Herschel (who has made the subject peculiarly his own) amounts
to seventy-six; although the four first detected still remain the
most conspicuous, and perhaps the only absolutely sure examples
of a relation as significant as it was, to most astronomers, unexpected.
There had, indeed, been anticipatory ideas. Not that Kepler’s
comparison of shooting stars to “minute comets,” or Maskelyne’s
“forse risulterà che essi sono comete,” in a letter to the Abate
Cesaris, December 12, 1774,[1217] need count for much. But Chladni,
in 1819,[1218] considered both to be fragments or particles of the same
primitive matter, irregularly scattered through space as nebulæ;
and Morstadt of Prague suggested about 1837[1219] that the meteors of[Pg 333]
November might be dispersed atoms from the tail of Biela’s comet,
the path of which is cut across by the earth near that epoch.
Professor Kirkwood, however, by a luminous intuition, penetrated
the whole secret, so far as it has yet been made known. In an
article published, or rather buried, in the Danville Quarterly Review
for December, 1861, he argued, from the observed division of Biela,
and other less noted instances of the same kind, that the sun
exercises a “divellent influence” on the nuclei of comets, which
may be presumed to continue its action until their corporate
existence (so to speak) ends in complete pulverisation. “May not,”
he continued, “our periodic meteors be the débris of ancient but
now disintegrated comets, whose matter has become distributed
round their orbits?”[1220]
The gist of Schiaparelli’s discovery could not be more clearly
conveyed. For it must be borne in mind that with the ultimate
destiny of comets’ tails this had nothing to do. The tenuous
matter composing them is, no doubt, permanently lost to the body
from which it emanated; but science does not pretend to track its
further wanderings through space. It can, however, state categorically
that these will no longer be conducted along the paths forsaken
under solar compulsion. From the central, and probably solid parts
of comets, on the other hand, are derived the granules by the swift
passage of which our skies are seamed with periodic fires. It is
certain that a loosely agglomerated mass (such as cometary nuclei
most likely are) must gradually separate through the unequal
action of gravity on its various parts—through, in short, solar tidal
influence. Thenceforward its fragments will revolve independently
in parallel orbits, at first as a swarm, finally—when time has been
given for the full effects of the lagging of the slower moving particles
to develop—as a closed ring. The first condition is still, more or
less, that of the November meteors; those of August have already
arrived at the second. For this reason, Leverrier pronounced,
in 1867, the Perseid to be of older formation than the Leonid
system. He even assigned a date at which the introduction of the
last-named bodies into their present orbit was probably effected
through the influence of Uranus. In 126 A.D. a close approach
must have taken place between the planet and the parent comet of
the November stars, after which its regular returns to perihelion,
and the consequent process of its disintegration, set in. Though
not complete, it is already far advanced.
The view that meteorites are the dust of decaying comets was now
to be put to a definite test of prediction. Biela’s comet had not
been seen since its duplicate return in 1852. Yet it had been carefully[Pg 334]
watched for with the best telescopes; its path was accurately
known; every perturbation it could suffer was scrupulously taken
into account. Under these circumstances, its repeated failure to
come up to time might fairly be thought to imply a cessation from
visible existence. Might it not, however, be possible that it would
appear under another form—that a star-shower might have sprung
from and would commemorate its dissolution?
An unusually large number of falling stars were seen by Brandes,
December 6, 1798. Similar displays were noticed in the years
1830, 1838, and 1847, and the point from which they emanated was
shown by Heis at Aix-la-Chapelle to be situated near the bright star
γ Andromedæ.[1221] Now this is precisely the direction in which the
orbit of Biela’s comet would seem to lie, as it runs down to cut the
terrestrial track very near the place of the earth at the above dates.
The inference was, then, an easy one, that the meteors were pursuing
the same path with the comet; and it was separately arrived at,
early in 1867, by Weiss, D’Arrest, and Galle.[1222] But Biela travels in
the opposite direction to Tempel’s comet and its attendant
“Leonids”; its motion is direct, or from west to east, while theirs is
retrograde. Consequently, the motion of its node is in the opposite
direction too. In other words, the meeting-place of its orbit with
that of the earth retreats (and very rapidly) along the ecliptic instead
of advancing. So that if the “Andromedes” stood in the supposed
intimate relation to Biela’s comet, they might be expected to anticipate
the times of their recurrence by as much as a week in half a
century. All doubt as to the fact may be said to have been
removed by Signor Zezioli’s observation of the annual shower in
more than usual abundance at Bergamo, November 30, 1867.
The missing comet was next due at perihelion in the year 1872,
and the probability was contemplated by both Weiss and Galle of its
being replaced by a copious discharge of falling stars. The precise
date of the occurrence was not easily determinable, but Galle thought
the chances in favour of November 28. The event anticipated the
prediction by twenty-four hours. Scarcely had the sun set in Western
Europe on November 27, when it became evident that Biela’s comet
was shedding over us the pulverised products of its disintegration.
The meteors came in volleys from the foot of the Chained Lady,
their numbers at times baffling the attempt to keep a reckoning.
At Moncalieri, about 8 p.m., they constituted (as Father Denza said[1223])
a “real rain of fire.” Four observers counted, on an average, four
hundred each minute and a half; and not a few fireballs, equalling
the moon in diameter, traversed the sky. On the whole, however,[Pg 335]
the stars of 1872, though about equally numerous, were less brilliant
than those of 1866; the phosphorescent tracks marking their passage
were comparatively evanescent and their movements sluggish. This
is easily understood when we remember that the Andromedes overtake
the earth, while the Leonids rush to meet it; the velocity of
encounter for the first class of bodies being under twelve, for the
second above forty-four miles a second. The spectacle was, nevertheless,
magnificent. It presented itself successively to various parts
of the earth, from Bombay and the Mauritius to New Brunswick
and Venezuela, and was most diligently and extensively observed.
Here it had well-nigh terminated by midnight.[1224]
It was attended by a slight aurora, and although Tacchini had
telegraphed that the state of the sun rendered some show of polar
lights probable, it has too often figured as an accompaniment of star-showers
to permit the coincidence to rank as fortuitous. Admiral
Wrangel was accustomed to describe how, during the prevalence of
an aurora on the Siberian coast, the passage of a meteor never failed
to extend the luminosity to parts of the sky previously dark;[1225] and
an enhancement of electrical disturbance may well be associated with
the flittings of such cosmical atoms.
A singular incident connected with the meteors of 1872 has now
to be recounted. The late Professor Klinkerfues, who had observed
them very completely at Göttingen, was led to believe that not
merely the débris strewn along its path, but the comet itself must
have been in immediate proximity to the earth during their appearance.[1226]
If so, it might be possible, he thought, to descry it as it retreated
in the diametrically opposite direction from that in which it
had approached. On November 30, accordingly, he telegraphed to
Mr. Pogson, the Madras astronomer, “Biela touched earth November
27; search near Theta Centauri”—the “anti-radiant,” as it is called,
being situated close to that star. Bad weather prohibited observation
during thirty-six hours, but when the rain clouds broke on the
morning of December 2, there a comet was, just in the indicated
position. In appearance it might have passed well enough for one
of the Biela twins. It had no tail, but a decided nucleus, and was
about 45 seconds across, being thus altogether below the range of
naked-eye discernment. It was again observed December 3, when a
short tail was perceptible; but overcast skies supervened, and it has
never since been seen. Its identity accordingly remains in doubt.
It seems tolerably certain, however, that it was not the lost comet,
which ought to have passed that spot twelve weeks earlier, and was[Pg 336]
subject to no conceivable disturbance capable of delaying to that
extent its revolution. On the other hand, there is the strongest
likelihood that it belonged to the same system[1227]—that it was a third
fragment, torn from the parent-body of the Andromedes at a period
anterior to our first observations of it.
In thirteen years Biela’s comet (or its relics) travels nearly twice
round its orbit, so that a renewal of the meteoric shower of 1872
was looked for on the same day of the year 1885, the probability
being emphasised by an admonitory circular from Dunecht.
Astronomers were accordingly on the alert, and were not disappointed.
In England, observation was partially impeded by
clouds; but at Malta, Palermo, Beyrout, and other southern
stations, the scene was most striking. The meteors were both
larger and more numerous than in 1872. Their numbers in the
densest part of the drift were estimated by Professor Newton at
75,000 per hour, visible from one spot to so large a group of
spectators that practically none could be missed. Yet each of these
multitudinous little bodies was found by him to travel in a clear
cubical space of which the edge measured twenty miles![1228] Thus the
dazzling effect of a luminous throng was produced without jostling
or overcrowding, by particles, it might almost be said, isolated in
the void.
Their aspect was strongly characteristic of the Andromede family
of meteors. “They invariably,” Mr. Denning wrote,[1229] “traversed
short paths with very slow motions, and became extinct in evolved
streams of yellowish sparks.” The conclusion seemed obvious “that
these meteors are formed of very soft materials, which expand while
incalescent, and are immediately crumbled and dissipated into
exiguous dust.”
The Biela meteors of 1885 did not merely gratify astronomers
with a fulfilled prediction, but were the means of communicating to
them some valuable information. Although their main body was
cut through by the moving earth in six hours, and was not more
than 100,000 miles across, skirmishers were thrown out to nearly
a million miles on either side of the compact central battalions.
Members of the system were, on the 26th of November, recorded
by Mr. Denning at the hourly rate of about 130; and they did not
wholly cease to be visible until December 1. They afforded besides
a particularly well-marked example of that diffuseness of radiation
previously observed in some less conspicuous displays. Their paths
seemed to diverge from an area rather than from a point in the sky.
They came so ill to focus that divergences of several degrees were[Pg 337]
found between the most authentically determined radiants. These
incongruities are attributed by Professor Newton to the irregular
shape of the meteoroids producing unsymmetrical resistance from
the air, and hence causing them to glance from their original
direction on entering it. Thus, their luminous tracks did not
always represent (even apart from the effects of the earth’s attraction)
the true prolongation of their course through space.
The Andromedes of 1872 were laggards behind the comet from
which they sprang; those of 1885 were its avant-couriers. That
wasted and disrupted body was not due at the node until January 26,
1886, sixty days, that is, after the earth’s encounter with its
meteoric fragments. These are now probably scattered over more
than five hundred million miles of its orbits;[1230] yet Professor Newton
considers that all must have formed one compact group with Biela
at the time of its close approach to Jupiter about the middle of 1841.
For otherwise both comet and meteorites could not have experienced,
as they seem to have done, the same kind and amount of
disturbance. The rapidity of cometary disintegration is thus
curiously illustrated.
A short-lived persuasion that the missing heavenly body itself had
been recovered, was created by Mr. Edwin Holms’s discovery, at
London, November 6, 1892, of a tolerably bright, tailless comet, just
in a spot which Biela’s comet must have traversed in approaching
the intersection of its orbit with that of the earth. A hasty
calculation by Berberich assigned elements to the newcomer
seeming not only to ratify the identity, but to promise a quasi-encounter
with the earth on November 21. The only effect of the
prediction, however, was to raise a panic among the negroes of the
Southern States of America. The comet quietly ignored it, and
moved away from instead of towards the appointed meeting-place.
Its projection, then, on the night of its discovery, upon a point of
the Biela-orbit was by a mere caprice of chance. North America,
nevertheless, was visited on November 23 by a genuine Andromede
shower. Although the meteors were less numerous than in 1885,
Professor Young estimated that 30,000, at the least, of their orange
fire-streaks came, during five hours, within the range of view at
Princeton.[1231] Brédikhine estimated the width of the space containing
them at about 2,700,000 miles.[1232] The anticipation of their due
time by four days implied—if they were a prolongation of the main
Biela group, the nucleus of which passed the spot of encounter five[Pg 338]
months previously—a recession of the node since 1885 by no less
than three degrees. Unless, indeed, Mr. Denning were right in
supposing the display to have proceeded from “an associated branch
of the main swarm through which we passed in 1872 and 1885.”[1233]
The existence of separated detachments of Biela meteors, due to
disturbing planetary action, was contemplated as highly probable
by Schiaparelli.[1234] Such may have been the belated flights met with
in 1830, 1838, 1841, and 1847, and such the advance flight plunged
through in 1892. A shower looked for November 23, 1899, did not
fall, and no further display from this quarter is probable until
November 17, 1905, although one is possible a year earlier.[1235]
The Leonids, through the adverse influence of Jupiter and Saturn,
inflicted upon multitudes of eager watchers a still more poignant
disappointment. A dense part of the swarm, having nearly completed
a revolution since 1866, should, travelling normally, have
met the earth November 15, 1899; in point of fact, it swerved sunward,
and the millions of meteorites which would otherwise have
been sacrificed for the illumination of our skies escaped a fiery
doom. The contingency had been forecast in the able calculations
of Dr. Johnstone Stoney and Dr. A. M. W. Downing,[1236] superintendent
of the Nautical Almanac Office; but the verification
scarcely compensated the failure. Nor was the situation retrieved
in the following years. Only ragged fringes of the great tempest-cloud
here and there touched our globe. As the same investigators
warned us to expect, the course of the meteorites had been not
only rendered sinuous by perturbation, but also broken and
irregular. We can no longer count upon the Leonids. Their
glory, for scenic purposes, is departed. The comet associated with
them also evaded observation. Although it doubtless kept its
tryst with the sun in the spring of 1899, the attendant circumstances
were too unfavourable to allow it to be seen from the earth.[1237]
By an almost fantastic coincidence, nevertheless, a faint comet was
photographed, November 14, 1898,[1238] by Dr. Chase, of the Yale
College Observatory, close to the Leonid radiant, whither a
“meteorograph” was directed with a view to recording trails left
by precursors of the main Leonid body. A promising start, too,
was made on the same occasion with meteoric researches from[Pg 339]
sensitive plates.[1239] Indeed, Schaeberle and Colton[1240] had already, in
1896, determined the height of a Leonid by means of photographs
taken at stations on different ridges of Mount Hamilton; and
Professor Pickering has prosecuted similar work at Harvard, with
encouraging results. Everything in this branch of science depends
upon how far they can be carried. Without the meteorograph,
rigid accuracy in the observation of shooting stars is unattainable,
and rigid accuracy is the sine quâ non for obtaining exact knowledge.
Biela does not offer the only example of cometary disruption.
Setting aside the unauthentic reports of early chroniclers, we
meet the “double comet” discovered by Liais at Olinda (Brazil),
February 27, 1860, of which the division appeared recent, and
about to be carried farther.[1241] But a division once established,
separation must continually progress. The periodic times of the
fragments will never be identical; one must drop a little behind the
other at each revolution, until at length they come to travel in
remote parts of nearly the same orbit. Thus the comet predicted
by Klinkerfues and discovered by Pogson had already lagged to the
extent of twelve weeks, and we shall meet instances farther on
where the retardation is counted, not by weeks, but by years.
Here original identity emerges only from calculation and comparison
of orbits.
Comets, then, die, as Kepler wrote long ago, sicut bombyces filo
fundendo. This certainty, anticipated by Kirkwood in 1861, we
have at least acquired from the discovery of their generative connection
with meteors. Nay, their actual materials become, in
smaller or larger proportions, incorporated with our globe. It is
not, indeed, universally admitted that the ponderous masses of which,
according to Daubrée’s estimate,[1242] at least 600 fall annually from
space upon the earth, ever formed part of the bodies known to us as
comets. Some follow Tschermak in attributing to aerolites a totally
different origin from that of periodical shooting-stars. That no
clear line of demarcation can be drawn is no valid reason for asserting
that no real distinction exists; and it is certainly remarkable
that a meteoric fusillade may be kept up for hours without a single
solid projectile reaching its destination. It would seem as if the
celestial army had been supplied with blank cartridges. Yet, since
a few detonating meteors have been found to proceed from ascertained
radiants of shooting-stars, it is difficult to suppose that any
generic difference separates them.
Their assimilation is further urged—though not with any demonstrative[Pg 340]
force—by two instances, the only two on record, of the
tangible descent of an aerolite during the progress of a star-shower.
On April 4, 1095, the Saxon Chronicle informs us that stars fell “so
thickly that no man could count them,” and adds that one of them
having struck the ground in France, a bystander “cast water upon
it, which was raised in steam with a great noise of boiling.”[1243] And
again, on November 27, 1885, while the skirts of the Andromede-tempest
were trailing over Mexico, “a ball of fire” was precipitated
from the sky at Mazapil, within view of a ranchman.[1244] Scientific
examination proved it to be a “siderite,” or mass of “nickel-iron”;
its weight exceeded eight pounds, and it contained many nodules of
graphite. We are not, however, authorised by the circumstances of
its arrival to regard the Mazapil fragment of cosmic metal as a
specimen torn from Biela’s comet. In this, as in the preceding case,
the coincidence of the fall with the shower may have been purely
casual, since no hint is given of any sort of agreement between the
tracks followed by the sample provided for curious study, and the
swarming meteors consumed in the upper air.
Professor Newton’s inquiries into the tracks pursued by meteorites
previous to their collisions with the earth tend to distinguish them,
at least specifically, from shooting-stars. He found that nearly all
had been travelling with a direct movement in orbits the perihelia
of which lay in the outer half of the space separating the earth from
the sun.[1245] Shooting-stars, on the contrary, are entirely exempt from
such limitations. The Yale Professor concluded “that the larger
meteorites moving in our solar system are allied much more closely
with the group of comets of short period than with the comets
whose orbits are nearly parabolic.” They would thus seem to be
more at home than might have been expected amid the planetary
family. Father Carbonelle has, moreover, shown[1246] that meteorites, if
explosion-products of the earth or moon, should, with rare exceptions,
follow just the kind of paths assigned to them, from data of
observation, by Professor Newton. Yet it is altogether improbable
that projectiles from terrestrial volcanoes should, at any geological
epoch, have received impulses powerful enough to enable them,
not only to surmount the earth’s gravity, but to penetrate its atmosphere.
A striking—indeed, an almost startling—peculiarity, on the other
hand, divides from their congeners a class of meteors identified by[Pg 341]
Mr. Denning during ten years’ patient watching of such phenomena
at Bristol.[1247] These are described as “meteors with stationary
radiants,” since for months together they seem to come from the
same fixed points in the sky. Now this implies quite a portentous
velocity. The direction of meteor-radiants is affected by a kind of
aberration, analogous to the aberration of light. It results from a
composition of terrestrial with meteoric motion. Hence, unless that
of the earth in its orbit be by comparison insignificant, the visual
line of encounter must shift, if not perceptibly from day to day, at
any rate conspicuously from month to month. The fixity, then, of
many systems observed by Mr. Denning seems to demand the
admission that their members travel so fast as to throw the earth’s
movement completely out of the account. The required velocity
would be, by Mr. Ranyard’s calculation, at least 880 miles a second.[1248]
But the aspect of the meteors justifies no such extravagant assumption.
Their seeming swiftness is very various, and—what is highly
significant—it is notably less when they pursue than when they
meet the earth. Yet the “incredible and unaccountable”[1249] fact of
the existence of these “long radiants,” although doubted by
Tisserand[1250] because of its theoretical refractoriness, must apparently
be admitted. The first plausible explanation of them was offered
by Professor Turner in 1899.[1251] They represent, in his view, the
cumulative effects of the earth’s attraction. The validity of his
reasoning is, however, denied by M. Brédikhine,[1252] who prefers to
regard them as a congeries of separate streams. The enigma they
present has evidently not yet received its definitive solution.
The Perseids afford, on the contrary, a remarkable instance of a
“shifting radiant.” Mr. Denning’s observations of these yellowish,
leisurely meteors extend over nearly six weeks, from July 8 to
August 16; the point of radiation meantime progressing no less
than 57° in right ascension. Doubts as to their common origin
were hence freely expressed, especially by Mr. Monck of Dublin.[1253]
But the late Dr. Kleiber[1254] showed, by strict geometrical reasoning,
that the forty-nine radiants successively determined for the shower
were all, in fact, comprised within one narrowly limited region of
space. In other words, the application of the proper correction for
the terrestrial movement, and the effects of attraction by which each
individual shooting-star is compelled to describe a hyperbola round
the earth’s centre, reduces the extended line of radiants to a compact[Pg 342]
group, with the cometary radiant for its central point; the cometary
radiant being the spot in the sky met by a tangent to the orbit of
the Perseid comet of 1862 at its intersection with the orbit of the
earth. The reality of the connection between the comet and the
meteors could scarcely be more clearly proved; while the vast
dimensions of the stream into which the latter are found to be
diffused cannot but excite astonishment not unmixed with perplexity.
The first successful application of the spectroscope to comets was
by Donati in 1864.[1255] A comet discovered by Tempel, July 4,
brightened until it appeared like a star somewhat below the second
magnitude, with a feeble tail 30° in length. It was remarkable as
having, on August 7, almost totally eclipsed a small star—a very
rare occurrence.[1256] On August 5 Donati admitted its light through
his train of prisms, and found it, thus analysed, to consist of three
bright bands—yellow, green, and blue—separated by wider dark
intervals. This implied a good deal. Comets had previously been
considered, as we have seen, to shine mainly, if not wholly, by
reflected sunlight. They were now perceived to be self-luminous,
and to be formed, to a large extent, of glowing gas. The next step
was to determine what kind of gas it was that was thus glowing in
them; and this was taken by Sir William Huggins in 1868.[1257]
A comet of subordinate brilliancy, known as comet 1868 ii., or
sometimes as Winnecke’s, was the subject of his experiment. On
comparing its spectrum with that of an olefiant-gas “vacuum tube”
rendered luminous by electricity, he found the agreement exact. It
has since been abundantly confirmed. All the eighteen comets
tested by light analysis, between 1868 and 1880, showed the typical
hydro-carbon spectrum[1258] common to the whole group of those compounds,
but probably due immediately to the presence of acetylene.
Some minor deviations from the laboratory pattern, in the shifting
of the maxima of light from the edge towards the middle of the
yellow and blue bands, have been experimentally reproduced by
Vogel and Hasselberg in tubes containing a mixture of carbonic
oxide with olefiant gas.[1259] Their illumination by disruptive electric
discharges was, however, a condition sine quâ non for the exhibition
of the cometary type of spectrum. When a continuous current was
Great Comet.
Photographed, May 5, 1901, with the thirteen-inch Astrographic Refractor of the
Royal Observatory, Cape of Good Hope.
[Pg 343]
employed, the carbonic oxide bands asserted themselves to the
exclusion of the hydro-carbons. The distinction has great significance
as regards the nature of comets. Of particular interest in this
connection is the circumstance that carbonic oxide is one of the
gases evolved by meteoric stones and irons under stress of heat.[1260]
For it must apparently have formed part of an aeriform mass in
which they were immersed at an earlier stage of their history.
In a few exceptional comets the usual carbon-bands have been
missed. Two such were observed by Sir William Huggins in 1866
and 1867 respectively.[1261] In each a green ray, approximating in
position to the fundamental nebular line, crossed an otherwise
unbroken spectrum. And Holmes’s comet of 1892 displayed only
a faint prismatic band devoid of any characteristic feature.[1262] Now
these three might well be set down as partially effete bodies; but a
brilliant comet, visible in southern latitudes in April and May, 1901,
so far resembled them in the quality of its light as to give a
spectrum mainly, if not purely, continuous. This, accordingly, is no
symptom of decay.
The earliest comet of first-class lustre to present itself for spectroscopic
examination was that discovered by Coggia at Marseilles,
April 17, 1874. Invisible to the naked eye till June, it blazed out
in July a splendid ornament of our northern skies, with a just
perceptibly curved tail, reaching more than half way from the
horizon to the zenith, and a nucleus surpassing in brilliancy the
brightest stars in the Swan. Brédikhine, Vogel, and Huggins[1263] were
unanimous in pronouncing its spectrum to be that of marsh or
olefiant gas. Father Secchi, in the clear sky of Rome, was able to
push the identification even closer than had heretofore been done.
The complete hydro-carbon spectrum consists of five zones of
variously coloured light. Three of these only—the three central
ones—had till then been obtained from comets; owing, it was
supposed, to their temperature not being high enough to develop
the others. The light of Coggia’s comet, however, was found to
contain all five, traces of the violet band emerging June 4, of the
red, July 2.[1264] Presumably, all five would show universally in
cometary spectra, were the dispersed rays strong enough to enable
them to be seen.
The gaseous surroundings of comets are, then, largely made up of
a compound of hydrogen with carbon. Other materials are also[Pg 344]
present; but the hydro-carbon element is probably unfailing and
predominant. Its luminosity is, there is little doubt, an effect of
electrical excitement. Zöllner showed in 1872[1265] that, owing to
evaporation and other changes produced by rapid approach to the
sun, electrical processes of considerable intensity must take place in
comets; and that their original light is immediately connected with
these, and depends upon solar radiation, rather through its direct or
indirect electrifying effects, than through its more obvious thermal
power, may be considered a truth permanently acquired to science.[1266]
They are not, it thus seems, bodies incandescent through heat, but
glowing by electricity; and this is compatible, under certain
circumstances, with a relatively low temperature.
The gaseous spectrum of comets is accompanied, in varying
degrees, by a continuous spectrum. This is usually derived most
strongly from the nucleus, but extends, more or less, to the nebulous
appendages. In part, it is certainly due to reflected sunlight; in
part, most likely, to the ignition of minute solid particles.
FOOTNOTES:
[1188] Month. Not., vol. xix., p. 27.
[1189] Mém. de l’Ac. Imp., t. ii., 1859, p. 46.
[1190] Harvard Annals, vol. iii., p. 368.
[1191] Ibid., p. 371.
[1192] Month. Not., vol. xxii., p. 306.
[1193] Stothard in Ibid., vol. xxi., p. 243.
[1194] Intell. Observer, vol. i., p. 65.
[1195] Comptes Rendus, t. lxi., p. 953.
[1196] Smiths. Report, 1881 (Holden); Nature, vol. xxv., p. 94; Observatory,
vol. xxi., p. 378 (W. T. Lynn).
[1197] Ueber den Ursprung der von Pallas gefundenen Eisenmassen, p. 24.
[1198] Arago, Annuaire, 1836, p. 294.
[1199] Humboldt had noticed the emanation of the shooting stars of 1799 from a
single point, or “radiant,” as Greg long afterwards termed it; but no reasoning
was founded on the observation.
[1200] Am. Journ. of Sc., vol. xxvi., p. 132.
[1201] Annuaire, 1836, p. 297.
[1202] Ann. de l’Observ., Bruxelles, 1839, p. 248.
[1203] Ibid., 1837, p. 272.
[1204] Astr. Nach., Nos. 385, 390.
[1205] Am. Jour. of Sc., vol. xxxviii. (2nd ser.), p. 377.
[1206] Ibid., vol. xxxviii., p. 61.
[1207] Month. Not., vol. xxvii., p. 247.
[1208] Am. Jour. of Sc., vol. xliii. (2nd ser.), p. 87.
[1209] Grant, Month. Not., vol. xxvii., p. 29.
[1210] P. Smyth, Ibid., p. 256.
[1211] Hind, Ibid., p. 49.
[1212] Reproduced in Les Mondes, t. xiii.
[1213] Comptes Rendus, t. lxiv., p. 96.
[1214] Astr. Nach., No. 1,626.
[1215] Ibid., No. 1,632.
[1216] Month. Not., vol. xxxviii., p. 369.
[1217] Schiaparelli, Le Stelle Cadenti, p. 54.
[1218] Ueber Feuer-Meteore, p. 406.
[1219] Astr. Nach., No. 347 (Mädler); see also Boguslawski, Die Kometen, p. 98.
1857.
[1220] Nature, vol. vi., p. 148.
[1221] A. S. Herschel, Month. Not., vol. xxxii., p. 355.
[1222] Astr. Nach., Nos. 1,632, 1,633, 1,635.
[1223] Nature, vol. vii., p. 122.
[1224] A. S. Herschel, Report Brit. Ass., 1873, p. 390.
[1225] Humboldt, Cosmos, vol. i., p. 114 (Otté’s trans.).
[1226] Month. Not., vol. xxxiii., p. 128.
[1227] Even this was denied by Bruhns, Astr. Nach., No. 2,054.
[1228] Am. Jour., vol. xxxi., p. 425.
[1229] Month. Not., vol. xlvi., p. 69.
[1230] In Schiaparelli’s opinion, centuries must have elapsed while the observed
amount of scattering was being produced. Le Stelle Cadenti, 1886, p. 112.
[1231] Astr. and Astroph., vol. xi., p. 943.
[1232] Bull. de l’Acad. St. Petersbourg, t. xxxv., p. 598. 1894.
[1233] Observatory, vol. xvi., p. 55.
[1234] Le Stelle Cadenti, p. 133; Rendiconti dell’ Istituto Lombardo, t. iii., ser. ii.,
p. 23.
[1235] Denning, Memoirs Roy. Astr. Soc., vol. liii., p. 214; Abelmann, Astr. Nach.,
No. 3,516.
[1236] Proc. Roy. Soc., March 2, 1899; Nature, November 9, 1899.
[1237] Berberich, Astr. Nach., No. 3,526.
[1238] Elkin, Astroph. Jour., vol. ix., p. 22.
[1239] Elkin, Astroph. Jour., vol. x., p. 24.
[1240] Pop. Astr., September, 1897, p. 232.
[1241] Month. Not., vol. xx., p. 336.
[1242] Revue des deux Mondes, December 15, 1885, p. 889.
[1243] Palgrave, Phil. Trans., vol. cxxv., p. 175.
[1244] W. E. Hidden, Century Mag., vol. xxxiv., p. 534.
[1245] Amer. Jour. of Science, vol. xxxvi., p. i., 1888.
[1246] Revue des Questions Scientifiques, January, 1899, p. 194; Tisserand, Bull.
Astr., t. viii., p. 460.
[1247] Month. Not., vol. xlv., p. 93.
[1248] Observatory, vol. viii., p. 4.
[1249] Denning, Month. Not., vol. xxxviii., p. 114.
[1250] Comptes Rendus, t. cix., p. 344.
[1251] Month. Not., vol. lix., p. 140.
[1252] Bull. de l’Acad. St. Petersb., t. xii., p. 95.
[1253] Publ. Astr. Pac. Soc., vol. iii., p. 114.
[1254] Month. Not., vol. lii., p. 341.
[1255] Astr. Nach., No. 1,488.
[1256] Annuaire, Paris, 1883, p. 185.
[1257] Phil. Trans., vol. clviii., p. 556.
[1258] Hasselberg, Mém. de l’Ac. Imp. de St. Pétersbourg, t. xxviii. (7th ser.), No. 2,
p. 66.
[1259] Scheiner, Die Spectralanalyse der Gestirne, p. 234. Kayser (Astr. and
Astroph., vol. xiii., p. 368) refers the anomalies of the carbon-spectrum in
comets wholly to instrumental sources.
[1260] Dewar, Proc. Roy. Inst., vol. xi., p. 541.
[1261] Proc. R. Soc., vol. xv., p. 5; Month. Not., vol. xxvii., p. 288.
[1262] Keeler, Astr. and Astrophysics, vol. xi., p. 929; Vogel, Astr. Nach.,
No. 3,142.
[1263] Proc. Roy. Soc., vol. xxiii., p. 154.
[1264] Hasselberg, loc. cit., p. 58.
[1265] Ueber die Natur der Cometen, p. 112.
[1266] Hasselberg, loc. cit., p. 38.
CHAPTER XI
RECENT COMETS (continued)
The mystery of comets’ tails had been to some extent penetrated;
so far, at least, that, by making certain assumptions strongly
recommended by the facts of the case, their forms can be, with
very approximate precision, calculated beforehand. We have, then,
the assurance that these extraordinary appendages are composed of
no ethereal or supersensual stuff, but of matter such as we know
it, and subject to the ordinary laws of motion, though in a state of
extreme tenuity.
Olbers, as already stated, originated in 1812 the view that the tails
of comets are made up of particles subject to a force of electrical
repulsion proceeding from the sun. It was developed and enforced
by Bessel’s discussion of the appearances presented by Halley’s
comet in 1835. He, moreover, provided a formula for computing
the movement of a particle under the influence of a repulsive force
of any given intensity, and thus laid firmly the foundation of a
mathematical theory of cometary emanations. Professor W. A.
Norton, of Yale College, considerably improved this by inquiries
begun in 1844, and resumed on the apparition of Donati’s comet;
and Dr. C. F. Pape at Altona[1267] gave numerical values for the impulses
outward from the sun, which must have actuated the materials
respectively of the curved and straight tails adorning the same
beautiful and surprising object.
The physical theory of repulsion, however, was, it might be said,
still in the air. Nor did it even begin to assume consistency until
Zöllner took it in hand in 1871.[1268] It is perfectly well ascertained that
the energy of the push or pull produced by electricity depends (other
things being the same) upon the surface of the body acted on; that of
gravity upon its mass. The efficacy of solar electrical repulsion
relatively to solar gravitational attraction grows, consequently, as
the size of the particle diminishes. Make this small enough, and it[Pg 346]
will virtually cease to gravitate, and will unconditionally obey the
impulse to recession.
This principle Zöllner was the first to realise in its application
to comets. It gives the key to their constitution. Admitting that
the sun and they are similarly electrified, their more substantially
aggregated parts will still follow the solicitations of his gravity,
while the finely divided particles escaping from them will, simply
by reason of their minuteness, fall under the sway of his repellent
electric power. They will, in other words, form “tails.” Nor is
any extravagant assumption called for as to the intensity of the
electrical charge concerned in producing these effects. Zöllner, in
fact, showed[1269] that it need not be higher than that attributed by the
best authorities to the terrestrial surface.
Forty years have elapsed since M. Brédikhine, director successively
of the Moscow and of the Pulkowa Observatories, turned his attention
to these curious phenomena. His persistent inquiries on the subject,
however, date from the appearance of Coggia’s comet in 1874. On
computing the value of the repulsive force exerted in the formation
of its tail, and comparing it with values of the same force arrived
at by him in 1862 for some other conspicuous comets, it struck him
that the numbers representing them fell into three well-defined
classes. “I suspect,” he wrote in 1877, “that comets are divisible
into groups, for each of which the repulsive force is perhaps the
same.”[1270] This idea was confirmed on fuller investigation. In 1882
the appendages of thirty-six well-observed comets had been reconstructed
theoretically, without a single exception being met with
to the rule of the three types. A further study of forty comets led,
in 1885, only to a modification of the numerical results previously
arrived at.
In the first of these, the repellent energy of the sun is fourteen
times stronger than his attractive energy;[1271] the particles forming the
enormously long straight rays projected outward from this kind of
comet leave the nucleus with a mean velocity of just seven kilometres
per second, which, becoming constantly accelerated, carries
them in a few days to the limit of visibility. The great comets of
1811, 1843, and 1861, that of 1744 (so far as its principal tail was
concerned), and Halley’s comet at its various apparitions, belonged
to this class. Less narrow limits were assigned to the values of the
repulsive force employed to produce the second type. For the
axis of the tail, it exceeds by one-tenth (= 1·1) the power of solar[Pg 347]
gravity; for the anterior edge, it is more than twice (2·2), for the
posterior only half as strong. The corresponding initial velocity
(for the axis) is 1,500 metres a second, and the resulting appendage
a scimitar-like or plumy tail, such as Donati’s and Coggia’s comets
furnished splendid examples of. Tails of the third type are constructed
with forces of repulsion from the sun ranging from one-tenth
to three-tenths that of his gravity, producing an accelerated
movement of attenuated matter from the nucleus, beginning at the
leisurely rate of 300 to 600 metres a second. They are short,
strongly bent, brush-like emanations, and in bright comets seem to
be only found in combination with tails of the higher classes.
Multiple tails, indeed—that is, tails of different types emitted simultaneously
by one comet—are perceived, as experience advances and
observation becomes closer, to be rather the rule than the exception.[1272]
Now what is the meaning of these three types? Is any translation
of them into physical fact possible? To this question Brédikhine
supplied, in 1879, a plausible answer.[1273] It was already a current
surmise that multiple tails are composed of different kinds of
matter, differently acted on by the sun. Both Olbers and Bessel
had suggested this explanation of the straight and curved emanations
from the comet of 1807; Norton had applied it to the faint
light tracks proceeding from that of Donati;[1274] Winnecke to the
varying deviations of its more brilliant plumage. Brédikhine
defined and ratified the conjecture. He undertook to determine
(provisionally as yet) the several kinds of matter appropriated
severally to the three classes of tails. These he found to be
hydrogen for the first, hydro-carbons for the second, and iron for
the third. The ground of this apportionment is that the atomic
weights of these substances bear to each other the same inverse
proportion as the repulsive forces employed in producing the
appendages they are supposed to form; and Zöllner had pointed out
in 1875 that the “heliofugal” power by which comets’ tails are
developed would, in fact, be effective just in that ratio.[1275] Hydrogen,
as the lightest known element—that is, the least under the influence
of gravity—was naturally selected as that which yielded most
readily to the counter-persuasions of electricity. Hydro-carbons
had been shown by the spectroscope to be present in comets, and
were fitted by their specific weight, as compared with that of
hydrogen, to form tails of the second type; while the atoms of iron
were just heavy enough to compose those of the third, and, from
the plentifulness of their presence in meteorites, might be presumed
to enter, in no inconsiderable proportion, into the mass of comets.[Pg 348]
These three substances, however, were by no means supposed to
be the sole constituents of the appendages in question. On the
contrary, the great breadth of what, for the present, were taken
to be characteristically “iron” tails was attributed to the presence
of many kinds of matter of high and slightly different specific weights;[1276]
while the expanded plume of Donati was shown to be, in reality, a
whole system of tails, made up of many substances, each spreading
into a separate hollow cone, more or less deviating from, and partially
superposed upon the others.
Yet these felicities of explanation must not make us forget that
the chemical composition attributed to the first type of cometary
trains has, so far, received no countenance from the spectroscope.
The emission lines of free, incandescent hydrogen have
never been derived from any part of these bodies. Dissentient
opinions, accordingly, were expressed as to the cause of their
structural peculiarities. Ranyard,[1277] Zenker, and others advocated
the agency of heat repulsion in producing them; Kiaer somewhat
obscurely explains them through the evolution of gases by colliding
particles;[1278] Herz of Vienna concludes tails to be mere illusory
appendages produced by electrical discharges through the rare
medium assumed to fill space.[1279] But Hirn[1280] conclusively showed
that no such medium could possibly exist without promptly bringing
ruin upon our “dædal earth” and its revolving companions.
On the whole, modern researches tend to render superfluous the
chemical diversities postulated by Brédikhine. Electricity alone
seems competent to produce the varieties of cometary emanation
they were designed to account for. The distinction of types rests
on a solid basis of fact, but probably depends upon differences
rather in the mode of action than in the kind of substance acted
upon. Suggestive sketches of electrical and “light-pressure” theories
of comets have been published respectively by Mr. Fessenden of
Alleghany,[1281] and by M. Arrhenius at Stockholm.[1282] Although evidently
of a tentative character, they possess great interest.
Brédikhine’s hypothesis was promptly and profusely illustrated.
Within three years of its promulgation, five bright comets made
their appearance, each presenting some distinctive peculiarity by
which knowledge of these curious objects was materially helped forward.
The first of these is remembered as the “Great Southern[Pg 349]
Comet.” It was never visible in these latitudes, but made a short
though stately progress through southern skies. Its earliest detection
was at Cordoba on the last evening of January, 1880; and it
was seen on February 1, as a luminous streak, extending just after
sunset from the south-west horizon towards the pole, in New South
Wales, at Monte Video, and the Cape of Good Hope. The head
was lost in the solar rays until February 4, when Dr. Gould, then
director of the National Observatory of the Argentine Republic at
Cordoba, caught a glimpse of it very low in the west; and on the
following evening, Mr. Eddie, at Graham’s Town, discovered a faint
nucleus, of a straw-coloured tinge, about the size of the annular
nebula in Lyra. Its condensation, however, was very imperfect,
and the whole apparition showed an exceedingly filmy texture. The
tail was enormously long. On February 5 it extended—large perspective
retrenchment notwithstanding—over an arc of 50°; but its
brightness nowhere exceeded that of the Milky Way in Taurus.
There was little curvature perceptible; the edges of the appendage
ran parallel, forming a nebulous causeway from star to star; and the
comparison to an auroral beam was appropriately used. The aspect
of the famous comet of 1843 was forcibly recalled to the memory of
Mr. Janisch, Governor of St. Helena; and the resemblance proved
not merely superficial. But the comet of 1880 was less brilliant,
and even more evanescent. After only eight days of visibility, it
had faded so much as no longer to strike, though still discoverable
by the unaided eye; and on February 20 it was invisible with the
great Cordoba equatoreal pointed to its known place.
But the most astonishing circumstance connected with this body
is the identity of its path with that of its predecessor in 1843. This
is undeniable. Dr. Gould,[1283] Mr. Hind, and Dr. Copeland,[1284] each computed
a separate set of elements from the first rough observations,
and each was struck with an agreement between the two orbits so
close as to render them virtually indistinguishable. “Can it be
possible,” Mr. Hind wrote to Sir George Airy, “that there is such a
comet in the system, almost grazing the sun’s surface in perihelion,
and revolving in less than thirty-seven years. I confess I feel a
difficulty in admitting it, notwithstanding the above extraordinary
resemblance of orbits.”[1285]
Mr. Hind’s difficulty was shared by other astronomers. It would,
indeed, be a violation of common-sense to suppose that a celestial
visitant so striking in appearance had been for centuries back an
unnoticed frequenter of our skies. Various expedients, accordingly,
were resorted to for getting rid of the anomaly. The most promising[Pg 350]
at first sight was that of the resisting medium. It was hard to
believe that a body, largely vaporous, shooting past the sun at a
distance of less than a hundred thousand miles from his surface,
should have escaped powerful retardation. It must have passed
through the very midst of the corona. It might easily have had an
actual encounter with a prominence. Escape from such proximity
might, indeed, very well have been judged beforehand to be
impossible. Even admitting no other kind of opposition than that
dubiously supposed to have affected Encke’s comet, the result in
shortening the period ought to be of the most marked kind. It was
proved by Oppolzer[1286] that if the comet of 1843 had entered our
system from stellar space with parabolic velocity it would, by the
action of a medium such as Encke postulated (varying in density
inversely as the square of the distance from the sun), have been
brought down, by its first perihelion passage, to elliptic movement
in a period of twenty-four years, with such rapid diminution that its
next return would be in about ten. But such restricted observations
as were available on either occasion of its visibility gave no sign of
such a rapid progress towards engulfment.
Another form of the theory was advocated by Klinkerfues.[1287] He
supposed that four returns of the same body had been witnessed
within historical memory—the first in 371 b.c., the next in 1668,
besides those of 1843 and 1880; an original period of 2,039 years
being successively reduced by the withdrawal at each perihelion
passage of 1/1320 of the velocity acquired by falling from the far
extremity of its orbit towards the sun, to 175 and 37 years. A
continuance of the process would bring the comet of 1880 back in
1897.
Unfortunately, the earliest of these apparitions cannot be identified
with the recent ones unless by doing violence to the plain meaning
of Aristotle’s words in describing it. He states that the comet was
first seen “during the frosts and in the clear skies of winter,” setting
due west nearly at the same time as the sun.[1288] This implies some
considerable north latitude. But the objects lately observed had
practically no north latitude. They accomplished their entire course
above the ecliptic in two hours and a quarter, during which space
they were barely separated a hand’s-breadth (one might say) from
the sun’s surface. For the purposes of the desired assimilation,
Aristotle’s comet should have appeared in March. It is not credible,
however, that even a native of Thrace should have termed March
“winter.”
With the comet of 1668 the case seemed more dubious. The
circumstances of its appearance are barely reconcilable with the
identity attributed to it, although too vaguely known to render
certainty one way or the other attainable. It might however, be
expected that recent observations would at least decide the questions
whether the comet of 1843 could have returned in less than thirty-seven,
and whether the comet of 1880 was to be looked for at the
end of 17-1/2 years. But the truth is that both these objects were
observed over so small an arc—8° and 3° respectively—that their
periods remained virtually undetermined. For while the shape and
position of their orbits could be and were fixed with a very close
approach to accuracy, the length of those orbits might vary enormously
without any very sensible difference being produced in the
small part of the curves traced out near the sun. Dr. Wilhelm
Meyer, however, arrived, by an elaborate discussion, at a period of
thirty-seven years for the comet of 1880,[1289] while the observations of
1843 were admittedly best fitted by Hubbard’s ellipse of 533 years;
but these Dr. Meyer supposed to be affected by some constant source
of error, such as would be produced by a mistaken estimate of the
position of the comet’s centre of gravity. He inferred finally that,
in spite of previous non-appearances, the two comets represented a
single regular denizen of our system, returning once in thirty-seven
years along an orbit of such extreme eccentricity that its movement
might be described as one of precipitation towards and rapid escape
from the sun, rather than of sedate circulation round it.
The geometrical test of identity has hitherto been the only one
which it was possible to apply to comets, and in the case before us it
may fairly be said to have broken down. We may, then, tentatively,
and with much hesitation, try a physical test, though scarcely yet,
properly speaking, available. We have seen that the comets of
1843 and 1880 were strikingly alike in general appearance, though
the absence of a formed nucleus in the latter, and its inferior
brilliancy, detracted from the convincing effect of the resemblance.
Nor was it maintained when tried by exact methods of inquiry. M.
Brédikhine found that the gigantic ray emitted in 1843 belonged to
his type No. 1; that of 1880 to type No. 2.[1290] The particles forming
the one were actuated by a repulsive force ten times as powerful as
those forming the other. It is true that a second noticeably curved
tail was seen in Chili, March 1, and at Madras, March 11, 1843;
and the conjecture was accordingly hazarded that the materials
composing on that occasion the principal appendage having become
exhausted, those of the secondary one remained predominant, and[Pg 352]
reappeared alone in the “hydro-carbon” train of 1880. But the one
known instance in point is against such a supposition. Halley’s
comet, the only great comet of which the returns have been securely
authenticated and carefully observed, has preserved its “type”
unchanged through many successive revolutions. The dilemma
presented to astronomers by the Great Southern Comet of 1880 was
unexpectedly renewed in the following year.
On the 22nd of May, 1881, Mr. John Tebbutt of Windsor, New
South Wales, scanning the western sky, discerned a hazy-looking
object which he felt sure was a strange one. A marine telescope at
once resolved it into two small stars and a comet, the latter of
which quickly attracted the keen attention of astronomers; for
Dr. Gould, computing its orbit from his first observations at
Cordoba, found it to agree so closely with that arrived at by Bessel
for the comet of 1807 that he telegraphed to Europe, June 1,
announcing the unexpected return of that body. So unexpected
that theoretically it was not possible before the year 3346; and
Bessel’s investigation was one which inspired and eminently deserved
confidence. Here, then, once more the perplexing choice had to be
made between a premature and unaccountable reappearance and
the admission of a plurality of comets moving nearly in the same
path. But in this case facts proved decisive.
Tebbutt’s comet passed the sun, June 16, at a distance of sixty-eight
millions of miles, and became visible in Europe six days later. It
was, in the opinion of some, the finest object of the kind since 1861.
In traversing the constellation Auriga on its début in these latitudes,
it outshone Capella. On June 24 and some subsequent nights, it
was unmatched in brilliancy by any star in the heavens. In the
telescope, the “two interlacing arcs of light” which had adorned the
head of Coggia’s comet were reproduced; while a curious dorsal spine
of strong illumination formed the axis of the tail, which extended
in clear skies over an arc of 20°. It belonged to the same “type”
as Donati’s great plume; the particles composing it being driven
from the sun by a force twice as powerful as that urging them
towards it.[1291] But the appendage was, for a few nights, and by two
observers perceived to be double. Tempel, on June 27, and Lewis
Boss, at Albany (N.Y.), June 26 and 28, saw a long straight ray
corresponding to a far higher rate of emission than the curved train,
and shown by Brédikhine to be a member of the (so-called) hydrogen
class. It had vanished by July 1, but made a temporary reappearance
July 22.[1292]
The appendages of this comet were of remarkable transparency.
[Pg 353]
Small stars shone wholly undimmed across the tail, and a very
nearly central transit of the head over one of the seventh magnitude
on the night of June 29, produced—if any change—an increase of
brilliancy in the object of this spontaneous experiment.[1293] Dr. Meyer,
indeed, at the Geneva Observatory, detected apparent signs of
refractive action upon rays thus transmitted;[1294] but his observations
remain isolated, and were presumably illusory.
The track pursued by this comet gave peculiar advantages for its
observation. Ascending from Auriga through Camelopardus, it
stood, July 19, on a line between the Pointers and the Pole, within
8° of the latter, and thus remained for a lengthened period constantly
above the horizon of northern observers. Its brightness, too, was
no transient blaze, but had a lasting quality which enabled it to be
kept steadily in view during nearly nine months. Visible to the
naked eye until the end of August, the last telescopic observation
of it was made February 14, 1882, when its distance from the earth
considerably exceeded 300 million miles. Under these circumstances,
the knowledge acquired of its orbit was of more than usual accuracy,
and showed conclusively that the comet was not a simple return of
Bessel’s; for this would involve a period of seventy-four years,
whereas Tebbutt’s comet cannot revisit the sun until after the lapse
of two and a half millenniums.[1295] Nevertheless, the twin bodies move
so nearly in the same path that an original connection of some kind
is obvious; and the recent example of Biela readily suggested a conjecture
as to what the nature of that connection might have been.
The comets of 1807 and 1881 are, then, regarded with much
probability as fragments of a primitive disrupted body, one following
in the wake of the other at an interval of seventy-four years.
Imperfect photographs were taken of Donati’s comet both in
England and America;[1296] but Tebbutt’s comet was the first to which
the process was satisfactorily applied. The difficulties to be overcome
were very great. The chemical intensity of cometary light is, to
begin with, extraordinarily small. Janssen estimated it at 1/300000
of moonlight.[1297] Hence, if the ordinary process by which lunar photographs
are taken had been applied to the comet of 1881, an exposure
of at least three days would have been required in order to get
an impression of the head with about a tenth part of the tail. But[Pg 354]
by that time a new method of vastly increased sensitiveness had
been rendered available, by which dry gelatine-plates were substituted
for the wet collodion-plates hitherto in use; and this improvement
alone reduced the necessary time of exposure to two hours. It was
brought down to half an hour by Janssen’s employment of a reflector
specially adapted to give an image illuminated eight or ten times as
strongly as that produced in the focus of an ordinary telescope.[1298]
The photographic feebleness of cometary rays was not the only
obstacle in the way of success. The proper motion of these bodies
is so rapid as to render the usual devices for keeping a heavenly
body steadily in view quite inapplicable. The machinery by which
the diurnal movement of the sphere is followed, must be especially
modified to suit each eccentric career. This, too, was done, and on
June 30, 1881, Janssen secured a perfect photograph of the brilliant
object then visible, showing the structure of the tail with beautiful
distinctness to a distance of 2-1/2° from the head. An impression to
nearly 10° was obtained about the same time by Dr. Henry Draper
at New York, with an exposure of 162 minutes.[1299]
Tebbutt’s (or comet 1881 iii.) was also the first comet of which the
spectrum was so much as attempted to be chemically recorded. Both
Huggins and Draper were successful in this respect, but Huggins
was more completely so.[1300] The importance of the feat consisted in
its throwing open to investigation a part of the spectrum invisible to
the eye, and so affording an additional test of cometary constitution.
The result was fully to confirm the origin from carbon-compounds
assigned to the visible rays, by disclosing additional bands belonging
to the same series in the ultra-violet; as well as to establish unmistakably
the presence of a not inconsiderable proportion of reflected
solar light by the clear impression of some of the principal Fraunhofer
lines. Thus the polariscope was found to have told the truth, though
not the whole truth.
The photograph so satisfactorily communicative was taken by
Sir William Huggins on the night of June 24; and on the 29th, at
Greenwich, the tell-tale Fraunhofer lines were perceived to interrupt
the visible range of the spectrum. This was at first so vividly continuous,
that the characteristic cometary bands could scarcely be
detached from their bright background. But as the nucleus faded
towards the end of June, they came out strongly, and were more
and more clearly seen, both at Greenwich and at Princeton, to agree,
not with the spectrum of hydro-carbons glowing in a vacuum tube,
but with that of the same substances burning in a Bunsen flame.[Pg 355][1301]
It need not, however, be inferred that cometary materials are really
in a state of combustion. This, from all that we know, may be
called an impossibility. The additional clue furnished was rather
to the manner of their electrical illumination.[1302]
The spectrum of the tail was, in this comet, found to be not essentially
different from that of the head. Professor Wright of Yale
College ascertained a large percentage of its light to be polarized in
a plane passing through the sun, and hence to be reflected sunlight.[1303]
A faint continuous spectrum corresponded to this portion of its
radiance; but gaseous emissions were also present. At Potsdam, on
June 30, the hydro-carbon bands were indeed traced by Vogel to the
very end of the tail;[1304] and they were kept in sight by Young at a
greater distance from the nucleus than the more equably dispersed
light. There seems little doubt that, as in the solar corona, the relative
strength of the two orders of spectra is subject to fluctuations.
The comet of 1881 iii. was thus of signal service to science. It
afforded, when compared with the comet of 1807, the first undeniable
example of two such bodies travelling so nearly in the same orbit as
to leave absolutely no doubt of the existence of a genetic tie between
them. Cometary photography came to its earliest fruition with it;
and cometary spectroscopy made a notable advance by means of it.
Before it was yet out of sight, it was provided with a successor.
At Ann Arbor Observatory, Michigan, on July 14, a comet was
discovered by Dr. Schaeberle, which, as his claim to priority is undisputed,
is often allowed to bear his name, although designated,
in strict scientific parlance, comet 1881 iv. It was observed in
Europe after three days, became just discernible by the naked eye
at the end of July, and brightened consistently up to its perihelion
passage, August 22, when it was still about fifty million miles from
the sun. During many days of that month, the uncommon spectacle
was presented of two bright comets circling together, though at
widely different distances, round the North pole of the heavens.
The newcomer, however, never approached the pristine lustre of its
predecessor. Its nucleus, when brightest, was comparable to the
star Cor Caroli, a narrow, perfectly straight ray proceeding from it to
a distance of 10°. This was easily shown by Brédikhine to belong to
the hydrogen type of tails;[1305] while a “strange, faint second tail, or
bifurcation of the first one,” observed by Captain Noble, August 24,[1306]
fell into the hydro-carbon class of emanations. It was seen, August
22 and 24, by Dr. F. Terby of Louvain,[1307] as a short nebulous brush,[Pg 356]
like the abortive beginning of a congeries of curving trains; but
appeared no more. Its well-attested presence was significant of the
complex constitution of such bodies, and the manifold kinds of action
progressing in them.
The only peculiarity in the spectrum of Schaeberle’s comet
consisted in the almost total absence of continuous light. The
carbon-bands were nearly isolated and very bright. Barely from
the nucleus proceeded a rainbow-tinted streak, indicative of solid or
liquid matter, which, in this comet, must have been of very scanty
amount. Its visit to the sun in 1881 was, so far as is known, the
first. The elements of its orbit showed no resemblance to those of
any previous comet, nor any marked signs of periodicity. So that,
although it may be considered probable, we do not know that it is
moving in a closed curve, or will ever again penetrate the precincts
of the solar system. It was last seen from the southern hemisphere,
October 19, 1881.
The third of a quartette of lucid comets visible within sixteen
months, was discovered by Mr. C. S. Wells at the Dudley Observatory,
Albany, March 17, 1882. Two days later it was described by
Mr. Lewis Boss as “a great comet in miniature,” so well defined
and regularly developed were its various parts and appendages. Discernible
with optical aid early in May, it was on June 5 observed
on the meridian at Albany just before noon—an astronomical event
of extreme rarity. Comet Wells, however, never became an object
so conspicuous as to attract general attention, owing to its immersion
in the evening twilight of our northern June.
But the study of its spectrum revealed new facts of the utmost
interest. All the comets till then examined had been found (with
the two transiently observed exceptions already mentioned) to
conform to one invariable type of luminous emission. Individual
distinctions there had been, but no specific differences. Now all
these bodies had kept at a respectful distance from the sun; for of
the great comet of 1880 no spectroscopic inquiries had been made.
Comet Wells, on the other hand, approached its surface within little
more than five million miles on June 10, 1882; and the vicinity had
the effect of developing a novel feature in its incandescence.
During the first half of April its spectrum was of the normal
type, though the carbon bands were unusually weak; but with
approach to the sun they died out, and the entire light seemed
to become concentrated into a narrow, unbroken, brilliant streak,
hardly to be distinguished from the spectrum of a star. This
unusual behaviour excited attention, and a strict watch was kept.
It was rewarded at the Dunecht Observatory, May 27, by the
discernment of what had never before been seen in a comet—the[Pg 357]
yellow ray of sodium.[1308] By June 1, this had kindled into a
blaze overpowering all other emissions. The light of the comet was
practically monochromatic; and the image of the entire head, with
the root of the tail, could be observed, like a solar prominence,
depicted, in its new saffron vesture of vivid illumination, within the
jaws of an open slit.
At Potsdam, the bright yellow line was perceived with astonishment
by Vogel on May 31, and was next evening identified with
Fraunhofer’s “D.” Its character led him to infer a very considerable
density in the glowing vapour emitting it.[1309] Hasselberg
founded an additional argument in favour of the electrical origin of
cometary light on the changes in the spectrum of comet Wells.[1310]
For they were closely paralleled by some earlier experiments of
Wiedemann, in which the gaseous spectra of vacuum tubes were at
once effaced on the introduction of metallic vapours. It seemed as
if the metal had no sooner been rendered volatile by heat, than it
usurped the entire office of carrying the discharge, the resulting
light being thus exclusively of its production. Had simple incandescence
by heat been in question, the effect would have been
different; the two spectra would have been superposed without
prejudice to either. Similarly, the replacement of the hydro-carbon
bands in the spectrum of the comet by the sodium line
proved electricity to be the exciting agent. For the increasing
thermal power of the sun might, indeed, have ignited the sodium,
but it could not have extinguished the hydro-carbons.
Sir William Huggins succeeded in photographing the spectrum of
comet Wells by an exposure of one hour and a quarter.[1311] The
result was to confirm the novelty of its character. None of the
ultra-violet carbon groups were apparent; but certain bright rays,
as yet unidentified, had imprinted themselves. Otherwise the
spectrum was strongly continuous, uninterrupted even by the
Fraunhofer lines detected in the spectrum of Tebbutt’s comet.
Hence it was concluded that a smaller proportion of reflected light
was mingled with the native emissions of the later arrival.
All that is certainly known about the extent of the orbit traversed
by the first comet of 1882 is that it came from, and is now retreating
towards, vastly remote depths of space. An American computer[1312]
found a period indicated for it of no less than 400,000 years;
A. Thraen of Dingelstädt arrived at one of 3617.[1313] Both are perhaps
equally insecure.
We have now to give some brief account of one of the most
remarkable cometary apparitions on record, and—with the single
exception of that identified with the name of Halley—the most
instructive to astronomers. The lessons learned from it were as
varied and significant as its aspect was splendid; although from the
circumstance of its being visible in general only before sunrise, the
spectators of its splendour were comparatively few.
The discovery of a great comet at Rio Janeiro, September 11, 1882,
became known in Europe through a telegram from M. Cruls, director
of the observatory at that place. It had, however (as appeared
subsequently), been already seen on the 8th by Mr. Finlay of the
Cape Observatory, and at Auckland as early as September 3. A
later, but very singularly conditioned detection, quite unconnected
with any of the preceding, was effected by Dr. Common at Ealing.
Since the eclipse of May 17, when a comet—named “Tewfik” in
honour of the Khedive of Egypt—was caught on Dr. Schuster’s
photographs, entangled, one might almost say, in the outer rays of
the corona, he had scrutinized the neighbourhood of the sun on the
infinitesimal chance of intercepting another such body on its rapid
journey thence or thither. We record with wonder that, after an
interval of exactly four months, that infinitesimal chance turned up
in his favour.
On the forenoon of Sunday, September 17, he saw a great comet
close to, and rapidly approaching the sun. It was, in fact, then
within a few hours of perihelion. Some measures of position were
promptly taken; but a cloud-veil covered the interesting spectacle
before mid-day was long past. Mr. Finlay at the Cape was more
completely fortunate. Divided from his fellow-observer by half the
world, he unconsciously finished, under a clearer sky, his interrupted
observation. The comet, of which the silvery radiance contrasted
strikingly with the reddish-yellow glare of the sun’s margin it drew
near to, was followed “continuously right into the boiling of the
limb”—a circumstance without precedent in cometary history.[1314]
Dr. Elkin, who watched the progress of the event with another
instrument, thought the intrinsic brilliancy of the nucleus scarcely
surpassed by that of the sun’s surface. Nevertheless it had no
sooner touched it than it vanished as if annihilated. So sudden
was the disappearance (at 4h. 50m. 58s., Cape mean time), that the
comet was at first believed to have passed behind the sun. But this
proved not to have been the case. The observers at the Cape had
witnessed a genuine transit. Nor could non-visibility be explained
The Great Comet of September, 1882.
Photographed at the Royal Observatory, Cape of Good Hope
by equality of lustre. For the gradations of light on the sun’s disc
are amply sufficient to bring out against the dusky background of
the limb any object matching the brilliancy of the centre; while an
object just equally luminous with the limb must inevitably show
dark at the centre. The only admissible view, then, is that the
bulk of the comet was of too filmy a texture, and its presumably
solid nucleus too small, to intercept any noticeable part of the solar
rays—a piece of information worth remembering.
On the following morning, the object of this unique observation
showed (in Sir David Gill’s words) “an astonishing brilliancy as it
rose behind the mountains on the east of Table Bay, and seemed in
no way diminished in brightness when the sun rose a few minutes
afterward. It was only necessary to shade the eye from direct
sunlight with the hand at arm’s length, to see the comet, with its
brilliant white nucleus and dense white, sharply bordered tail of
quite half a degree in length.”[1315] All over the world, wherever the
sky was clear during that day, September 18, it was obvious to
ordinary vision. Since 1843 nothing had been seen like it. From
Spain, Italy, Algeria, Southern France, despatches came in announcing
the extraordinary appearance. At Cordoba, in South
America, the “blazing star near the sun” was the one topic of discourse.[1316]
Moreover—and this is altogether extraordinary—the
records of its daylight visibility to the naked eye extend over three
days. At Reus, near Tarragona, it showed bright enough to be seen
through a passing cloud when only three of the sun’s diameters from
his limb, just before its final rush past perihelion on September 17;
while at Carthagena in Spain, on September 19, it was kept in view
during two hours before and two hours after noon, and was similarly
visible in Algeria on the same day.[1317]
But still more surprising than the appearance of the body itself
were the nature and relations of the path it moved in. The first
rough elements computed for it by Mr. Tebbutt, Dr. Chandler,
and Mr. White, assistant at the Melbourne Observatory, showed
at once a striking resemblance to those of the twin comets of
1843 and 1880. This suggestive fact became known in this
country, September 27, through the medium of a Dunecht circular.
It was fully confirmed by subsequent inquiries, for which ample
opportunities were luckily provided. The likeness was not, indeed,
so absolutely perfect as in the previous case; it included some slight,
though real differences; but it bore a strong and unmistakable stamp,
broadly challenging explanation.
Two hypotheses only were really available. Either the comet of[Pg 360]
1882 was an accelerated return of those of 1843 and 1880, or it was
a fragment of an original mass to which they also had belonged.
For the purposes of the first view the “resisting medium” was
brought into full play; the opinion of its efficacy was for some time
both prevalent and popular, and formed the basis, moreover, of something
of a sensational panic. For a comet which, at a single passage
through the sun’s atmosphere, encountered sufficient resistance to
shorten its period from thirty-seven to two years and eight months,
must, in the immediate future, be brought to rest on his surface;
and the solar conflagration thence ensuing was represented in some
quarters, with more licence of imagination than countenance from
science, as likely to be of catastrophic import to the inhabitants of
our little planet.
But there was a test available in 1882 which it had not been
possible to apply either in 1843 or in 1880. The two bodies visible
in those years had been observed only after they had already passed
perihelion;[1318] the third member of the group, on the other hand, was
accurately followed for a week before that event, as well as during
many months after it. Finlay’s and Elkin’s observation of its disappearance
at the sun’s edge formed, besides, a peculiarly delicate test
of its motion. The opportunity was thus afforded, by directly comparing
the comet’s velocity before and after its critical plunge through
the solar surroundings, of ascertaining with approximate certainty
whether any considerable retardation had been experienced in the
course of that plunge. The answer distinctly given was that there
had not. The computed and observed places on both sides of the sun
fitted harmoniously together. The effect, if any were produced, was
too small to be perceptible.
This result is, in itself, a memorable one. It seems to give the
coup de grâce to Encke’s theory—discredited, in addition, by
Backlund’s investigation—of a resisting medium growing rapidly
denser inwards. For the perihelion distance of the comet of 1882,
though somewhat greater than that of its predecessors, was nevertheless
extremely small. It passed at less than 300,000 miles of the
sun’s surface. But the ethereal substance long supposed to obstruct
the movement of Encke’s comet would there be nearly 2,000 times
denser than at the perihelion of the smaller body, and must have
exerted a conspicuous retarding influence. That none such could be
detected seems to argue that no such medium exists.
Further evidence of a decisive kind was not wanting on the
question of identity. The “Great September Comet” of 1882 was
in no hurry to withdraw itself from curious terrestrial scrutiny. It[Pg 361]
was discerned with the naked eye at Cordoba as late as March 7,
1883, and still showed in the field of the great equatoreal on June 1
as an “excessively faint whiteness.”[1319] It was then about 480 millions
of miles from the earth—a distance to which no other comet—not
even excepting the peculiar one of 1729—had been pursued.[1320] Moreover,
an arc of 340 out of the entire 360 degrees of its circuit had
been described under the eyes of astronomers; so that its course
came to be very well known. That its movement is in a very
eccentric ellipse, traversed in several hundred years, was ascertained.[1321]
The later inquiries of Dr. Kreutz,[1322] completed in a volume published
in 1901,[1323] demonstrated the period to be of about 800 years, while that
of its predecessor in 1843 might possibly agree with it, but is much
more probably estimated at 512 years. The hypothesis that they,
or any of the comets associated with them, were returns of an individual
body is peremptorily excluded. They may all, however, have
been separated from one original mass by the divellent action of the
sun at close quarters. Each has doubtless its own period, since each
has most likely suffered retardations or accelerations special to itself,
which, though trifling in amount, would avail materially to alter the
length of the major axis, while leaving the remaining elements of
the common orbit virtually unchanged.[1324]
A fifth member was added to the family in 1887. On the 18th
of January in that year, M. Thome discovered at Cordoba a comet
reproducing with curious fidelity the lineaments of that observed in
the same latitudes seven years previously. The narrow ribbon of
light, contracting towards the sun, and running outward from it to
a distance of thirty-five degrees; the unsubstantial head—a veiled
nothingness, as it appeared, since no distinct nucleus could be made
out; the quick fading into invisibility, were all accordant peculiarities,
and they were confirmed by some rough calculations of its orbit,
showing geometrical affinity to be no less unmistakable than physical
likeness. The observations secured were indeed, from the nature
of the apparition, neither numerous nor over-reliable; and the earliest
of them dated from a week after perihelion, passed, almost by a
touch-and-go escape, January 11. On January 27, this mysterious
object could barely be discerned telescopically at Cordoba.[1325] That
it belonged to the series of “southern comets” can scarcely be[Pg 362]
doubted; but the inference that it was an actual return of the comet
of 1880, improbable in itself, was negatived by its non-appearance in
1894. Meyer’s incorporation with this extraordinary group of the
“eclipse-comet” of 1882[1326] has been approved by Kreutz, after searching
examination.
The idea of cometary systems was first suggested by Thomas
Clausen in 1831.[1327] It was developed by the late M. Hoek, director
of the Utrecht Observatory, in 1865 and some following years.[1328] He
found that in quite a considerable number of cases, the paths of two
or three comets had a common point of intersection far out in space,
indicating with much likelihood a community of origin. This consisted,
according to his surmise, in the disruption of a parent mass
during its sweep round the star latest visited. Be this as it may,
the fact is undoubted that numerous comets fall into groups, in
which similar conditions of motion betray a pre-existent physical
connection. Never before, however, had geometrical relationship
been so notorious as between the comets now under consideration;
and never before, in a comet still, it might be said, in the prime of
life, had physical peculiarities tending to account for that affinity
been so obvious as in the chief member of the group.
Observation of a granular structure in cometary nuclei dates far
back into the seventeenth century, when Cysatus and Hevelius
described the central parts of the comets of 1618 and 1652
respectively as made up of a congeries of minute stars. Analogous
symptoms of a loose state of aggregation have of late been not
unfrequently detected in telescopic comets, besides the instances of
actual division offered by those connected with the names of Biela
and Liais. The forces concerned in producing these effects seem to
have been peculiarly energetic in the great comet of 1882.
The segmentation of the nucleus was first noticed in the United
States and at the Cape of Good Hope, September 30. It proceeded
rapidly. At Kiel, on October 5 and 7, Professor Krüger perceived
two centres of condensation. A definite and progressive separation
into three masses was observed by Professor Holden, October 13 and
17.[1329] A few days later, M. Tempel found the head to consist of four
lucid aggregations, ranged nearly along the prolongation of the
caudal axis;[1330] and Dr. Common, January 27, 1883, saw five nuclei
in a line “like pearls on a string.”[1331] This remarkable character was
preserved to the last moment of the comet’s distinct visibility. It[Pg 363]
was a consequence, according to Dr. Kreutz, of violent interior
action in the comet itself While close to the sun.
There were, however, other curious proofs of a disaggregative
tendency in this body. On October 9, Schmidt discovered at
Athens a nebulous object 4° south-west of the great comet, and
travelling in the same direction. It remained visible for a few days,
and, from Oppenheim’s and Hind’s calculations, there can be little
doubt that it was really the offspring by fission of the body it
accompanied.[1332] This is rendered more probable by the unexampled
spectacle offered, October 14, to Professor Barnard, then of Nashville,
Tennessee, of six or eight distinct cometary masses within 6° south
by west of the comet’s head, none of which reappeared on the next
opportunity for a search.[1333] A week later, however, one similar object
was discerned by Mr. W. R. Brooks, in the opposite direction from
the comet. Thus space appeared to be strewn with the filmy débris
of this beautiful but fragile structure all along the track of its
retreat from the sun.
Its tail was only equalled (if it were equalled) in length by that
of the comet of 1843. It extended in space to the vast distance
of 200 millions of miles from the head; but, so imperfectly
were its proportions displayed to terrestrial observers, that it at no
time covered an arc of the sky of more than 30°. This apparent
extent was attained, during a few days previous to September 25,
by a faint, thin, rigid streak, noticed only by a few observers—by
Elkin at the Cape Observatory, Eddie at Grahamstown, and Cruls
at Rio Janeiro. It diverged at a low angle from the denser curved
train, and was produced, according to Brédikhine,[1334] by the action of
a repulsive force twelve times as strong as the counter-pull of gravity.
It belonged, that is, to type 1; while the great bifurcate appendage,
obvious to all eyes, corresponded to the lower rate of emission
characteristic of type 2. This was remarkable for the perfect
definiteness of its termination, for its strongly-forked shape, and for
its unusual permanence. Down to the end of January, 1883, its
length, according to Schmidt’s observations, was still 93 million
miles; and a week later it remained visible to the naked eye, without
notable abridgment.
Most singular of all was an anomalous extension of the appendage
towards the sun. During the greater part of October and November,
a luminous “tube” or “sheath,” of prodigious dimensions, seemed
to surround the head, and project in a direction nearly opposite to
that of the usual outpourings of attentuated matter. (See Plate III.)
Its diameter was computed by Schmidt to be, October 15, no less[Pg 364]
than four million miles, and it was described by Cruls as a “truncated
cone of nebulosity,” stretching 3° or 4° sunwards.[1335] This, and the
entire anterior part of the comet, were again surrounded by a thin,
but enormously voluminous paraboloidal envelope, observed by
Schiaparelli for a full month from October 19.[1336] There can be little
doubt that these abnormal effluxes were a consequence of the
tremendous physical disturbance suffered at perihelion; and it is
worth remembering that something analogous was observed in the
comet of 1680 (Newton’s), also noted for its excessively close
approach to the sun, and possibly moving in a related orbit. The
only plausible hypothesis as to the mode of their production is that
of an opposite state of electrification in the particles composing the
ordinary and extraordinary appendages.
The spectrum of the great comet of 1882 was, in part, a repetition
of that of its immediate predecessor, thus confirming the inference
that the previously unexampled sodium-blaze was in both a direct
result of the intense solar action to which they were exposed. But
the D line was, this time, not seen alone. At Dunecht, on the
morning of September 18, Drs. Copeland and J. G. Lohse succeeded
in identifying six brilliant rays in the green and yellow with as
many prominent iron-lines;[1337] a very significant addition to our
knowledge of cometary constitution, and one which lent countenance
to Brédikhine’s assumption of various kinds of matter issuing
from the nucleus with velocities inversely as their atomic weights.
All the lines equally showed a slight displacement, indicating a
recession from the earth of the radiating body at the rate of 37 to
46 miles a second. A similar observation, made by M. Thollon at
Nice on the same day, gave emphatic sanction to the spectroscopic
method of estimating movement in the line of sight. Before
anything was as yet known of the comet’s path or velocity, he
announced, from the position of the double sodium-line alone, that
at 3 p.m. on September 18 it was increasing its distance from our
planet by from 61 to 76 kilometres per second.[1338] M. Bigourdan’s
subsequent calculations showed that its actual swiftness of recession
was at that moment 73 kilometres.
Changes in the inverse order to those seen in the spectrum of
comet Wells soon became apparent. In the earlier body, carbon
bands had died out with approach to perihelion, and had been
replaced by sodium emissions; in its successor, sodium emissions
became weakened and disappeared with retreat from perihelion,
and found their substitute in carbon bands. Professor Riccò was,
in fact, able to infer, from the sequence of prismatic phenomena,[Pg 365]
that the comet had already passed the sun; thus establishing a
novel criterion for determining the position of a comet in its orbit
by the varying quality of its radiations.
Recapitulating what was learnt from the five conspicuous comets
of 1880-82, we find that the leading facts acquired to science
were these three. First, that comets may be met with pursuing
each other, after intervals of many years, in the same, or nearly the
same, track; so that identity of orbit can no longer be regarded as
a sure test of individual identity. Secondly, that at least the outer
corona may be traversed by such bodies with perfect apparent
impunity. Finally, that their chemical constitution is highly complex,
and that they possess, in some cases at least, a metallic core
resembling the meteoric masses which occasionally reach the earth
from planetary space.
A group of five comets, including Halley’s, own a sort of cliental
dependence upon the planet Neptune. They travel out from the
sun just to about his distance from it, as if to pay homage to a
powerful protector, who gets the credit of their establishment as
periodical visitors to the solar system. The second of these bodies
to affect a looked-for return was a comet—the sixteenth within ten
years—discovered by Pons, July 20, 1812, and found by Encke to
revolve in an elliptic orbit, with a period of nearly 71 years. It
was not, however, until September 1, 1883, that Mr. Brooks caught
its reappearance; it passed perihelion January 25, and was last seen
June 2, 1884. At its brightest, it had the appearance of a second
magnitude star, furnished with a poorly developed double tail, and
was fairly conspicuous to the naked eye in Southern Europe, from
December to March. One exceptional feature distinguished it. Its
fluctuations in form and luminosity were unprecedented in rapidity
and extent. On September 21, Dr. Chandler[1339] observed it at
Harvard as a very faint, diffused nebulosity, with slight central
condensation. On the next night, there was found in its place a
bright star of the eighth magnitude, scarcely marked out, by a bare
trace of environing haze, from the genuine stars it counterfeited.
The change was attended by an eight-fold augmentation of light,
and was proved by Schiaparelli’s confirmatory observations[1340] to have
been accomplished within a few hours. The stellar disguise was
quickly cast aside. The comet appeared on September 23 as a
wide nebulous disc, and soon after faded down to its original
dimness. Its distance from the sun was then no less than 200
million miles, and its spectrum showed nothing unusual. These
strange variations recurred slightly on October 15, and with marked
emphasis on January 1, when they were witnessed with amazement,[Pg 366]
and photometrically studied by Müller of Potsdam.[1341] The entire
cycle this time was run through in less than four hours—the
comet having, in that brief space, condensed, with a vivid outburst
of light, into a seeming star, and the seeming star having expanded
back again into a comet. Scarcely less transient, though not
altogether similar, changes of aspect were noted by M. Perrotin,[1342]
January 13 and 19, 1884. On the latter date, the continuous
spectrum given by a reddish-yellow disc surrounding the true
nucleus seemed intensified by bright knots corresponding to the
rays of sodium.
A comet discovered by Mr. Sawerthal at the Royal Observatory,
Cape of Good Hope, February 19, 1888, distinguished itself by
blazing up, on May 19, to four or five times its normal brilliancy,
at the same time throwing out from the head two lustrous lateral
branches.[1343] These had, on June 1, spread backward so as to join
the tail, with an effect like the playing of a fountain; ten or eleven
days later, they had completely disappeared, leaving the comet in
its former shape and insignificance. Its abrupt display of vitality
occurred two full months after perihelion.
On the morning of July 7, 1889, Mr. W. R. Brooks, of Geneva,
New York, eminent as a successful comet-hunter, secured one
of his customary trophies. The faint object in question was
moving through the constellation Cetus, and turned out to be
a member of Jupiter’s numerous family of comets, revolving
round the sun in a period of seven years. Its past history
came then, to a certain extent, within the scope of investigation,
and proved to have been singularly eventful; nor had the
body escaped scatheless from the vicissitudes to which it had
been exposed. Observing from Mount Hamilton, August 2 and 5,
Professor Barnard noticed this comet (1889, v.) to be attended in its
progress through space by four outriders, “The two brighter companions”
(the fainter pair survived a very short time) “were perfect
miniatures,” Professor Barnard tells us,[1344] “of the larger comet, each
having a small, fairly defined head and nucleus, with a faint, hazy
tail, the more distant one being the larger and less developed. The
three comets were in a straight line, nearly east and west, their
tails lying along this line. There was no connecting nebulosity
between these objects, the tails of the two smaller not reaching
each other, or the large comet. To all appearance they were
absolutely independent comets.” Nevertheless, Spitaler, at Vienna,[Pg 367]
in the early days of August, perceived, as it were, a thin cocoon of
nebulosity woven round the entire trio.[1345] One of them faded from
view September 5; the other actually outshone the original comet
on August 31, but was plainly of inferior vitality. It was last seen
by Barnard on November 25, with the thirty-six inch refractor,
while its primary afforded an observation for position with the
twelve-inch, March 20, 1890.[1346] A cause for the disruption it had
presumably undergone had, before then, been plausibly assigned.
The adventures of Lexell’s comet have long served to exemplify
the effects of Jupiter’s despotic sway over such bodies. Although
bright enough in 1770 to be seen with the naked eye, and ascertained
to be circulating in five and a half years, it had never
previously been seen, and failed subsequently to present itself.
The explanation of this anomaly, suggested by Lexell, and fully
confirmed by the analytical inquiries both of Laplace and Leverrier,[1347]
was that a very close approach to Jupiter in 1767 had completely
changed the character of its orbit, and brought it within the range
of terrestrial observation; while in 1779, after having only twice
traversed its new path (at its second return it was so circumstanced
as to be invisible from the earth), it was, by a fresh encounter,
diverted into one entirely different. Yet the possibility was not
lost sight of that the great planet, by inverting its mode of action,
might undo its own work, and fling the comet once more into the
inner part of the solar system. This possibility seemed to be
realized by Chandler’s identification of Brooks’s and Lexell’s comet.[1348]
An exceedingly close approach to Jupiter in 1886 had, he found
reason to believe, produced such extensive alterations in the
elements of its motion as to bring the errant body back to our
neighbourhood in 1889. But his inference, though ratified by
Mr. Charles Lane Poor’s preliminary calculations, proved dubious
on closer inquiry, and was rendered wholly inadmissible by the
circumstances attending the return of Brooks’s comet in 1896.[1349]
The companion-objects watched by Barnard in 1889 had by that
time, perhaps, become dissipated in space, for they were not redetected.
They represented, in all likelihood, wreckage from a
collision with Jupiter, dating, perhaps, so far back as 1791, when
Mr. Lane Poor found that one of the fateful meetings to which
short-period comets are especially subject had taken place.
The Lexell-Brooks case was almost duplicated by the resemblance
to De Vico’s lost comet of 1844[1350] of one detected November 20, 1894,[Pg 368]
by Edward, son of Lewis Swift. Schulhof[1351] announced the identity,
and Chandler,[1352] under reserve, vouched for it. Had the comet continued
to pursue the track laboriously laid down for it at Boston,
and shown itself at the due epoch in 1900, its individuality might
have been considered assured; but the formidable vicegerent of
the sun once more interposed, and, in 1897, swept it out of the
terrestrial range of view. Hence the recognition remains ambiguous.
On the morning of March 7, 1892, Professor Lewis Swift
discovered the brightest comet that had been seen by northern
observers since 1882. About the time of perihelion, which occurred
on April 6, it was conspicuous, as it crossed the celestial equator
from Aquarius towards Pegasus, with a nucleus equal to a third
magnitude star, and a tail twenty degrees long. This tail was
multiple, and multiple in a most curiously variable manner. It
divided up into many thin nebulous streaks, the number and
relative lustre of which underwent rapid and marked changes.
Their permanent record on Barnard’s and W. H. Pickering’s plates
marked a noteworthy advance in cometary photography. Plate IV.
reproduces two of the Lick pictures, taken with a six-inch camera,
on April 5 and 7 respectively, with, in each case, an exposure of
about one hour. The tail is in the first composed of three main
branches, the middle one having sprung out since the previous
morning, and the branches are, in their turn, split up into finer
rays, to the number of perhaps a dozen in all. In the second a
very different state of things is exhibited. “The southern component,”
Professor Barnard remarked, “which was the brightest
on the 5th, had become diffused and fainter, while the middle
tail was very bright and broad. Its southern side, which was
the best defined, was wavy in numerous places, the tail appearing
as if disturbing currents were flowing at right angles to it. At 42°
from the head the tail made an abrupt bend towards the south, as if
its current was deflected by some obstacle. In the densest portion
of the tail, at the point of deflection, are a couple of dark holes,
similar to those seen in some of the nebulæ. The middle portion
of the tail is brighter, and looks like crumpled silk in places.”[1353] Next
morning the southern was the prominent branch, and it was loaded,
at 1° 42′ from the head, with a strange excrescence, suggesting the
budding-out of a fresh comet in that incongruous situation.[1354] Some
of these changes, Professor Barnard thought, might possibly be
explained by a rotation of the tail on an axis passing through the
nucleus, and Pickering, who formed a similar opinion on independent
1 2
Photographs of Swift’s Comet.
By Professor E. E. Barnard.
No. 1. Taken April 4, 1892; exposure 1 h. No. 2. Taken April 6, 1892; exposure 1 h. 5 m
[Pg 369]
grounds, assigned about 94 hours as the period of
the gyrating movement.[1355] He, moreover, determined accelerative
velocities outward from the sun of definite condensations in the
tail, indicating for its materials, on Brédikhine’s theory, a density
less than one half that of hydrogen.[1356] This conclusion applied also
to Rordame’s comet, which exhibited a year later phenomena
analogous to those remarked in Swift’s. Their photographic study
led Professor Hussey[1357] to significant inferences as to the structure and
rapid changes of cometary appendages.
Seven comets were detected in 1892, and all, strange to say, were
visible together towards the close of the year.[1358] Among them was a
faint object, which unexpectedly left a trail on a plate exposed by
Professor Barnard to the stars in Aquila[1359] on October 12. This was
the first comet actually discovered by photography, the Sohag comet
having been simultaneously seen and pictured. It has a period of
about six years. Holmes’s comet is likewise periodical, in rather less
than seven years. Its path, which is wholly comprised between the
orbits of Mars and Jupiter, is less eccentric than that of any other
known comet. Subsequently to its discovery, on November 6, it
underwent some curious vicissitudes. At first bright and condensed,
it expanded rapidly with increasing distance from the sun (to which
it had made its nearest approach on June 13), until, by the middle of
December, it was barely discernible with powerful telescopes as “a
feebly luminous mist on the face of the sky.”[1360] But on January 16,
1893, observers in Europe and America were bewildered to find,
as if substituted for it, a yellow star of the seventh magnitude,
enveloped in a thin nebulous husk, which enclosed a faint miniature
tail.[1361] This condensation and recovery of light lasted in its full
intensity only a couple of days. The almost evanescent faintness
of Holmes’s comet at its next return accounted for its invisibility
previous to 1892, when it was evidently in a state of peculiar
excitement. Mr. Perrine was barely able, with the Lick 36-inch, to
find the vague nebulous patch which occupied its predicted place on
June 10, 1899.
The origin of comets has been long and eagerly inquired into, not
altogether apart from the cheering guidance of ascertained facts.
Sir William Herschel regarded them as fragments of nebulæ[1362]—scattered[Pg 370]
débris of embryo worlds; and Laplace approved of and
adopted the idea.[1363] But there was a difficulty. No comet has yet
been observed to travel in a decided hyperbola. The typical
cometary orbit, apart from disturbance, is parabolic—that is to
say, it is indistinguishable from an enormously long ellipse. But
this circumstance could only be reconciled with the view that the
bodies thus moving were casual visitors from outer space, by making,
as Laplace did, the tacit assumption that the solar system was at rest.
His reasoning was, indeed, thereby completely vitiated, as Gauss
pointed out in 1815;[1364] and the objections then urged were reiterated
by Schiaparelli,[1365] who demonstrated in 1871 that a large preponderance
of well-marked hyperbolic orbits should result if comets were
picked up en route by a swiftly-advancing sun. The fact that their
native movement is practically parabolic shows it to have been wholly
imparted from without. They passively obeyed the pull exerted
upon them. In other words, their condition previous to being
attracted by the sun was one very nearly of relative repose.[1366] They
shared, accordingly, the movement of translation through space of
the solar system.
This significant conclusion had been indicated, on other grounds,
as the upshot of researches undertaken independently by Carrington[1367]
and Mohn[1368] in 1860, with a view to ascertaining the anticipated
existence of a relationship between the general lie of the paths of
comets and the direction of the sun’s journey. It is tolerably
obvious that if they wander at haphazard through interstellar
regions their apparitions should markedly aggregate towards
the vicinity of the constellation Lyra; that is to say, we should
meet considerably more comets than would overtake us, for the
very same reason that falling stars are more numerous after than
before midnight. Moreover, the comets met by us should be,
apparently, swifter-moving objects than those coming up with us
from behind; because, in the one case, our own real movement
would be added to, in the other subtracted from, theirs. But
nothing of all this can be detected. Comets approach the sun indifferently
from all quarters, and with velocities quite independent of
direction.
We conclude, then, that the “cosmical current” which bears the
solar system towards its unknown goal carries also with it nebulous
masses of undefined extent, and at an undefined remoteness, fragments[Pg 371]
detached from which, continually entering the sphere of the
sun’s attraction, flit across our skies under the form of comets.
These are, however, almost certainly so far strangers to our system
that they had no part in the long processes of development by which
its present condition was attained. They are, perhaps, survivals of
an earlier, and by us scarcely and dimly conceivable state of things,
when the swirling chaos from which sun and planets were, by a
supreme edict, to emerge, had not as yet separately begun to be.[Pg 372]
FOOTNOTES:
[1267] Astr. Nach., Nos. 1,172-4.
[1268] Berichte Sächs. Ges., 1871, p. 174.
[1269] Natur der Cometen, p. 124; Astr. Nach., No. 2,086.
[1270] Annales de l’Obs. de Moscou, t. iii., pt. i., p. 37.
[1271] Bull. Astr., t. iii., p. 598. The value of the repellent force for the comet of
1811 (which offered peculiar facilities for its determination) was found = 17·5.
[1272] Faye, Comptes Rendus, t. xciii., p. 13.
[1273] Annales, t. v., pt. ii., p. 137.
[1274] Am. Jour. of Sc., vol. xxxii. (2nd ser.), p. 57.
[1275] Astr. Nach., No. 2,082.
[1276] Annales de l’Obs. de Moscou, t. vi., pt. i., p. 60.
[1277] Astr. Register, March, 1883.
[1278] Astr. Nach., No. 3,018.
[1279] Ibid., No. 3,093.
[1280] Constitution de l’Espace Céleste, p. 224.
[1281] Astroph. Jour., vol. iii., p. 36.
[1282] Physikalische Zeitschrift, November 10 and 17, 1900; Astroph. Jour.,
vol. xiii., p. 344. Cf. Schwarzschild, Sitzungsb., München, 1901, Heft iii.;
J. Hahn, Nature, vols. lxv., p. 415; lxvi., p. 55.
[1283] Astr. Nach., No. 2,307.
[1284] Ibid., No. 2,304.
[1285] Observatory, vol. iii., p. 390.
[1286] Astr. Nach., No. 2,319.
[1287] Ueber die Kometen von 371 v. Chr., 1668, 1843, I. und 1880 I. Göttingen, 1880.
[1288] Meteor., lib. i., cap. 6.
[1289] Mém. Soc. Phys. de Genève, t. xxviii., p. 23.
[1290] Annales de l’Obs. de Moscou, t. vii., pt. i., p. 60.
[1291] Brédikhine, Annales, t. viii., p. 68.
[1292] Am. Jour. of Sc., vol. xxii., p. 305.
[1293] Messrs. Burton and Green observed a dilatation of the stellar image into a
nebulous patch by the transmission of its rays through a nuclear jet of the
comet. Am. Jour. of Sc., vol. xxii., p. 163.
[1294] Archives des Sciences, t. viii., p. 535. Cf. Perrine’s negative results for
Swift’s comet in 1899, Astr. Nach., No. 3,602.
[1295] Riem concluded in 1896 for a definitive period of 2,429 years; Observatory,
vol. xix., p. 282.
[1296] Holden, Publ. Astr. Pac. Soc., vol. ix., p. 89.
[1297] Annuaire, Paris, 1882, p. 781.
[1298] Annuaire, 1882, p. 766.
[1299] Am. Jour. of Sc., vol. xxii., p. 134.
[1300] Report Brit. Assoc., 1881, p. 520.
[1301] Month. Not., vol. xlii., p. 14; Am. Jour. of Sc., vol. xxii., p. 136.
[1302] Piazzi Smyth, Nature, vol. xxiv., p. 430.
[1303] Astr. Nach., No. 2,395.
[1304] Ibid.
[1305] Astr. Nach., No. 2,411.
[1306] Month. Not., vol. xlii., p. 49.
[1307] Astr. Nach., No. 2,414.
[1308] Copernicus, vol. ii., p. 229.
[1309] Astr. Nach., Nos. 2,434, 2,437.
[1310] Ibid., No. 2,441.
[1311] Report Brit. Assoc., 1882, p. 442.
[1312] J. J. Parsons, Am. Jour. of Science, vol. xxvii., p. 34.
[1313] Astr. Nach., No. 2,441.
[1314] Observatory, vol. v., p. 355. The transit had been foreseen by Mr. Tebbutt,
but it occurred after sunset in New South Wales.
[1315] Observatory, vol. v., p. 354.
[1316] Gould, Astr. Nach., No. 2,481.
[1317] Flammarion, Comptes Rendus, t. xcv., p. 558.
[1318] Captain Ray’s sextant observation of the comet of 1843, a few hours before
perihelion, was too rough to be of use.
[1319] Astr. Nach., No. 2,538.
[1320] Nature, vol. xxix., p. 135.
[1321] Astr. Nach., No. 2,482.
[1322] Vierteljahrsschrift Astr. Ges., Jahrg. xxiv., p. 308; Bull. Astr., t. vii.,
p. 513.
[1323] Observatory, vol. xxiv., p. 167.
[1324] The attention of the author was kindly directed to this point by Professor
Young of Princeton (N. J.). Cf. Rebeur-Paschwitz, Sirius, Bd. xvi., p. 233.
[1325] Oppenheim, Astr. Nach., No. 2,902.
[1326] Astr. Nach., No. 2,717.
[1327] Gruithuisen’s Analekten, Heft 7, p. 48.
[1328] Month. Not., vols. xxv., xxvi., xxviii. Cf. Plummer, Observatory, vol. xiii.,
p. 263.
[1329] Nature, vol. xxvii., p. 246.
[1330] Astr. Nach., No. 2,468.
[1331] Athenæum, February 3, 1883.
[1332] Astr. Nach., Nos. 2,462, 2,466.
[1333] Ibid., No. 2,489.
[1334] Annales, Moscow, t. ix., pt. ii., p. 52.
[1335] Comptes Rendus, t. xcvii., p. 797.
[1336] Astr. Nach., No. 2,966.
[1337] Copernicus, vol. ii., p. 235.
[1338] Comptes Rendus, t. xcvi., p. 371.
[1339] Astr. Nach., No. 2,553.
[1340] Ibid.
[1341] Astr. Nach., No. 2,568.
[1342] Annales de l’Observatoire de Nice, t. ii., c. 53.
[1343] Fényi, Astr. Nach., No. 2,844; Kammermann, Ibid., No. 2,849.
[1344] Publ. Astr. Pac. Soc., vol. i., p. 72.
[1345] Annuaire, Paris, 1891, p. 301.
[1346] Astr. Nach., No. 2,989.
[1347] Comptes Rendus, t. xxv., p. 564.
[1348] Astr. Journ., Nos. 205, 231.
[1349] Ibid., Nos. 228, 244, 380.
[1350] Observatory, vol. xviii., pp. 60, 163 (Denning and Lynn).
[1351] Astr. Nach., No. 3,267; Plummer, Knowledge, vol. xix., p. 156.
[1352] Astr. Jour., Nos. 333, 338.
[1353] Astr. and Astroph., vol. xi., p. 387.
[1354] Knowledge, vol. xv., p. 299.
[1355] Harvard Annals, vol. xxxii., pt. ii., p. 272.
[1356] Ibid., p. 287.
[1357] Publ. Astr. Pac. Soc., vol. vii., p. 161.
[1358] H. C. Wilson, Astr. and Astroph., vol. xii., p. 121.
[1359] Observatory, vol. xvi., p. 92.
[1360] Barnard, Astr. and Astroph., vol. xii., p. 180; Astroph. Jour., vol. iii., p. 41.
[1361] Palisa, Astr. Nach., No. 3,147; Denning, Observatory, vol. xvi., p. 142.
[1362] Phil. Trans., vol. ci., p. 306.
[1363] Conn. des Temps, 1816, p. 213.
[1364] Œuvres, t. vi., p. 581.
[1365] Mem. dell’ Istit. Lombardo, t. xii., p. 164; Rendiconti, t. vii., p. 77, 1874.
[1366] W. Förster, Pop. Mitth., 1879, p. 7; Fabry, Étude sur la Probabilité des
Comètes Hyperboliques, Marseille, 1893, p. 158.
[1367] Mem. R. A. Soc., vol. xxix., p. 335.
[1368] Month. Not., vol. xxiii., p. 203.
CHAPTER XII
STARS AND NEBULÆ
That a science of stellar chemistry should not only have become
possible, but should already have made material advances, is
assuredly one of the most amazing features in the swift progress
of knowledge our age has witnessed. Custom can never blunt the
wonder with which we must regard the achievement of compelling
rays emanating from a source devoid of sensible magnitude through
immeasurable distance, to reveal, by its distinctive qualities, the
composition of that source. The discovery of revolving double stars
assured us that the great governing force of the planetary movements,
and of our own material existence, sways equally the courses
of the farthest suns in space; the application of prismatic analysis
certified to the presence in the stars of the familiar materials, no less
of the earth we tread, than of the human bodies built up out of its
dust and circumambient vapours.
We have seen that, as early as 1823, Fraunhofer ascertained the
generic participation of stellar light in the peculiarity by which
sunlight, spread out by transmission through a prism, shows
numerous transverse rulings of interrupting darkness. No sooner
had Kirchhoff supplied the key to the hidden meaning of those
ciphered characters than it was eagerly turned to the interpretation
of the dim scrolls unfolded in the spectra of the stars. Donati
made at Florence in 1860 the first efforts in this direction; but
with little result, owing to the imperfections of the instrumental
means at his command. His comparative failure, however, was a
prelude to others’ success. Almost simultaneously, in 1862, the
novel line of investigation was entered upon by Huggins near
London, by Father Secchi at Rome, and by Lewis M. Rutherfurd in
New York. Fraunhofer’s device of using a cylindrical lens for the
purpose of giving a second dimension to stellar spectra was adopted
by all, and was, indeed, indispensable. For a luminous point, such
as a star appears, becomes, when viewed through a prism, a[Pg 373]
variegated line, which, until broadened into a band by the intervention
of a cylindrical lens, is all but useless for purposes of research.
This process of rolling out involves, it is true, much loss of light—a
scanty and precious commodity, as coming from the stars; but
the loss is an inevitable one. And so fully is it compensated by
the great light-grasping power of modern telescopes that important
information can now be gained from the spectroscopic examination
of stars far below the range of the unarmed eye.
The effective founders of stellar spectroscopy, then (since
Rutherfurd shortly turned his efforts elsewhither), were Father
Secchi, the eminent Jesuit astronomer of the Collegio Romano,
where he died, February 26, 1878, and Sir William Huggins, with
whom the late Professor W. A. Miller was associated. The work
of each was happily directed so as to supplement that of the other.
With less perfect appliances, the Roman astronomer sought to render
his extensive rather than precise; at Tulse Hill searching accuracy
over a narrow range was aimed at and attained. To Father Secchi
is due the merit of having executed the first spectroscopic survey of
the heavens. Above 4,000 stars were passed in review by him, and
classified according to the varying qualities of their light. His provisional
establishment (1863-67) of four types of stellar spectra[1369] has
proved a genuine aid to knowledge through the facilities afforded
by it for the arrangement and comparison of rapidly accumulating
facts. Moreover, it is scarcely doubtful that these spectral distinctions
correspond to differences in physical condition of a marked
kind.
The first order comprises more than half the visible and probably
an overwhelming proportion of the faintest stars. Sirius, Vega,
Regulus, Altair, are amongst its leading members. Their spectra
are distinguished by the breadth and intensity of the four dark bars
due to the absorption of hydrogen, and by the extreme faintness of
the metallic lines, of which, nevertheless, hundreds are disclosed by
careful examination. The light of these “Sirian” orbs is white or
bluish; and it is found to be rich in ultra-violet rays.
Capella and Arcturus belong to the second, or solar type of stars,
which is about one-sixth less numerously represented than the first.
Their spectra are quite closely similar to that of sunlight, in being
ruled throughout by innumerable fine dark lines; and they share its
yellowish tinge.
The third class includes most red and variable stars (commonly
synonymous), of which Betelgeux in the shoulder of Orion, and[Pg 374]
“Mira” in the Whale, are noted examples. Their characteristic
spectrum is of the “fluted” description. It shows like a strongly
illuminated range of seven or eight variously tinted columns seen in
perspective, the light falling from the red end towards the violet.
This kind of absorption is produced by the vapours of metalloids or
of compound substances.
To the fourth order of stars belongs also a colonnaded spectrum,
but reversed; the light is thrown the other way. The three broad
zones of absorption which interrupt it are sharp towards the red,
insensibly gradated towards the violet end. The individuals composing
Class IV. are few and apparently insignificant, the brightest
of them not exceeding the fifth magnitude. They are commonly
distinguished by a deep red tint, and gleam like rubies in the field
of the telescope. Father Secchi, who in 1867 detected the peculiarity
of their analyzed light, ascribed it to the presence of carbon in some
form in their atmospheres; and this was confirmed by the researches
of H. C. Vogel,[1370] director of the Astro-physical Observatory at
Potsdam. The hydro-carbon bands, in fact, seen bright in comets,
are dark in these singular objects—the only ones in the heavens
(save one bright-line star and a rare meteor)[1371] which display a
cometary analogy of the fundamental sort revealed by the spectroscope.
The members of all four orders are, however, emphatically suns.
They possess, it would appear, photospheres radiating all kinds of
light, and differ from each other mainly in the varying qualities of
their absorptive atmospheres. The principle that the colours of
stars depend, not on the intrinsic nature of their light, but on the
kinds of vapours surrounding them, and stopping out certain
portions of that light, was laid down by Huggins in 1864.[1372] The
phenomena of double stars seem to indicate a connection between
the state of the investing atmospheres, by the action of which their
often brilliantly contrasted tints are produced, and their mutual
physical relations. A tabular statement put forward by Professor
Holden in June, 1880,[1373] made it, at any rate, clear that inequality of
magnitude between the components of binary systems accompanies
unlikeness in colour, and that stars more equally matched in one
respect are pretty sure to be so in the other. Besides, blue and
green stars of a decided tinge are never solitary; they invariably
form part of systems. So that association has undoubtedly a
predominant influence upon colour.
Nevertheless, the crude notion thrown out by Zöllner in 1865,[1374]
that yellow and red stars are simply white stars in various stages of
cooling, obtained for a time undeserved currency. D’Arrest, indeed,
protested against it, and Ångström, in 1868,[1375] substituted atmospheric
quality for mere colour[1376] as a criterion of age and temperature. His
lead was followed by Lockyer in 1873,[1377] and by Vogel in 1874.[1378]
The scheme of classification due to the Potsdam astro-physicist
differed from Father Secchi’s only in presenting his third and
fourth types as subdivisions of the same order, and in inserting
three subordinate categories; but their variety was “rationalised”
by the addition of the seductive idea of progressive development.
Thus, the white Sirian stars were represented as the youngest
because the hottest of the sidereal family; those of the solar
pattern as having already wasted much of their store by radiation,
and being well advanced in middle life; while the red stars with
banded spectra figured as effete suns, hastening rapidly down the
road to final extinction.
Vogel’s scheme is, however, incomplete. It traces the downward
curve of decay, but gives no account of the slow ascent to maturity.
The present splendour of Vega, for instance, was prepared, according
to all creative analogy, by almost endless processes of gradual
change. What was its antecedent condition? The question has
been variously answered. Dr. Johnstone Stoney advocated, in 1867,
the comparative youth of red stars;[1379] A. Ritter, of Aix-la-Chapelle,
divided them, in 1883,[1380] into two squadrons, posted, the one on the
ascending, the other on the descending branch of the temperature-curve,
and corresponding, presumably, with Secchi’s third and
fourth orders of stars with banded spectra. Whether, in the
interim, they should display spectra of the Sirian or of the solar type
was made to depend on their greater or less massiveness.[1381] But the
relation actually existing perhaps inverts that contemplated by
Ritter. Certainly, the evidence collected by Mr. Maunder in 1891
strongly supports the opinion that the average solar star is a
weightier body than the average Sirian star.[1382]
On November 17, 1887, Sir Norman Lockyer communicated to
the Royal Society the first of a series of papers embodying his
“Meteoritic Hypothesis” of cosmical constitution, stated and
supported more at large in a separate work bearing that name,
published in 1890. The fundamental proposition wrought out in it
was that “all self-luminous bodies in the celestial space are composed
either of swarms of meteorites or of masses of meteoric vapour
produced by heat.”[1383] On the basis of this supposed community of
origin, sidereal objects were distributed in seven groups along a
temperature-curve ascending from nebulæ and gaseous, or bright-line
stars, through red stars of the third type, and a younger division
of solar stars, to the high Sirian level; then descending through the
more strictly solar stars to red stars of the fourth type (“carbon-stars”),
below which lay only the caput mortuum entitled Group vii.
The ground-work of this classification was, however, insecure, and
has given way. Certain spectroscopic coincidences, avowedly only
approximate, suggesting that stars and nebulæ of every species
might be formed out of variously aggregated meteorites, failed of
verification by exact inquiry. And spectroscopic coincidences
admit of no compromise. Those that are merely approximate are,
as a rule, unmeaning.
In his Presidential Address at the Cardiff Meeting of the British
Association in 1891, Dr. Huggins adhered in the main to the line
of advance traced by Vogel. The inconspicuousness of metallic lines
in the spectra of the white stars he attributed, not to the paucity,
but to the high temperature of the vapours producing them, and
the consequent deficiency of contrast between their absorption-rays
and the continuous light of the photospheric background.
“Such a state of things would more probably,” in his opinion, “be
found in conditions anterior to the solar stage,” while “a considerable
cooling of the sun would probably give rise to banded spectra due
to compounds.” He adverted also to the influential effects upon
stellar types of varying surface gravity, which being a function of
both mass and bulk necessarily gains strength with wasting heat
and consequent shrinkage. The same leading ideas were more fully
worked out in “An Atlas of Representative Stellar Spectra,” published
by Sir William and Lady Huggins in 1899. They were, moreover,
splendidly illustrated by a set of original spectrographic plates,
while precision was added to the adopted classification by the
separation of helium from hydrogen stars. The spectrum of the
exotic substance terrestrially captured in 1895 is conspicuous by
absorption, as Vogel, Lockyer, and Deslandres promptly recognised
in a considerable number of white stars, among them the Pleiades[Pg 377]
and most of the brilliants in Orion. Mr. McClean, whose valuable
spectrographic survey of the heavens was completed at the Cape in
1897, found reason to conclude that they are in the first stage of
development from gaseous nebulæ;[1384] and in this the Tulse Hill
investigators unhesitatingly concur.
The strongest evidence for the primitive state of white stars is
found in their nebular relations. The components of groups, still
involved and entangled with “silver braids” of cosmic mist, show,
perhaps invariably, spectra of the helium type, occasionally crossed
by bright rays. Possibly all such stars have passed through a bright-line
stage; but further evidence on the point is needed. Relative
density furnishes another important test of comparative age, and
Sirian stars are, on the whole, undoubtedly more bulky proportionately
to their mass than solar stars. The rule, however, seems to
admit of exceptions; hence the change from one kind of spectrum
to the other is not inevitably connected with the attainment of a
particular degree of condensation. There is reason to believe that
it is anticipated in the more massive globes, despite their comparatively
slow cooling, as a consequence of the greater power of
gravity over their investing vaporous envelopes. This conclusion
is enforced by the relations of double-star spectra. The fact that,
in unequal pairs, the chief star most frequently shows a solar, its
companion a Sirian, spectrum can scarcely be otherwise explained
than by admitting that, while the sequence of types is pursued in
an invariable order, it is pursued much more rapidly in larger
than in small orbs. It need not, indeed, be supposed that all stars
are identical in constitution, and present identical life-histories.[1385]
Individualities in the one, and divergencies in the other, must be
allowed for. Yet the main track is plainly continuous, and leads
by insensible gradations from nebulæ through helium stars to the
Sirian, and onward to the solar type, whence, by an inevitable
transition, fluted, or “Antarian,”[1386] spectra develop.
The first-known examples of the class of gaseous stars—β Lyræ
and γ Cassiopeiæ—were noticed by Father Secchi at the outset of
his spectroscopic inquiries. Both show bright lines of hydrogen and
helium, so that the peculiarity of their condition probably consists
in the intense ignition of their chromospheric surroundings. Their
entire radiating surfaces might be described as faculous. That is
to say, brilliant formations, such as have been photographed by
Professor Hale on the sun’s disc,[1387] cover, perhaps, the whole, instead[Pg 378]
of being limited to a small portion of the photospheric area. But
this state of things is more or less inconstant. Some at least of the
bright rays indicative of it are subject to temporary extinctions.
Already in 1871-72, Dr. Vogel[1388] suspected the prevalence of such
vicissitudes; and their reality was ascertained by M. Eugen von
Gothard. After the completion of his new astrophysical observatory
at Herény in the autumn of 1881, he repeatedly observed the
spectra of both stars without perceiving a trace of bright lines; and
was thus taken quite by surprise when he caught a twinkling of the
crimson C in γ Cassiopeiæ, August 13, 1883.[1389] A few days later, the
whole range including D_3 was lustrous. Duly apprised of the recurrence
of a phenomenon he had himself vainly looked for during some
years, M. von Konkoly took the opportunity of the great Vienna
refractor being placed at his disposal to examine with it the relighted
spectrum on August 27.[1390] In its wealth of light C was dazzling; D_3
and the green and blue hydrogen rays shone somewhat less vividly;
D and the group b showed faintly dark; while three broad absorption-bands,
sharply terminated towards the red, diffuse towards the
violet, shaded the spectrum near its opposite extremities.
The previous absence of bright lines from the spectrum of this
star was, however, by no means so protracted or complete as M.
von Gothard supposed. At Dunecht, C was “superbly visible”
December 20, 1879[1391]; F was seen bright on October 28 of the same
year, and frequently at Greenwich in 1880-81. The curious fact has,
moreover, been adverted to by Dr. Copeland, that C is much more
variable than F. To Vogel, June 18, 1872, the first was invisible, while
the second was bright; at Dunecht, January 11, 1887, the conditions
were so far inverted that C was resplendent, F comparatively dim.
No spectral fluctuations were detected in γ Cassiopeiæ by Keeler
in 1889; but even with the giant telescope of Mount Hamilton,
the helium-ray was completely invisible.[1392] It made, nevertheless,
capricious appearances at South Kensington during that autumn, and
again October 21, 1894,[1393] while in September, 1892, Bélopolsky could
obtain no trace of it on orthochromatic plates exposed with the 30-inch
Pulkowa refractor.[1394] Still more noteworthy is the circumstance that
the well-known green triplet of magnesium (b), recorded as dark by
Keeler in 1889, came out bright on fifty-two spectrographs of the
star taken by Father Sidgreaves during the years 1891-99.[1395] No[Pg 379]
fluctuations in the hydrogen-spectrum were betrayed by them; but
subordinate lines of unknown origin showed alternate fading and
vivification.
The spectrum of β Lyræ undergoes transitions to some extent
analogous, yet involving a different set of considerations. First
noticed by Von Gothard in 1882,[1396] they were imperfectly made out,
two years later, to be of a cyclical character.[1397] This, however, could
only be effectively determined by photographic means. Beta Lyræ
is a “short-period variable.” Its light changes with great regularity
from 3·4 to 4·4 magnitude every twelve days and twenty-two hours,
during which time it attains a twofold maximum, with an intervening
secondary minimum. The question, then, is of singular interest,
whether the changes of spectral quality visible in this object correspond
to its changes in visual brightness. A distinct answer in
the affirmative was supplied through Mrs. Fleming’s examination
of the Harvard plates of the star’s spectrum, upon which, in 1891,
she found recorded diverse complex changes of bright and dark lines
obviously connected with the phases of luminous variation, and
obeying, in the long-run, precisely the same period.[1398] Something
more will be said presently as to the import of this discovery.
Bright hydrogen lines have so far been detected—for the most
part photographically at Harvard College—in about sixty stars, including
Pleione, the surmised lost Pleiad, P Cygni, noted for instability
of light in the seventeenth century, and the extraordinary
southern variable, η Carinæ. In most of these objects other vivid
rays are associated with those due to hydrogen. A blaze of
hydrogen, moreover, accompanies the recurring outbursts of about
one hundred and fifty “long-period variables,” giving banded spectra
of the third type. Professor Pickering discovered the first example
of this class, towards the close of 1886, in Mira Ceti; further detections
were made visually by Mr. Espin; and the conjunction of
bright hydrogen-lines with dusky bands has been proved by
Mrs. Fleming’s long experience in studying the Harvard photographs,
to indicate unerringly the subjection of the stars thus
characterised to variations of lustre accomplished in some months.
A third variety of gaseous star is named after MM. Wolf and
Rayet, who discovered, at Paris in 1867,[1399] its three typical representatives,
close together in the constellation Cygnus. Six further
specimens were discovered by Dr. Copeland, five of them in the[Pg 380]
course of a trip for the exploration of visual facilities in the Andes
in 1883;[1400] and a large number have been made known through
spectral photographs taken in both hemispheres under Professor
Pickering’s direction. At the close of the nineteenth century, over
a hundred such objects had been registered, none brighter than the
sixth magnitude, with the single exception of γ Argûs, the resplendent
continuous spectrum of which, first examined by Respighi and Lockyer
in 1871, is embellished with the yellow and blue rays distinctive of
the type.[1401] Here, then, we have a stellar globe apparently at the
highest point of sunlike incandescence, sharing the peculiarities of
bodies verging towards the nebulous state. Examined with instruments
of adequate power, their spectra are seen to be highly complex.
They include a fairly strong continuous element, a numerous set of
absorption-lines, and a range of emission-lines, more or less completely
represented in different stars. Especially conspicuous is a
broad effluence of azure light, found by Dr. Vogel in 1883,[1402] and by
Sir William and Lady Huggins in 1890,[1403] to be of multiple structure,
and hence to vary in its mode of display. Its suggested identification
with the blue carbon-fluting was disproved at Tulse Hill.
Metallic vapours give no certain sign of their presence in the
atmospheres of these remarkable bodies; but nebulum is stated to
shine in some.[1404] Hydrogen and helium account for a large proportion
of their spectral rays. Thirty-two Wolf-Rayet stars were
investigated, spectroscopically and spectrographically, by Professor
Campbell with the great Lick refractor in 1892-94;[1405] and several
disclosed the singularity, already noticed by him in γ Argûs, of
giving out mixed series, the members of which change from vivid
to obscure with increase of refrangibility. It is difficult to imagine
by what chromospheric machinery this curious result can be produced.
Alcyone in the Pleiades presents the same characteristic.
Alone among the hydrogen lines, crimson C glows in its spectrum,
while all the others are dark. Luminosity of the Wolf-Rayet
kind is particularly constant, both in quantity and quality. It
seems to be incapable of developing save under galactic conditions.
All the stars marked by it lie near the central line of the Milky
Way, or in the Magellanic Clouds. They tend also to gather into
groups. Circles of four degrees radius include respectively seven in
Argo, eight in Cygnus.
The first spectroscopic star catalogue was published by Dr. Vogel
at Potsdam in 1883.[1406] It included 4,051 stars, distributed over a zone
of the heavens extending from 20° north to 20° south of the celestial
equator.[1407] More than half of these were white stars, while red stars
with banded spectra occurred in the proportion of about one-thirteenth
of the whole. To the latter genus, M. Dunér, then of
Lund, now Director of the Upsala Observatory, devoted a work of
standard authority, issued at Stockholm in 1884. This was a
catalogue with descriptive particulars of 352 stars showing banded
spectra, 297 of which belong to Secchi’s third, 55 to his fourth
class (Vogel’s iii. a and iii. b). Since then discovery has progressed
so rapidly, at first through the telescopic reviews of Mr. Espin, then
in the course of the photographic survey carried on at Harvard
College, that considerably over one thousand stars are at present
recognised as of the family of Betelgeux and Mira, while about 250
have so far exhibited the spectral pattern of 19 Piscium. One fact
well ascertained as regards both species is the invariability of the type.
The prismatic flutings of the one, and the broader zones of the other,
are as if stereotyped—they undergo, in their fundamental outlines,
no modification, though varying in relative intensity from star to
star. They are always accompanied by, or superposed upon, a
spectrum of dark lines, in producing which sodium and iron have an
obvious share; and certain bright rays, noticed by Secchi with imperfect
appliances as enhancing the chiaroscuro effects in carbon-stars,
came out upon plates exposed by Hale and Ellerman in 1898 with the
stellar spectrograph of the Yerkes Observatory.[1408] Their genuineness
was shortly afterwards visually attested by Keeler, Campbell, and
Dunér;[1409] but no chemical interpretation has been found for them.
A fairly complete preliminary answer to the question, What are
the stars made of? was given by Sir William Huggins in 1864.[1410] By
laborious processes of comparison between stellar dark lines and the
bright rays emitted by terrestrial substances, he sought to assure
his conclusions, regardless of cost in time and pains. He averred,
indeed, that—taking into account restrictions by weather and
position—the thorough investigation of a single star-spectrum
would be the work of some years. Of two, however—those of
Betelgeux and Aldebaran—he was able to furnish detailed and[Pg 382]
accurate drawings. The dusky flutings in the prismatic light of the
first of these stars have not been identified with the absorption of
any particular substance; but associated with them are metallic lines,
of which 78 were measured, and a good many identified by Huggins,
while the wave-lengths of 97 were determined by Vogel in 1871.[1411]
A photographic research, made by Keeler at the Alleghany Observatory
in 1897, convinced him that the linear spectrum of third-type
stars of the Betelgeux pattern essentially repeats that of the sun,
but with marked differences in the comparative strength of its components.[1412]
Hydrogen rays are inconspicuously present. That an
exalted temperature reigns, at least in the lower strata of the atmosphere,
is certified by the vaporisation there of matter so refractory
to heat as iron.[1413]
Nine elements—among them iron, sodium, calcium, and magnesium—were
recognised by Huggins as having stamped their signature
on the spectrum of Aldebaran; while the existence in Sirius, and
nearly all the other stars inspected, of hydrogen, together with
sundry metals, was rendered certain or highly probable. This
was admitted to be a bare gleaning of results; nor is there
reason to suppose any of his congeners inferior to our sun in complexity
of constitution. Definite knowledge on the subject, however,
made little advance beyond the point to which it was brought by
Huggins’s early experiments until spectroscopic photography became
thoroughly effective as a means of research.
In this, as in so many other directions, Sir William Huggins acted
as pioneer. In March, 1863, he obtained microscopic prints of the
spectra of Sirius and Capella.[1414] But they told nothing. No lines
were visible in them. They were mere characterless streaks of light.
Nine years later Dr. Henry Draper of New York got an impression
of four lines in the spectrum of Vega. Then Huggins attacked
the subject again in 1876, when the 18-inch speculum of the Royal
Society had come into his possession, using prisms of Iceland spar
and lenses of rock crystal; and this time with better success. A
photograph of the spectrum of Vega showed seven strong lines.[1415]
Still he was not satisfied. He waited and worked for three years
longer. At length, on December 18, 1879, he was able to communicate
to the Royal Society[1416] results answering to his expectations.
The delicacy of eye and hand needed to obtain them may be estimated
from the single fact that the image of a star had to be kept,
by continual minute adjustments, exactly projected upon a slit[Pg 383]
1/350 of an inch in width during nearly an hour, in order to give it
time to imprint the characters of its analyzed light upon a gelatine
plate raised to the highest pitch of sensitiveness. But by this time
he had secured in his wife a rarely qualified assistant.
The ultra-violet spectrum of the white stars—of which Vega was
taken as the type—was thus shown to be a very remarkable one.
A group of broad dark lines intersected it, arranged at intervals
diminishing regularly upward, and falling into a rhythmical succession
with the visible hydrogen lines. All belonged presumably to the
same substance; and the presumption was rendered a certainty by
direct photographs of the hydrogen spectrum taken by H. W. Vogel
at Berlin a few months earlier.[1417] In them seven of the white-star
series of grouped lines were visible; and the full complement of
twelve appeared on Cornu’s plates in 1886.[1418]
In yellow stars, such as Capella and Arcturus, the same rhythmical
series was partially represented, but associated with a great number
of other lines; their state, as regards ultra-violet absorption, approximating
to that of the sun; while the redder stars betrayed so
marked a deficiency in actinic rays that from Betelgeux, with an
exposure forty times that required for Sirius, only a faint spectral
impression could be obtained, and from Aldebaran, in the strictly
invisible region, almost none at all.
Thus, by the means of stellar light-analysis, acquaintance was
first made with the ultra-violet spectrum of hydrogen;[1419] and its
harmonic character, as expressed by “Balmer’s Law,” supplies a
sure test for discriminating, among newly discovered lines, those
that appertain from those that are unrelated to it. Deslandres’ five
additional prominence-rays, for instance, were at once seen to make
part of the series, because conforming to its law;[1420] while a group of
six dusky bands, photographed by Sir William and Lady Huggins,
April 4, 1890,[1421] near the extreme upper end of the spectrum of Sirius,
were pronounced without hesitation, for the opposite reason, to have
nothing to do with hydrogen. Their true affinities are still a matter
for inquiry.
As regards the hydrogen spectrum, however, the stars had further
information in reserve. Until recently, it was supposed to consist of
a single harmonic series, although, by analogy, three should co-exist.
In 1896, accordingly, a second, bound to the first by unmistakable
numerical relationships, was recognised by Professor Pickering in
spectrographs of the 2·5 magnitude star ζ Puppis,[1422] and the identification[Pg 384]
was shortly afterwards extended to prominent Wolf-Rayet
emission lines. The discovery was capped by Dr. Rydberg’s indication
of the Wolf-Rayet blue band at λ 4,688 as the fundamental
member of the third, and principal, hydrogen series.[1423] None of the
“Pickering lines” (as they may be called to distinguish them from
the “Huggins series”) can be induced to glimmer in vacuum-tubes.
They seem to characterise bodies in a primitive state,[1424] and are in
many cases associated with absorption rays of oxygen, the identification
of which by Mr. McClean in 1897[1425] was fully confirmed by
Sir David Gill.[1426] The typical “oxygen star” is β Crucis, one of the
brilliants of the Southern Cross; but the distinctive notes of its
spectrum occur in not a few specimens of the helium class. Thus,
Sir William and Lady Huggins photographed several ultra-violet
oxygen lines in β Lyræ,[1427] and found in Rigel signs of the presence of
nitrogen,[1428] which, as well as silicium, proves to be a tolerably frequent
constituent of such orbs.[1429] For some unknown reason, metalloids
tend to become effaced, as metals, in the normal course of stellar
development, exert a more and more conspicuous action.
Dr. Scheiner’s spectrographic researches at Potsdam in 1890 and
subsequently, exemplify the immense advantages of self-registration.
In a restricted section of the spectrum of Capella, he was enabled
to determine nearly three hundred lines with more precision than
had then been attained in the measurement of terrestrial spectra.
This star appeared to be virtually identical with the sun in physical
constitution, although it emits, according to the best available data,
about 140 times as much light, and is hence presumably 1,600 times
more voluminous. An equally close examination of the spectrum
of Betelgeux showed the predominance in it of the linear absorption
of iron;[1430] but its characteristic flutings do not extend to the photographic
region. Spectra of the second and third orders are for this
reason not easily distinguished on the sensitive plate.
A spectrographic investigation of all the brighter northern stars
was set on foot in 1886 at the observatory of Harvard College,
under the form of a memorial to Dr. H. Draper, whose promising
work in that line was brought to a close by his premature death
in 1882. No individual exertions could, however, have realized a
tithe of what has been and is being accomplished under Professor[Pg 385]
Pickering’s able direction, with the aid of the Draper and other
instruments, supplemented by Mrs. Draper’s liberal provision of
funds. A novel system was adopted, or, rather, an old one—originally
used by Fraunhofer—was revived.[1431] The use of a slit
was discarded as unnecessary for objects like the stars, devoid of
sensible dimensions, and giving hence a naturally pure spectrum;
and a large prism, placed in front of the object-glass, analysed at
once, with slight loss of light, the rays of all the stars in the field.
Their spectra were taken, as it were, wholesale. As many as two
hundred stars down to the eighth magnitude were occasionally
printed on a single plate with a single exposure. No cylindrical
lens was employed. The movement of the stars themselves was
turned to account for giving the desirable width to their spectra.
The star was allowed—by disconnecting or suitably regulating the
clock—to travel slowly across the line of its own dispersed light, so
broadening it gradually into a band. Excellent results were secured
in this way. About fifty lines appear in the photographed spectrum
of Aldebaran, and eight in that of Vega. On January 26, 1886,
with an exposure of thirty-four minutes, a simultaneous impression
was obtained of the spectra (among many others) of close upon
forty Pleiades. With few and doubtful exceptions, they all proved
to belong to the same type. An additional argument for the
common origin of the stars forming this beautiful group was thus
provided.[1432]
The “Draper Catalogue” of stellar spectra was published in
1890.[1433] It gives the results of a rapid analytical survey of the
heavens north of 25° of southern declination, and includes 10,351
stars, down to about the eighth magnitude. The telescope used
was of eight inches aperture and forty-five focus, its field of view—owing
to the “portrait-lens” or “doublet” form given to it—embracing
with fair definition no less than one hundred square
degrees. An objective prism eight inches square was attached, and
exposures of a few minutes were given to the most sensitive plates
that could be procured. In this way the sky was twice covered
in duplicate, each star appearing, as a rule, on four plates. The
registration of their spectra was sought to be made more distinctive
than had previously been attempted, Secchi’s first type being
divided into four, his second into five subdivisions; but the differences
regarded in them could be confidently established only for
stars above the sixth magnitude. The work supplies none the less
valuable materials for general inferences as to the distribution and
relations of the spectral types. The labour of its actual preparation[Pg 386]
was borne by a staff of ladies under the direction of Mrs. Fleming.
Materials for its completion to the southern pole have been accumulated
with the identical instrument used at Cambridge, transferred for
the purpose in 1889 to Peru, and the forthcoming “Second Draper
Catalogue” will comprise 30,000 stars in both hemispheres. As
supplements to this great enterprise, two important detailed discussions
of stellar spectra were issued in 1897 and 1901 respectively.[1434]
The first, by Miss A. C. Maury, dealt with 681 bright stars visible
in the northern hemisphere; the second, by Miss A. J. Cannon,
with 1,122 southern stars. Both authors traced, with care and
ability, the minute gradations by which the long process of stellar
evolution appears to be accomplished.
The progress of the Draper Memorial researches was marked by
discoveries of an unexampled kind.
The principle upon which “motion in the line of sight” can
be detected and measured with the spectroscope has already been
explained.[1435] It depends, as our readers will remember, upon the
removal of certain lines, dark or bright (it matters not which), from
their normal places by almost infinitesimal amounts. The whole
spectrum of the moving object, in fact, is very slightly shoved hither
or thither, according as it is travelling towards or from the eye;
but, for convenience of measurement, one line is usually picked out
from the rest, and attention concentrated upon it. The application
of this method to the stars, however, is encompassed with difficulties.
It needs a powerfully dispersive spectroscope to show line-displacements
of the minute order in question; and powerful dispersion
involves a strictly proportionate enfeeblement of light. This, where
the supply is already to a deplorable extent niggardly, can ill be
afforded; for which reason the operation of determining a star’s
approach or recession is, even apart from atmospheric obstacles, an
excessively delicate one.
It was first executed by Sir William Huggins early in 1868.[1436]
Selecting the brightest star in the heavens as the most promising
subject of experiment, he considered the F line in the spectrum of
Sirius to be just so much displaced towards the red as to indicate
(the orbital motion of the earth being deducted) recession at the
rate of twenty-nine miles a second; and the reality and direction of
the movement were ratified by Vogel and Lohse’s observation,
March 22, 1871, of a similar, but even more considerable displacement.[1437]
The inquiry was resumed by Huggins with improved
apparatus in the following year, when the velocities of thirty stars[Pg 387]
were approximately determined.[1438] The retreat of Sirius, which
proved slower than had at first been supposed, was now announced
to be shared, at rates varying from twelve to twenty-nine miles, by
Betelgeux, Rigel, Castor, Regulus, and five of the principal stars in
the Plough. Arcturus, on the contrary, gave signs of rapid approach,
as well as Pollux, Vega, Deneb in the Swan, and the brightness of
the Pointers.
Numerically, indeed, these results were encompassed with uncertainty.
Thus, Arcturus is now fully ascertained to be travelling
towards the sun at the comparatively slow pace of less than five
miles a second; and Sirius moves twice as fast in the same direction.
The great difficulty of measuring so distended a line as the
Sirian F might, indeed, well account for some apparent anomalies.
The scope of Sir William Huggins’s achievement was not, however,
to provide definitive data, but to establish as practicable the method
of procuring them. In this he was thoroughly successful, and his
success was of incalculable value. Spectroscopic investigations of
stellar movements may confidently be expected to play a leading
part in the unravelment of the vast and complex relations which we
can dimly detect as prevailing among the innumerable orbs of the
sidereal world; for it supplements the means which we possess of
measuring by direct observation movements transverse to the line
of sight, and thus completes our knowledge of the courses and
velocities of stars at ascertained distances, while supplying for all
a valuable index to the amount of perspective foreshortening of
apparent movement. Thus some, even if an imperfect, knowledge
may at length be gained of the revolutions of the stars—of the
systems they unite to form, of the paths they respectively pursue,
and of the forces under the compulsion of which they travel.
The applicability of the method to determining the orbital motions
of double stars was pointed out by Fox Talbot in 1871;[1439] but its use
for their discovery revealed itself spontaneously through the Harvard
College photographs. In “spectrograms” of ζ Ursæ Majoris
(Mizar), taken in 1887, and again in 1889, the K line was seen to be
double; while on other plates it appeared single. A careful study of
Miss A. C. Maury of a series of seventy impressions indicated for
the doubling a period of fifty-two days, and showed it to affect
all the lines in the spectrum.[1440] The only available, and no doubt the
true, explanation of the phenomenon was that two similar and nearly
equal stars are here merged into one telescopically indivisible; their
combined light giving a single or double spectrum, according as their[Pg 388]
orbital velocities are directed across or along our line of sight. The
movements of a revolving pair of stars must always be opposite in
sense, and proportionately equal in amount. That is, they at all
times travel with speeds in the inverse ratio of their masses. Hence,
unless the plane of their orbits be perpendicular to a plane passing
through the eye, there must be two opposite points where their
velocities in the line of sight reach a maximum, and two diametrically
opposite points where they touch zero. The lines in their common
spectrum would thus appear alternately double and single twice in
the course of each revolution. To that of Mizar, at first supposed
to need 104 days for its completion, a period of only twenty days
fourteen hours was finally assigned by Vogel.[1441] Anomalous spectral
effects, probably due to the very considerable eccentricity of the
orbit, long impeded its satisfactory determination. The mean distance
apart of the component stars, as now ascertained, is just twenty-two
million miles, and their joint mass quadruples that of the sun. But
these are minimum estimates. For if the orbital plane be inclined,
much or little, to the line of sight, the dimensions and mass of the
system should be proportionately increased.
An analogous discovery was made by Miss Maury in 1889. But
in the spectrum of β Aurigæ, the lines open out and close up on
alternate days, indicating a relative orbit[1442] with a radius of less than
eight million miles, traversed in about four days. This implies a
rate of travel for each star of sixty-five miles a second, and a combined
mass 4·7 times that of the sun. The components are approximately
equal, both in mass and light,[1443] and the system formed by
them is transported towards us with a speed of some sixteen miles
a second. The line-shiftings so singularly communicative proceed,
in this star, with perfect regularity.
This new class of “spectroscopic binaries” could never have been
visually disclosed. The distance of β Aurigæ from the earth, as
determined, somewhat doubtfully, by Professor Pritchard, is nearly
three and a third million times that of the earth from the sun
(parallax = 0·06′); whence it has been calculated that the greatest
angular separation of the revolving stars is only five-thousandths of
a second of arc.[1444] To make this evanescent interval perceptible, a
telescope eighty feet in aperture would be required.
The zodiacal star, Spica (α Virginis), was announced by Dr.
Vogel, April 24, 1890,[1445] to belong to the novel category, with the
difference, however, of possessing a nearly dark, instead of a brilliantly
lustrous companion. In this case, accordingly, the tell-tale spectroscopic
variations consist merely in a slight swinging to and fro of
single lines. No second spectrum leaves a legible trace on the
plate. Spica revolves in four days at the rate of fifty-seven miles a
second,[1446] or quicker, in proportion as its orbit is more inclined to
the line of sight, round a centre at a minimum distance of three
millions of miles. But the position of the second star being
unknown, the mass of the system remains indeterminate. The
lesser component of the splendid, slowly revolving binary, Castor,
is also closely double. Its spectral lines were found by Bélopolsky
in 1896[1447] to oscillate once in nearly three days, the secondary globe
being apparently quite obscure. Further study of the movements
thus betrayed elicited the fact that the major axis of the eclipse
traversed revolves in a period of 2,100 days, as a consequence, most
likely, of the flattened shape of the stars.[1448] Still more unexpected
was the simultaneous assignment, by Campbell and Newall, of a
duplex character to Capella.[1449] Here both components shine, though
with a different quality of light, one giving a pure solar spectrum,
the other claiming prismatic affinity with Procyon. Their mutual
circulation is performed in 104 days, and the radius of their orbit
cannot be less, and may be a great deal more, than 51,000,000 miles.
Hence the possibility is not excluded that the star—which has an
authentic parallax of 0·08′—may be visually resolved. Indeed,
signs of “elongation” were thought to be perceptible with the
Greenwich 28-inch refractor,[1450] while only round images could be
seen at Lick.[1451] Another noteworthy case is that of Polaris, found
by Campbell to have certainly one, and probably two obscure
attendants.[1452] Through his systematic investigations of stellar radial
velocities with the Mills spectrograph, knowledge in this department
has, since 1897, progressed so rapidly that the spectroscopic
binaries of our acquaintance already number half a hundred, and
ten times as many more doubtless lie within easy range of
detection.
Now it is evident that a spectroscopic binary, if the plane of its[Pg 390]
motion made a very small angle with the line of sight, would be a
variable star. For, during a few hours of each revolution, some at
least of its light should be cut off by a transit of its dusky companion.
Such “eclipse-stars” are actually found in the heavens.
The best and longest-known member of the group is Algol in the
Head of Medusa, the “Demon-star” of the Arabs.[1453] This remarkable
object, normally above the third magnitude, loses and regains
three-fifths of its light once in 68·8 hours, the change being completed
in about twelve hours. Its definite and limited nature, and punctual
recurrence, suggested to Goodricke of York, by whom the periodicity
of the star was discovered in 1783,[1454] the interposition of a large dark
satellite. But the conditions involved by the explanation were first
seriously investigated by Pickering in 1880.[1455] He found that the
phenomena could be satisfactorily accounted for by supposing an
obscure body 0·764 the bright star’s diameter to revolve round
it in a period identical with that of its observed variation. This
theoretical forecast was verified with singular exactitude at Potsdam
in 1889.[1456] A series of spectral photographs taken there showed
each of Algol’s minima to be preceded by a rapid recession from the
earth, and succeeded by a rapid movement of approach towards it.
They take place, accordingly, when the star is at the furthest point
from ourselves of an orbit described round an invisible companion,
the transits of which across its disc betray themselves to notice by
the luminous vicissitudes they occasion. The diameter of this orbit,
traversed at the rate of twenty-six miles a second, is just 2,000,000
miles; and it is an easy further inference from the duration and
extent of the phases exhibited that Algol itself must be (in round
numbers) one million, its attendant 830,000 miles in diameter.
Assuming both to be of the same density, Vogel found their
respective masses to be four-ninths and two-ninths that of the sun,
and their distance asunder to be 3,230,000 miles.
This singularly assorted pair of stars possibly form part of a
larger system. Their period of revolution is shorter now by six
seconds than it was in Goodricke’s time; and Dr. Chandler has
shown, by an exhaustive discussion, that its inequalities are
comprised in a cycle of about 130 years.[1457] They arise, in his view,
from a common revolution, in that period, of the close couple about[Pg 391]
a third distant body, emitting little or no light, in an orbit inclined
20° to our line of vision, and of approximately the size of that
described by Uranus round the sun. The time spent by light in
crossing this orbit causes an apparent delay in the phases of the
variable, when Algol and its eclipsing satellite are on its further
side from ourselves, balanced by acceleration while they traverse
its hither side. Dr. Chandler derives confirmation for his plausible
and ingenious theory from a supposed undulation in the line traced
out by Algol’s small proper motion; but the reality of this disturbance
has yet to be established.[1458] Meanwhile, M. Tisserand,[1459]
late Director of the Paris Observatory, preferred to account for
Algol’s inequalities on the principle later applied by Bélopolsky
to those of Castor. That is to say, he assumed a revolving line of
apsides in an elliptical orbit traversed by a pretty strongly compressed
pair of globes. The truth of this hypothesis can be
tested by close observation of the phases of the star during the next
few years.
The variable in the Head of Medusa is the exemplar of a class
including 26 recognised members, all of which doubtless represent
occulting combinations of stars. But their occultations result
merely from the accident of their orbital planes passing through our
line of sight; hence the heavens must contain numerous systems
similarly constituted, though otherwise situated as regards ourselves,
some of which, like Spica Virginis, will become known through their
spectroscopic changes, while others, because revolving in planes nearly
tangent to the sphere, or at right angles to the visual line, may never
disclose to us their true nature. Among eclipsing stars should
probably be reckoned the peculiar variables, β Lyræ and V Puppis,
each believed to consist of a pair of bright stars revolving almost in
contact.[1460] Three stars, on the other hand, distinguished by rapid and
regular fluctuations, have been proved by Bélopolsky to be attended
by non-occulting satellites, which circulate, nevertheless, in the
identical periods of light-change.
Gore’s “Catalogue of Known Variables”[1461] included, in 1884,
190 entries, and the number was augmented to 243 on its revision
in 1888.[1462] Chandler’s first list of 225 such objects,[1463] published about
the same time, received successive expansions in 1893 and 1896,[1464] and
finally included 400 entries. A new “Catalogue of Variable Stars,”[Pg 392]
still wider in scope, will shortly be issued by the German Astronomische
Gesellschaft. Mr. A. W. Roberts’s researches on southern
variables[1465] have greatly helped to give precision, while adding to the
extent of knowledge in this branch. Dr. Gould held the opinion
that most stars fluctuate slightly in brightness through surface-alterations
similar to, but on a larger scale than those of the sun;
and the solar analogy might be pushed somewhat further. It perhaps
affords a clue to much that is perplexing in stellar behaviour. Wolf
pointed out in 1852 the striking resemblance in character between
curves representing sun-spot frequency and curves representing the
changing luminous intensity of many variable stars. There were
the same steep ascent to maximum and more gradual decline to
minimum, the same irregularities in heights and hollows, and, it may
be added, the same tendency to a double maximum, and complexity
of superposed periods.[1466] It is impossible to compare the two sets of
phenomena thus graphically portrayed without reaching the conclusion
that they are of closely related origin. But the correspondence
indicated is not, as has often been hastily assumed, between
maxima of sun-spots and minima of stellar brightness, but just the
reverse. The luminous outbursts, not the obscurations of variable
stars, obey a law analogous to that governing the development of
spots on the sun. Objects of the kind do not, then, gain light through
the closing-up of dusky chasms in their photospheres, but by an
actual increase of surface-brilliancy, together with an immense
growth of these brilliant formations—prominences and faculæ—which,
in the sun, accompany, or are appended to spots. A comparison
of light-curves with curves of spot-frequency leaves no doubt
on this point, and the strongest corroborative evidence is derived
from the emergence of bright lines in the spectra of long-period
variables rising to their recurring maxima.
Every kind and degree of variability is exemplified in the heavens.
At the bottom of the scales are stars like the sun, of which the lustre
is—tried by our instrumental means—sensibly steady. At the
other extreme are ranged the astounding apparitions of “new,” or
“temporary” stars. Within the last thirty-six years eleven of these
stellar guests (as the Chinese call them) have presented themselves,
and we meet with a twelfth no farther back than April 27, 1848.
But of the “new star” in Ophiuchus found by Mr. Hind on that
night, little more could be learnt than of the brilliant objects of the
same kind observed by Tycho and Kepler. The spectroscope had
not then been invented. Let us hear what it had to tell of later
arrivals.
About thirty minutes before midnight of May 12, 1866, Mr. John[Pg 393]
Birmingham of Millbrook, near Tuam, in Ireland, saw with astonishment
a bright star of the second magnitude unfamiliarly situated
in the constellation of the Northern Crown. Four hours earlier,
Schmidt of Athens had been surveying the same part of the heavens,
and was able to testify that it was not visible there. That is to say,
a few hours, or possibly a few minutes, sufficed to bring about a
conflagration, the news of which may have occupied hundreds of
years in travelling to us across space. The rays which were its
messengers, admitted within the slit of Sir William Huggins’s
spectroscope, May 16, proved to be of a composition highly significant
as to the nature of the catastrophe. The star—which had
already declined below the third magnitude—showed what was
described as a double spectrum. To the dusky flutings of Secchi’s
third type four brilliant rays were added.[1467] The chief of these agreed
in position with lines of hydrogen; so that the immediate cause of
the outburst was inferred to have been the eruption, or ignition, of
vast masses of that subtle kind of matter, the universal importance
of which throughout the cosmos is one of the most curious facts
revealed by the spectroscope.
T Coronæ (as the new star was called) quickly lost its adventitious
splendour. Nine days after its discovery it was again invisible to
the naked eye. It is now a pale yellow, slightly variable star near
the tenth magnitude, and finds a place as such in Argelander’s
charts.[1468] It was thus obscurely known before it made its sudden
leap into notoriety.
The next “temporary,” discovered by Dr. Schmidt at Athens,
November 24, 1876, could lay no claim to previous recognition even
in that modest rank. It was strictly a parvenu. There was no
record of its existence until it made its appearance as a star of nearly
the third magnitude, in the constellation of the Swan. Its spectrum
was examined, December 2, by Cornu at Paris,[1469] and a few days later
by Vogel and O. Lohse at Potsdam.[1470] It proved of a closely similar
character to that of T Coronæ. A range of bright lines, including
those of hydrogen, and probably of helium, stood out from a continuous
background impressed with strong absorption. It may be
presumed that in reality the gaseous substances, which, by their
sudden incandescence, had produced the apparent conflagration, lay
comparatively near the surface of the star, while the screen of cooler
materials intercepting large portions of its light was situated at a
considerable elevation in its atmosphere.
The object, meanwhile, steadily faded. By the end of the year it[Pg 394]
was of no more than seventh magnitude. After the second week of
March, 1877, strengthening twilight combined with the decline of
its radiance to arrest further observation. It was resumed, September
2, at Dunecht, with a strange result. Practically the whole of
its scanty light (it had then sunk below the tenth magnitude) was
perceived to be gathered into a single bright line in the green, and
that the most characteristic line of gaseous nebulæ.[1471] The star had,
in fact, so far as outward appearance was concerned, become transformed
into a planetary nebula, many of which are so minute as
to be distinguishable from small stars only by the quality of their
radiations. It is now, having sunk to about the fourteenth magnitude,[1472]
entirely beyond the reach of spectroscopic scrutiny.
Perhaps none of the marvellous changes witnessed in the heavens
has given a more significant hint as to their construction than the
stellar blaze kindled in the heart of the great Andromeda nebula
some undetermined number of years or centuries before its rays
reached the earth in the month of August, 1885. The first
published discovery was by Dr. Hartwig at Dorpat on August 31;
but it was found to have been already seen, on the 19th, by
Mr. Isaac W. Ward of Belfast, and on the 17th by M. Ludovic
Gully of Rouen. The negative observations, on the 16th, of Tempel[1473]
and Max Wolf, limited very narrowly the epoch of the apparition.
Nevertheless, it did not, like most temporaries, attain its maximum
brightness all at once. When first detected, it was of the ninth, by
September 1 it had risen to the seventh magnitude, from which it
so rapidly fell off that in March it touched the limit of visibility
(sixteenth magnitude) with the Washington 26-inch. Its light
bleached very perceptibly as it faded.[1474] During the earlier stages of
its decline, the contrast was striking between the sharply defined,
ruddy disc of the star, and the hazy, greenish-white background
upon which it was projected,[1475] and with which it was inevitably
suggested to be in some sort of physical connection.
Let us consider what evidence was really available on this point.
To begin with, the position of the star was not exactly central. It
lay sixteen seconds of arc to the south-west of the true nebular
nucleus. Its appearance did not then signify a sudden advance of
the nebula towards condensation, nor was it attended by any visible
change in it save the transient effect of partial effacement through
superior brightness.
Equally indecisive information was derived from the spectroscope.[Pg 395]
To Vogel, Hasselberg, and Young, the light of the “Nova” seemed
perfectly continuous; but Huggins caught traces of bright lines on
September 2, confirmed on the 9th;[1476] and Copeland succeeded, on
September 30, in measuring three bright bands with an acute-angled
prism specially constructed for the purpose.[1477] A shimmer of F was
suspected, and had also been perceived by Mr. O. T. Sherman of
Yale College. Still, the effect was widely different from that of the
characteristic blazing spectrum of a temporary star, and prompted
the surmise that here, too, a variable might be under scrutiny. The
star, however, was certainly so far “new” that its rays, until their
sudden accession of strength, were too feeble to affect even our
reinforced senses. Not one of the 1,283 small stars recorded in
charts of the nebula could be identified with it; and a photograph
taken by Dr. Common, August 16, 1884, on which a multitude of
stars down to the fifteenth magnitude had imprinted themselves,
showed the uniform, soft gradation of nebulous light to be absolutely
unbroken by a stellar indication in the spot reserved for the future
occupation of the “Nova.”[1478]
So far, then, the view that its relation to the nebula was a merely
optical one might be justified; but it became altogether untenable
when it was found that what was taken to be a chance coincidence
had repeated itself within living memory. On the 21st of May,
1860, M. Auwers perceived at Königsberg a seventh magnitude star
shining close to the centre of a nebula in Scorpio, numbered 80 in
Messier’s Catalogue.[1479] Three days earlier it certainly was not there,
and three weeks later it had vanished. The effect to Mr. Pogson
(who independently discovered the change, May 28)[1480] was as if the
nebula had been replaced by a star, so entirely were its dim rays
overpowered by the concentrated blaze in their midst. Now, it is
simply incredible that two outbursts of so uncommon a character
should have accidentally occurred just on the line of sight between
us and the central portions of two nebulæ; we must, then, conclude
that they showed on these objects because they took place in them.
The most favoured explanation is that they were what might be
called effects of overcrowding—that some of the numerous small
bodies, presumably composing the nebulæ, jostled together, in their
intricate circlings, and obtained compensation in heat for their
sacrifice of motion. But this is scarcely more than a plausible
makeshift of perplexed thought. Mr. W. H. S. Monck, on the
other hand, has suggested that new stars appear when dark bodies
are rendered luminous by rushing through the gaseous fields of[Pg 396]
space,[1481] just as meteors kindle in our atmosphere. The idea, which
has been revived and elaborated by Dr. Seeliger of Munich,[1482] is
ingenious, but was not designed to apply to our present case.
Neither of the objects distinguished by the striking variations just
described is of gaseous constitution. That in Scorpio appears under
high magnifying powers as a “compressed cluster”; that in
Andromeda is perhaps, as Sir J. Herschel suggested, “optically
nebulous through the smallness of its constituent stars”[1483]—if stars
they deserve to be called.
On the 8th of December, 1891, Dr. Max Wolf took a photograph
of the region about χ Aurigæ. No stranger so bright as the
eighth magnitude was among the stars depicted upon it. On the
10th, nevertheless, a stellar object of the fifth magnitude, situated
a couple of degrees to the north-east of β Tauri and previously
unrecorded, where eleventh magnitude stars appeared, imprinted
itself upon a Harvard negative. Subsequent photographs taken at
the same place showed it to have gained about half a magnitude by
the 20th; but the plates were not then examined, and the discovery
was left to be modestly appropriated by an amateur, the Rev. Dr.
Anderson of Edinburgh, by whom it was announced, February 1,
1892, through the medium of an anonymous postcard, to Dr. Copeland,
the Astronomer Royal for Scotland.[1484] By him and others, the
engines of modern research were promptly set to work. And to
good purpose. Nova Aurigæ was the first star of its kind studied
by the universal chemical method. It is the first, accordingly, of
which authentic records can be handed down to posterity. They
are of a most remarkable character. The spectrum of the new
object was photographed at Stonyhurst and South Kensington on
February 3; a few days later, at Harvard and Lick in America, at
Potsdam and Hérény on the Continent of Europe. But by far the
most complete impression was secured, February 22, with an exposure
of an hour and three-quarters, by Sir William and Lady
Huggins, through whose kindness it is reproduced in Plate V.,
Fig. 1. The range of bright lines displayed in it is of astonishing
vividness and extent. It includes all the hydrogen rays dark in the
spectrum of Sirius (separately printed for comparison), besides many
others still more refrangible, as yet unidentified. Very significant,
too, is the marked character of the great prominence lines H and K.
The visual spectrum of the Nova was splendidly effective. A
Photographic and Visual Spectrum of Nova Aurigæ.”
Fig. 1.—From a Photograph taken by Sir William and Lady Huggins, Feb. 22, 1892.
Fig. 2.—From a Drawing made by Lady Huggins, Feb. 2 to 6, 1892.
[Pg 397]
quartette of brilliant green rays, two of them due to helium, caught
the eye; and they had companions too numerous to be easily
counted. The hydrogen lines were broad and bright; C blazed, as
Mr. Espin said, “like a danger-signal on a dark night”; the sodium
pair were identified at Tulse Hill, and the yellow helium ray was
suspected to lurk close beside them. Fig. 2 in the same plate shows
the spectrum as it was seen and mapped by Lady Huggins, February 2
to 6, together with the spectra employed to test the nature of the emissions
dispersed in it. One striking feature will be at once remarked.
It is that of the pairing of bright with dark lines. Both in the
visible and the photographic regions this singular peculiarity was
unmistakable; and since the two series plainly owned the same
chemical origin, their separate visibility implied large displacement.
Otherwise they would have been superposed, not juxtaposed.
Measurements of the bright rays, accordingly, showed them to be
considerably pushed down towards the red, while their dark companions
were still more pushed up towards the blue end. Thus
the spectrum of Nova Aurigæ, like that of β Lyræ, with which it
had many points in common, appeared to be really double. It was
supposed to combine the light of two distinct bodies, one, of a
gaseous nature, moving rapidly away from the earth, the other,
giving a more sunlike spectrum, approaching it with even higher
speed. The relative velocity determined at Potsdam for these
oppositely flying masses amounted to 550 miles a second.[1485] And
this prodigious rate of separation was fully maintained during six
weeks! It did not then represent a mere periastral rush-past.[1486] To
the bodies exhibiting its effects, and parting company for ever under
its stress, it must have belonged, with slight diminution, in perpetuity.
The luminous outburst by which they became visible was
explained by Sir William Huggins, in a lecture delivered at the
Royal Institution, May 13, 1892, on the tidal theory of Klinkerfues
and Wilsing. Disturbances and deformations due to the mutual
attraction of two bulky globes at a close approach would, he considered,
“give rise to enormous eruptions of the hotter matters
from within, immensely greater, but similar in kind, to solar
eruptions; and accompanied, probably, by large electrical disturbances.”
The multiple aspect and somewhat variable character of
both bright and dark lines were plausibly referred to processes of
“reversal,” such as are nearly always in progress above sun-spots;
but the long duration of the star’s suddenly acquired lustre did not
easily fit in with the adopted rationale. A direct collision, on the[Pg 398]
other hand, was out of the question, since there had obviously been
little, if any, sacrifice of motion; and the substitution of a nebula
for one of the “stars”[1487] compelled recourse to scarcely conceivable
modes of action for an explanation of the perplexing peculiarities of
the compound spectrum.
An unexpected dénouement, however, threw all speculations off
the track. The Nova contained most of its brightness, fluctuations
notwithstanding, until March 9; after which date it ran swiftly
and uniformly down towards what was apprehended to be total
extinction. No marked change of spectrum attended its decline.
When last examined at Tulse Hill, March 24, all the more essential
features of its prismatic light were still faintly recognisable.[1488]
The object was steadily sinking on April 26, when a (supposed)
final glimpse of it was caught with the Lick 36-inch.[1489] It was
then of about the sixteenth magnitude. But on August 17 it had
sprung up to the tenth, as Professors Holden, Schaeberle, and
Campbell perceived with amazement on turning the same instrument
upon its place. And to Professor Barnard it appeared, two nights
later, not only revived, but transformed into the nucleus of a
planetary nebula, 3′ across.[1490] The reality of this seeming distension,
however, at once disputed, was eventually disproved. It unquestionably
arose from the imperfect focussing power of the telescope for
rays of unusual quality.[1491]
The rekindled Nova was detected in this country by Mr. H. Corder,
on whose notification Mr. Espin, on August 21, examined its nearly
monochromatic spectrum.[1492] The metamorphosis of Nova Cygni
seemed repeated.[1493] The light of the new object, like that of its
predecessor, was mainly concentrated in a vivid green band,
identified with the chief nebular line by Copeland,[1494] Von Gothard,[1495]
and Campbell.[1496] The second nebular line was also represented.
Indeed, the last-named observer recognised nearly all the eighteen
lines measured by him in the Nova as characteristic of planetary
nebulæ.[1497] Of particular interest is the emergence in the star-spectrum
photographed by Von Gothard of an ultra-violet line
originally discovered at Tulse Hill in the Orion nebula, which is
also very strong in the Lyra annular nebula,[Pg 399]
Obviously, then, the physical constitution of Nova Aurigæ became
profoundly modified during the four months of its invisibility. The
spectrum of February was or appeared compound; that of August
was simple; it could be reasonably associated only with a single
light-source. Many of the former brilliant lines, too, had vanished,
and been replaced by others, at first inconspicuous or absent. As
a result, the solar-prominence type, to which the earlier spectrum
had seemed to conform, was completely effaced in the later. The
cause of these alterations remains mysterious, yet its effects
continue. The chromatic behaviour of the semi-extinct Nova,
when scrutinised with great refractors, shows its waning light to
be distinctly nebular.[1498] Like nearly all its congeners, the star is
situated in the full stream of the Milky Way, and we learn without
surprise that micrometrical measures by Burnham and Barnard[1499]
failed to elicit from it any sign of parallactic shifting. It is hence
certain that the development of light, of which the news reached
the earth in December, 1891, must have been on a vast scale, and
of ancient date. Nova Aurigæ at its maximum assuredly exceeded
the sun many times in brightness; and its conflagration can scarcely
have occurred less, and may have occurred much more, than a
hundred years ago.
By means of the photographic surveys of the skies, carried on in
both hemispheres under Professor Pickering’s superintendence, such
amazing events have been proved to be of not infrequent occurrence.
Within six years five new stars were detected from Draper Memorial,
or chart-plates by Mrs Fleming, besides the retrospective discovery
of a sixth which had rapidly burnt itself out, eight years previously,
in Perseus.[1500] Nova Normæ was the immediate successor of Nova
Aurigæ; Nova Carinæ and Nova Centauri lit up in 1895, the latter
in a pre-existent nebula; Nova Sagittarii and Nova Aquilæ attained
brief maxima in 1898 and 1899 respectively. Now, three out of
these five stars reproduced with singular fidelity the spectrum of
Nova Aurigæ; they displayed the same brilliant rays shadowed,
invariably on their blue sides, by dark ones. Palpably, then, the
arrangement was systematic and significant; it could not result
merely from the casually directed, opposite velocities of bodies
meeting in space. The hypothesis of stellar encounters accordingly
fell to the ground, and has been provided with no entire satisfactory
substitute. Most speculators now fully recognise that motion-displacements[Pg 400]
cannot be made to account for the doubled spectra of
Novæ, and seek recourse instead to some kind of physical agency
for producing the observed effect.[1501] And since this is also visible in
certain permanent, though peculiar objects—notably in P Cygni,
β Lyræ, and η Carinæ—the acting cause must also evidently be
permanent and inherent.
The “new star of the new century”[1502] was a visual discovery.
Dr. Anderson duplicated, with added éclat, his performance of nine
years back. In the early morning of February 22, 1901, he
perceived that Algol had a neighbour of nearly its own brightness,
which a photograph taken by Mr. Stanley Williams, at Brighton,
proved to have risen from below the twelfth magnitude within the
preceding 28 hours. And it was still swiftly ascending. On the
23rd, it outshone Capella; for a brief space it took rank as the
premier star of the northern hemisphere. A decline set in promptly,
but was pursued hesitatingly. The light fluctuated continually over
a range of a couple of magnitudes, and with a close approach, during
some weeks, to a three-day periodicity. A year after the original
outburst, the star was still conspicuous with an opera-glass. The
spectrum underwent amazing changes. At first continuous, save for
fine dark lines of hydrogen and helium, it unfolded within forty-eight
hours a composite range of brilliant and dusky bands disposed in the
usual fashion of Novæ. These lasted until far on in March, when
hydrogen certainly, and probably other substances as well, ceased to
exert any appreciable absorptive action. Blue emissions of the
Wolf-Rayet type then became occasionally prominent, in remarkable
correspondence with the varying lustre of the star;[1503] finally, a band
at λ 3969, found by Wright at Lick to characterise nebular spectra,[1504]
assumed abnormal importance; and in July the nebular transformation
might be said to be complete. Striking alterations of colour
attended these spectral vicissitudes. White to begin with, the star
soon turned deep red, and its redness was visibly intensified at each
of its recurring minima of light. Blanching, however, ensued upon
the development of its nebulous proclivities; and its surviving rays
are of a steely hue.
All the more important investigations of Nova Persei were
conducted by photographic means. Libraries of spectral plates were
collected at the Yerkes and Lick Observatories, at South Kensington,
Stonyhurst, and Potsdam, and await the more exhaustive interpretation
of the future. Meanwhile, extraordinary revelations have been
supplied by immediate photographic delineation. On August 22[Pg 401]
and 23, 1901, Professor Max Wolf, by long exposures with the 16-inch
Bruce twin objectives of the Königstuhl Observatory (Heidelberg),
obtained indications of a large nebula finely ramified, extending
south-east of the Nova;[1505] and the entire formation came out in four
hours with the Yerkes 2-foot reflector, directed to it by Mr. Ritchey
on September 20.[1506] It proved to be a great spiral encircling, and
apparently emanating from, the star. But if so, tumultuously, and
under stress of catastrophic impulsions. A picture obtained by
Mr. Perrine with the Crossley refractor, in 7h. 19m., on November 7
and 8, disclosed the progress of a startling change.[1507] Comparison
with the Yerkes photograph showed that during the intervening
48 days four clearly identifiable condensations had become displaced,
all to the same extent of about 90 seconds of arc, and in fairly concordant
directions, suggesting motion round the Nova as well as away
from it. The velocity implied, however, is so prodigious as virtually
to exclude the supposition of a bodily transport of matter. It
should be at the rate of no less than twenty thousand miles a second,
admitting the object to be at a distance from us corresponding to an
annual parallax of one-tenth of a second, and actual measurements
show it to be indefinitely more remote. The fact of rapid variations
in the nebula was reaffirmed, though with less precision, from
Yerkes photographs of November 9 and 13, Mr. Ritchey inferring a
general expansion of its southern portions.[1508] Much further evidence
must be at hand before a sane judgment can be formed as to the
nature of the strange events taking place in that secluded corner of
the Galaxy.[1509] And it is highly probable that the illumination of the
nebulous wreaths round the star will prove no less evanescent than
the blazing of the star itself.
We have been compelled somewhat to anticipate our narrative as
regards inquiries into the nature of nebulæ. The excursions of
opinion on the point were abruptly restricted and defined by the
application to them of the spectroscope. On August 29, 1864, Sir
William Huggins sifted through his prisms the rays of a bright
planetary nebula in Draco.[1510] To his infinite surprise, they proved
to be mainly of one colour. In other words, they avowed their
origin from a mass of glowing vapour. As to what kind of vapour
it might be by which Herschel’s conjecture of a “shining fluid”
diffused at large throughout the cosmos was thus unexpectedly
verified, an answer only partially satisfactory could be afforded.[Pg 402]
The conspicuous bright line of the Draco nebula seemed to agree in
position with one emitted by nitrogen, but has since proved to be
distinct from it; of its two fainter companions, one was unmistakably
the F line of hydrogen, while the other, in position intermediate
between the two, still remains unidentified.
By 1868 Huggins had satisfactorily examined the spectra of about
seventy nebulæ, of which one-third displayed a gaseous character.[1511]
All of these gave the green ray fundamental to the nebular spectrum,
and emanating from an unknown form of matter named by Sir
William Huggins “nebulum.” It is associated with seven or eight
hydrogen lines, with three of “yellow” helium, and with a good
many of undetermined origin. The absence of the crimson radiation
of hydrogen—perceived with difficulty only in some highly condensed
objects—is an anomaly very imperfectly explained as a physiological
effect connected with the extreme faintness of nebular light.[1512] An
approximate coincidence between the chief nebular line and a
“fluting” of magnesium having been alleged by Lockyer in support
of his meteoritic hypothesis of nebular constitution, it became of
interest to ascertain its reality. The task was accomplished by Sir
William and Lady Huggins in 1889 and 1890,[1513] and by Professor
Keeler, with the advantages of the Mount Hamilton apparatus and
atmosphere, in 1890-91.[1514] The upshot was to show a slight but sure
discrepancy as to place, and a marked diversity as to character,
between the two qualities of light. The nebular ray (wave-length
5,007 millionths of a millimetre) is slightly more refrangible than
the magnesium fluting-edge, and it is sharp and fine, with no trace of
the unilateral haze necessarily clinging even to the last “remnant”
of a banded formation.
Planetary and annular nebulæ are, without exception, gaseous, as
well as those termed “irregular,” which frequent the region of the
Milky Way. Their constitution usually betrays itself to the eye by
their blue or greenish colour; while those yielding a continuous
spectrum are of a dull white. Among the more remarkable of these
are the well-known nebula in Andromeda, and the great spiral in
Canes Venatici; and, as a general rule, the emissions of all such
nebulæ as present the appearance of star-clusters grown misty
through excessive distance are of the same kind. It would, however,[Pg 403]
be eminently rash to conclude thence that they are really
aggregations of sun-like bodies. The improbability of such an
inference has been greatly enhanced by the occurrence, at an interval
of a quarter of a century, of stellar outbursts in the midst of two of
them. For it is practically certain that the temporary stars were
equally remote with the hazy formations they illuminated; hence, if
the constituent particles of the latter be suns, the incomparably vaster
orbs by which their feeble light was well-nigh obliterated must, as
was argued by Mr. Proctor, have been on a scale of magnitude such
as the imagination recoils from contemplating. Nevertheless, Dr.
Scheiner, not without much difficulty, obtained, in January, 1899,
spectrographic prints of the Andromeda nebula, indicative, he
thought, of its being a cluster of solar stars.[1515] Sir William and Lady
Huggins, on the other hand, saw, in 1897, bright intermixed with
dark bands in the spectrum of the same object.[1516] And Mr. Maunder
conjectures all “white” nebulæ to be made up of sunlets in which
the coronal element predominates, while chromospheric materials
assert their presence in nebulæ of the “green” variety.[1517]
Among the ascertained analogies between the stellar and nebular
systems is that of variability of light. On October 11, 1852,
Mr. Hind discovered a small nebula in Taurus. Chacornac observed
it at Marseilles in 1854, but was confounded four years later to
find it vanished. D’Arrest missed it October 3, and redetected it
December 29, 1861. It was easily seen in 1865-66, but invisible in
the most powerful instruments from 1877 to 1880.[1518] Barnard, however,
made out an almost evanescent trace of it, October 15, 1890, with
the great Lick telescope,[1519] and saw it easily in the spring of 1895, while
six months later it evaded his most diligent search.[1520] Then again,
on September 28, 1897, the Yerkes 40-inch disclosed it to him as a
mere shimmer at the last limit of visibility; and it came out in three
diffuse patches on plates to which, on December 6 and 27, 1899,
Keeler gave prolonged exposures with the Crossley reflector.[1521]
Moreover, a fairly bright adjacent nebula, perceived by O. Struve
in 1868, and observed shortly afterwards by d’Arrest, has totally
vanished, and was most likely only a temporary apparition. These
are the most authentic instances of nebular variability. Many others
have been more or less plausibly alleged;[1522] but Professor Holden’s persuasion,
acquired from an exhaustive study of the records since 1758,[1523]
that the various parts of the Orion nebula fluctuate continually in[Pg 404]
relative lustre, has not been ratified by photographic evidence.
The case of the “trifid” nebula in Sagittarius, investigated by
Holden in 1877,[1524] is less easily disposed of. What is certain is that
a remarkable triple star, centrally situated, according to the observations
of both the Herschels, 1784-1833, in a dark space between the
three great lobes of the nebula, is now, and has been since 1839,
densely involved in one of them; and since the hypothesis of relative
motion is on many grounds inadmissible, the change that has apparently
taken place must be in the distribution of light. One no
less conspicuous was adduced by Mr. H. C. Russell, director of the
Sydney Observatory.[1525] A particularly bright part of the great Argo
nebula, as drawn by Sir John Herschel, has, it would seem, almost
totally disappeared. He noticed its absence in 1871, using a 7-inch
telescope, failed equally later on to find it with an 11-1/2-inch, and
his long-exposure photographs show no vestige of it. The same
structure is missing from, or scarcely traceable in, a splendid picture
of the nebula taken by Sir David Gill in twelve hours distributed
over four nights in March, 1892.[1526] An immense gaseous expanse has,
it would seem, sunk out of sight. Materially it is no doubt there;
but the radiance has left it.
Nebulæ have no ascertained proper motions. No genuine change
of place in the heavens has yet been recorded for any one of them.
All equally hold aloof, so far as telescopic observation shows, from
the busy journeyings of the stars. This seeming immobility is
partly an effect of vast distance. Nebular parallax has, up to the
present, proved evanescent, and nebular parallactic drift, in response
to the sun’s advance through space, remains likewise imperceptible.[1527]
It may hence be presumed that no nebulæ occur within the sphere
occupied by the nearer stars. But the difficulty of accurately
measuring such objects must also be taken into account. Displacements
which would be conspicuous in stars might easily escape
detection in ill-defined, hazy masses. Thus the measures executed
by d’Arrest in 1857[1528] have not yet proved effective for their designed
purpose of contributing to the future detection of proper motions.
Some determinations made by Mr. Burnham with the Lick refractor
in 1891,[1529] will ultimately afford a more critical test. He found that
nearly all planetary nebulæ include a sharp stellar nucleus, the[Pg 405]
position of which with reference to neighbouring stars could be fixed
no less precisely than if it were devoid of nebulous surroundings.
Hence, the objects located by him cannot henceforward shift, were
it only to the extent of a small fraction of a second, without the
fact coming to the knowledge of astronomers.
The spectroscope, however, here as elsewhere, can supplement the
telescope; and what it has to tell, it tells at once, without the necessity
of waiting on time to ripen results. Sir William Huggins
made, in 1874,[1530] the earliest experiments on the radial movements of
nebulæ. But with only a negative upshot. None of the six objects
examined gave signs of spectral alteration, and it was estimated
that they must have done so had they been in course of recession
from or approach towards the earth by as much as twenty-five
miles a second. With far more powerful appliances, Professor
Keeler renewed the attempt at Lick in 1890-91. His success was
unequivocal. Ten planetary nebulæ yielded perfectly satisfactory
evidence of line-of-sight motion,[1531] the swiftest traveller being the
well-known greenish globe in Draco,[1532] found to be hurrying
towards the earth at the rate of forty miles a second. For the
Orion nebula, a recession of about eleven miles was determined,[1533]
the whole of which may, however, very well belong to the solar
system itself, which, by its translation towards the constellation Lyra,
is certainly leaving the great nebula pretty rapidly behind. The
anomaly of seeming nebular fixity has nevertheless been removed;
and the problem of nebular motion has begun to be solved through
the demonstrated possibility of its spectroscopic investigation.
Keeler’s were the first trustworthy determinations of radial
motion obtained visually. That the similar work on the stars
begun at Greenwich in 1874, and carried on for thirteen years,
remained comparatively unfruitful, was only what might have been
expected, the instruments available there being altogether inadequate
for the attainment of a high degree of accuracy.
The various obstacles in the way of securing it were overcome by
the substitution of the sensitive plate for the eye. Air-tremors are
thus rendered comparatively innocuous; and measurements of stellar
lines displaced by motion with reference to fiducial lines from
terrestrial sources, photographed on the same plates, can be
depended upon within vastly reduced limits of error. Studies for
the realisation of the “spectrographic” method were begun by
Dr. Vogel and his able assistant, Dr. Scheiner, at Potsdam in 1887.
Their preliminary results, communicated to the Berlin Academy of[Pg 406]
Sciences, March 15, 1888, already showed that the requirements for
effective research in this important branch were at last about to be
complied with. An improved instrument was erected in the autumn
of the same year, and the fifty-one stars, bright enough for determination
with a refractor of 11 inches aperture, were promptly
taken in hand. A list of their motions in the line of sight,
published in 1892,[1534] was of high value, both in itself and for what
it promised. One noteworthy inference from the data it collected
was that the eye tends, under unfavourable circumstances, to
exaggerate the line-displacements it attempts to estimate. The
velocities photographically arrived at were of much smaller amounts
than those visually assigned. The average speed of the Potsdam
stars came out only 10·4 miles a second, the quickest among them
being Aldebaran, with a recession of thirty miles a second. More
lately, however, Deslandres and Campbell have determined for
ζ Herculis and η Cephei respectively approaching rates of forty-four
and fifty-four miles a second.
The installation, in 1900, of a photographic refractor 31-1/2 inches
in aperture, coupled with a 20-inch guiding telescope, will enable
Dr. Vogel to investigate spectrographically some hundreds of stars
fainter than the second magnitude; and the materials thus accumulated
should largely help to provide means for a definite and complete
solution of the more than secular problem of the sun’s advance
through space. The solution should be complete, because including
a genuine determination of the sun’s velocity, apart from assumptions
of any kind. M. Homann’s attempt, in 1885,[1535] to extract some
provisional information on the subject from the radial movements
of visually determined stars gave a fair earnest of what might be
done with materials of a better quality. He arrived at a goal for
the sun’s way shifted eastward to the constellation Cygnus—a result
congruous with the marked tendency of recently determined apexes
to collect in or near Lyra; and the most probable corresponding
velocity seemed to be about nineteen miles a second, or just that of
the earth in its orbit. A more elaborate investigation of the same
kind, based by Professor Campbell in 1900[1536] upon the motions of
280 stars, determined with extreme precision, suffered in completeness
through lack of available data from the southern hemisphere.
The outcome, accordingly, was an apex most likely correctly placed
as regards right ascension, but displaced southward by some fifteen
degrees. The speed of twelve miles a second, assigned to the solar
translation, approximates doubtless very closely to the truth.
A successful beginning was made in nebular spectrography by
Sir William Huggins, March 7, 1882.[1537] Five lines in all stamped
themselves upon the plate during forty-five minutes of exposure to
the rays of the strange object in Orion. Of these, four were the
known visible lines, and a fifth, high up in the ultra-violet, at
wave-length 3,727, has evidently peculiar relationships, as yet imperfectly
apprehended. It is strong in the spectra of many
planetaries; it helped to characterise the nebular metamorphosis of
Nova Aurigæ, yet failed to appear in Nova Persei. Two additional
hydrogen lines, making six in all, were photographed at Tulse Hill,
from the Orion nebula, in 1890;[1538] and Dr. Copeland’s detection in
1886[1539] of the yellow ray D3 gave the first hint of the presence of
helium in this prodigious formation. Nor are there wanting spectroscopic
indications of its physical connection with the stars visually
involved in it. Sir William and Lady Huggins found a plate exposed
February 5, 1888, impressed with four groups of fine bright lines,
originating in the continuous light of two of the trapezium-stars,
but extending some way into the surrounding nebula.[1540] And Dr.
Scheiner[1541] argued a wider relationship from the common possession,
by the nebula and the chief stars in the constellation Orion, of a
blue line, bright in the one case, dark in the others, since identified
as a member of one of the helium series.
The structural unity of the stellar and nebular orders in this
extensive region of the sky has also, by direct photographic means,
been unmistakably affirmed.
The first promising autographic picture of the Orion nebula was
obtained by Draper, September 30, 1880.[1542] The marked approach
towards a still more perfectly satisfactory result shown by his plates
of March, 1881 and 1882, was unhappily cut short by his death.
Meanwhile, M. Janssen was at work in the same field from 1881,
with his accustomed success.[1543] But Dr. A. Ainslie Common left all
competitors far behind with a splendid picture, taken January 30,
1883, by means of an exposure of thirty-seven minutes in the focus
of his 3-foot silver-on-glass mirror.[1544] Photography may thereby be
said to have definitely assumed the office of historiographer to the
nebulæ, since this one impression embodies a mass of facts hardly to[Pg 408]
be compassed by months of labour with the pencil, and affords a
record of shape and relative brightness in the various parts of the
stupendous object it delineates which must prove invaluable to the
students of its future condition. Its beauty and merit were officially
recognised by the award of the Astronomical Society’s Gold Medal
in 1884.
A second picture of equal merit, obtained by the same means,
February 28, 1883, with an exposure of one hour, is reproduced in
the frontispiece. The vignette includes two specimens of planetary
photography. The Jupiter, with the great red spot conspicuous
in the southern hemisphere, is by Dr. Common. It dates from
September 3, 1879, and was accordingly one of the earliest results
with his 36-inch, the direct image in which imprinted itself in a
fraction of a second, and was subsequently enlarged on paper about
twelve times. The exquisite little picture of Saturn was taken at
Paris by MM. Paul and Prosper Henry, December 21, 1885, with
their 13-inch photographic refractor. The telescopic image was in
this case magnified eleven times previous to being photographed, an
exposure of about five seconds being allowed; and the total enlargement,
as it now appears, is nineteen times. A trace of the dusky
ring perceptible on the original negative is lost in the print.
A photograph of the Orion nebula taken by Dr. Roberts in
67 minutes, November 30, 1886, made a striking disclosure of the
extent of that prodigious object. More than six times the nebulous
area depicted on Dr. Common’s plates is covered by it, and it plainly
shows an adjacent nebula, separately catalogued by Messier, to
belong to the same vast formation.
This disposition to annex and appropriate has come out more
strongly with every increase of photographic power. Plates
exposed at Harvard College in March, 1888, with an 8-inch
portrait-lens (the same used in the preparation of the Draper
Catalogue) showed the old-established “Fish-mouth” nebula not
only to involve the stars of the sword-handle, but to be in tolerably
evident connection with the most easterly of the three belt-stars,
from which a remarkable nebulous appendage was found to proceed.[1545]
A still more curious discovery was made by W. H. Pickering in
1889.[1546] Photographs taken in three hours from the summit of
Wilson’s Peak in California revealed the existence of an enormous,
though faint spiral structure, enclosing in its span of nearly
seventeen degrees the entire stellar and nebulous group of the
Belt and Sword, from which it most likely, although not quite
traceably, issues as if from a nucleus. A startling glimpse is thus
afforded of the cosmical importance of that strange “hiatus” in the[Pg 409]
heavens which excited the wonder of Huygens in 1656. The inconceivable
attenuation of the gaseous stuff composing it was virtually
demonstrated by Mr. Ranyard.[1547]
In March, 1885, Sir Howard Grubb mounted for Dr. Isaac
Roberts, at Maghull, near Liverpool (his observatory has since been
transferred to Crowborough in Sussex), a silver-on-glass reflector of
twenty inches aperture, constructed expressly for use in celestial
photography. A series of nebula-pictures, obtained with this fine
instrument, have proved highly instructive both as to the structure
and extent of these wonderful objects; above all, one of the great
Andromeda nebula, to which an exposure of three hours was given
on October 1, 1888.[1548] In it a convoluted structure replaced and
rendered intelligible the anomalously rifted mass seen by Bond
in 1847.[1549] The effects of annular condensation appeared to have
stamped themselves upon the plate, and two attendant nebulæ
presented the aspect of satellites already separated from the parent
body, and presumably revolving round it. The ring-nebula in Lyra
was photographed at Paris in 1886, and shortly afterwards by Von
Gothard with a 10-inch reflector,[1550] and he similarly depicted in 1888
the two chief spiral and other nebulæ.[1551] Photographs of the Lyra
nebula taken at Algiers in 1890,[1552] and at the Vatican observatory in
1892,[1553] were remarkable for the strong development of a central star,
difficult of telescopic discernment, but evidently of primary importance
to the annular structure around.
The uses of photography in celestial investigations become every
year more manifold and more apparent. The earliest chemical star-pictures
were those of Castor and Vega, obtained with the Cambridge
refractor in 1850 by Whipple of Boston under the direction of
W. C. Bond. Double-star photography was inaugurated under the
auspices of G. P. Bond, April 27, 1857, with an impression, obtained
in eight seconds, of Mizar, the middle star in the handle of the
Plough. A series of measures from sixty-two similar images gave
the distance and position-angle of its companion with about the
same accuracy attainable by ordinary micrometrical operations; and
the method and upshot of these novel experiments were described
in three papers remarkably forecasting the purposes to be served by
stellar photography.[1554] The matter next fell into the able hands of
Rutherfurd, who completed in 1864 a fine object glass (of 11-1/2 inches)[Pg 410]
corrected for the ultra-violet rays, consequently useless for visual
purposes. The sacrifice was recompensed by conspicuous success.
A set of measurements from his photographs of nearly fifty stars
in the Pleiades, and their comparison with Bessel’s places, enabled
Dr. Gould to announce, in 1866, that during the intervening third of
a century no changes of importance had occurred in their relative
positions.[1555] And Mr. Harold Jacoby[1556] similarly ascertained the fixity
of seventy-five of Rutherfurd’s Atlantids, between the epoch 1873
and that of Dr. Elkin’s heliometric triangulation of the cluster in
1886,[1557] extending the interval to twenty-seven years by subsequent
comparisons with plates taken at Lick, September 27, 1900.[1558] Positive,
however, as well as negative results have ensued from the application
of modern methods to that antique group.
On October 19, 1859, Wilhelm Tempel, a Saxon peasant by origin,
later a skilled engraver, discovered with a small telescope, bought out
of his scanty savings, an elliptical nebulosity, stretching far to the
southward from the star Merope. It attracted the attention of many
observers, but was so often missed, owing to its extreme susceptibility
to adverse atmospheric influences, as to rouse unfounded
suspicions of its variability. The detection of this evasive object
gave a hint, barely intelligible at the time, of further revelations of
the same kind by more cogent means.
A splendid photograph of 1,421 stars in the Pleiades, taken by
the MM. Henry with three hours’ exposure, November 16, 1885,
showed one of the brightest of them to have a small spiral nebula,
somewhat resembling a strongly-curved comet’s tail, attached to it.
The reappearance of this strange appurtenance on three subsequent
plates left no doubt of its real existence, visually attested at Pulkowa,
February 5, 1886, by one of the first observations made with
the 30-inch equatoreal.[1559] Much smaller apertures, however, sufficed
to disclose the “Maia nebula,” once it was known to be there. Not
only did it appear greatly extended in the Vienna 27-inch,[1560] but
MM. Perrotin and Thollon saw it with the Nice 15-inch, and
M. Kammermann of Geneva, employing special precautions, with
a refractor of only ten inches aperture.[1561] The advantage derived
by him for bringing it into view, from the insertion into the eye-piece
of a uranium film, gives, with its photographic intensity,
valid proof that a large proportion of the light of this remarkable
object is of the ultra-violet kind.
The beginning thus made was quickly followed up. A picture of
the Pleiades procured at Maghull in eighty-nine minutes, October 23,
1886, revealed nebulous surroundings to no less than four leading
stars of the group, namely, Alcyone, Electra, Merope, and Maia;
and a second impression, taken in three hours on the following night,
showed further “that the nebulosity extends in streamers and fleecy
masses till it seems almost to fill the spaces between the stars, and
to extend far beyond them.”[1562] The coherence of the entire mixed
structure was, moreover, placed beyond doubt by the visibly close
relationship of the stars to the nebulous formations surrounding them
in Dr. Roberts’s striking pictures. Thus Goldschmidt’s notion that
all the clustered Pleiades constitute, as it were, a second Orion
trapezium in the midst of a huge formation of which Tempel’s
nebula is but a fragment,[1563] has been to some extent verified. Yet
it seemed fantastic enough in 1863.
Then in 1888 the MM. Henry gave exposures of four hours each
to several plates, which exhibited on development some new features
of the entangled nebulæ. The most curious of these was the linking
together of stars by nebulous chains. In one case seven aligned
stars appeared strung on a silvery filament, “like beads on a rosary.”[1564]
The “rows of stars,” so often noticed in the sky, may, then, be concluded
to have more than an imaginary existence. Of the 2,326 stars
recorded in these pictures, a couple of hundred among the brightest
can, at the outside, be reckoned as genuine Pleiades. The great
majority were relegated, by Pickering’s[1565] and Stratonoff’s[1566] counts of
the stellar populace in and near the cluster, to the position of outsiders
from it. They are undistinguished denizens of the abysmal
background upon which it is projected.
Investigations of its condition were carried a stage further by
Barnard. On November 14, 1890,[1567] he discovered visually with the
Lick refractor a close nebulous satellite to Merope, photographs of
which were obtained by Keeler in 1898.[1568] It appears in them of
a rudely pentagonal shape, a prominent angle being directed towards
the adjacent star. Finally, an exposure of ten hours made by Barnard
with the Willard lens indicated the singular fact that the entire group
is embedded in a nebulous matrix, streaky outliers of which blur a
wide surface of the celestial vault.[1569] The artist’s conviction of the
reality of what his picture showed was confirmed by negatives
obtained by Bailey at Arequipa in 1897, and by H. C. Wilson at
Northfield (Minnesota) in 1898.[1570]
With the Ealing 3-foot reflector, sold by Dr. Common to
Mr. Crossley, and by him presented to the Lick Observatory, Professor
Keeler took in 1899 a series of beautiful and instructive
nebula[1571] photographs; One of the Trifid may be singled out as of
particular excellence. An astonishing multitude of new nebulæ
were revealed by trial-exposures with this instrument. A “conservative
estimate” gave 120,000 as the number coming within its
scope. Moreover, the majority of those actually recorded were of
an unmistakable spiral character, and they included most of Sir
John Herschel’s “double nebulæ,” previously supposed to exemplify
the primitive history of binary stellar systems.[1572] Dr. Max Wolf’s
explorations with a 6-inch Voigtländer lens in 1901 emphatically
reaffirmed the inexhaustible wealth of the nebular heavens. In
one restricted region, midway between Præsepe and the Milky
Way, he located 135 nebulæ, where only three had until then
been catalogued; and he counted 108 such objects clustering round
the star 31 Comæ Berenices,[1573] and so closely that all might be
occulted together by the moon. The general photographic Catalogue
of Nebulæ which Dr. Wolf has begun to prepare[1574] will thus be a
most voluminous work.
The history of celestial photography at the Cape of Good Hope
began with the appearance of the great comet of 1882. No special
apparatus was at hand; so Sir David Gill called in the services of
a local artist, Mr. Allis of Mowbray, with whose camera, strapped
to the Observatory equatoreal, pictures of conspicuous merit were
obtained. But their particular distinction lay in the multitude of
stars begemming the background. (See Plate III.) The sight of
them at once opened to the Royal Astronomer a new prospect. He
had already formed the project of extending Argelander’s “Durchmusterung”
from the point where it was left by Schönfeld to the
southern pole; and his ideas regarding the means of carrying it into
execution crystallised at the needle touch of the cometary experiments.
He resolved to employ photography for the purpose. The
exposure of plates was accordingly begun, under the care of
Mr. Ray Woods, in 1885; and in less than six years, the sky, from
19° of south latitude to the pole, had been covered in duplicate.
Their measurement, and the preparation of a catalogue of the stars
imprinted upon them, were generously undertaken by Professor
Kapteyn, and his laborious task has at length been successfully
completed. The publication, in 1900, of the third and concluding
volume of the “Cape Photographic Durchmusterung”[1575] placed at[Pg 413]
the disposal of astronomers a photographic census of the heavens
fuller and surer than the corresponding visual enumeration executed
at Bonn. It includes 454,875 stars, nearly to the tenth magnitude,
and their positions are reliable to about one second of arc.
The production of this important work was thus a result of the
Cape comet-pictures; yet not the most momentous one. They
turned the scale in favour of recourse to the camera when the
MM. Henry encountered, in their continuation of Chacornac’s
half-finished enterprise of ecliptical charting, sections of the Milky
Way defying the enumerating efforts of eye and hand. The perfect
success of some preliminary experiments made with an instrument
constructed by them expressly for the purpose was announced to
the Academy of Sciences at Paris, May 2, 1885. By its means
stars estimated as of the sixteenth magnitude clearly recorded their
presence and their places; and the enormous increase of knowledge
involved may be judged of from the fact that, in a space of the
Milky Way in Cygnus 2° 15′ by 3°, where 170 stars had been
mapped by the old laborious method, about five thousand stamped
their images on a single Henry plate.
These results suggested the grand undertaking of a general
photographic survey of the heavens, and Gill’s proposal, June 4,
1886, of an International Congress for the purpose of setting it on
foot was received with acclamation, and promptly acted upon.
Fifty-six delegates of seventeen different nationalities met in Paris,
April 16, 1887, under the presidentship of Admiral Mouchez, to
discuss measures and organise action. They resolved upon the
construction of a Photographic Chart of the whole heavens, comprising
stars of a fourteenth magnitude, to the surmised number
of twenty millions; to be supplemented by a Catalogue, framed
from plates of comparatively short exposure, giving start to the
eleventh magnitude. These will probably amount to about one
million and a quarter. For procuring both sets of plates, instruments
were constructed precisely similar to that of the MM. Henry,
which is a photographic refractor, thirteen inches in aperture, and
eleven feet focus, attached to a guiding telescope of eleven inches
aperture, corrected, of course, for the visual rays. Each place covers
an area of four square degrees, and since the series must be duplicated
to prevent mistakes, about 22,000 plates will be needed for the
Chart alone. The task of preparing them was apportioned among
eighteen observatories scattered over the globe, from Mexico to
Melbourne; but three in South America having become disabled or
inert, were replaced in 1900 by those at Cordoba, Montevideo, and
Perth, Western Australia. Meanwhile, the publication of results
has begun, and is likely to continue for at least a quarter of a[Pg 414]
century. The first volume of measures from the Potsdam Catalogue-plates
was issued in 1899, and its successors, if on the same scale,
must number nearly 400. Moreover, ninety-six heliogravure enlargements
from the Paris Chart-plates, distributed in the same year,
supplied a basis for the calculation that the entire Atlas of the sky,
composed of similar sheets, will form a pile thirty feet high and two
tons in weight![1576] It will, however, possess an incalculable scientific
value. For millions of stars can be determined by its means, from
their imprinted images, with an accuracy comparable to that attainable
by direct meridian observations.
One of the most ardent promoters of the scheme it may be
expected to realise was Admiral Mouchez, the successor of Leverrier
in the direction of the Paris Observatory. But it was not granted
to him to see the fruition of his efforts. He died suddenly June 25,
1892.[1577] Although not an astronomer by profession, he had been
singularly successful in pushing forward the cause of the science he
loved, while his genial and open nature won for him wide personal
regard. He was replaced by M. Tisserand, whose mathematical
eminence fitted him to continue the traditions of Delaunay and
Leverrier. But his career, too, was unhappily cut short by an
unforeseen death on October 20, 1896; and the more eminent
among the many qualifications of his successor, M. Maurice Loewy,
are of the practical kind.
The sublime problem of the construction of the heavens has not
been neglected amid the multiplicity of tasks imposed upon the
cultivators of astronomy by its rapid development. But data of a
far higher order of precision, and indefinitely greater in amount,
than those at the disposal of Herschel or Struve must be accumulated
before any definite conclusions on the subject are possible.
The first organised effort towards realising this desideratum was
made by the German Astronomical Society in 1865, two years after
its foundation at Heidelberg. The original programme consisted in
the exact determination of the places of all Argelander’s stars to the
ninth magnitude (exclusive of the polar zone), from the reobservation
of which, say, in the year 1950, astronomers of two generations
hence may gather a vast store of knowledge—directly of the
apparent motions, indirectly of the mutual relations binding together
the suns and systems of space. Thirteen observatories in Europe
and America joined in the work, now virtually terminated. Its
scope was, after its inception, widened to include southern zones as
far as the Tropic of Capricorn; this having been rendered feasible
by Schönfeld’s extension (1875-1885) of Argelander’s survey.
Thirty thousand additional stars thus taken in were allotted in[Pg 415]
zones to five observatories. Another important undertaking of the
same class is the reobservation of the 47,300 stars in Lalande’s
Histoire Céleste. Begun under Arago in 1855, its upshot has been
the publication of the great Paris Catalogue, issued in eight volumes,
between 1887 and 1902. From a careful study of their secular
changes in position, M. Bossert has already derived the proper
motions of a couple of thousand out of nearly fifty thousand stars
enumerated in it.
Through Dr. Gould’s unceasing labours during his fifteen years’
residence at Cordoba, a detailed acquaintance with southern stars
was brought about. His Uranometria Argentina (1879) enumerates
the magnitudes of 8,198 out of 10,649 stars visible to the naked eye
under those transparent skies; 33,160 down to 9-1/2 magnitude are
embraced in his “zones”; and the Argentine General Catalogue of
32,468 southern stars was published in 1886. Valuable work of the
same kind has been done at the Leander McCormick Observatory,
Virginia, by Professor O. Stone; while the late Redcliffe observer’s
“Cape Catalogue for 1880′ affords inestimable aid to the practical
astronomer south of the line, which has been reinforced with several
publications issued by the present Astronomer Royal at the Cape.
Moreover, the gigantic task entered upon in 1860 by Dr. C. H. F.
Peters, director of the Litchfield Observatory, Clinton (N.Y.), and
of which a large instalment was finished in 1882, deserves honourable
mention. It was nothing less than to map all stars down to, and
even below, the fourteenth magnitude, situated within 30° on either
side of the ecliptic, and so to afford “a sure basis for drawing
conclusions with respect to the changes going on in the starry
heavens.”[1578]
It is tolerably safe to predict that no work of its kind and for its
purpose will ever again be undertaken. In a small part of one
night stars can now be got to register themselves more numerously
and more accurately than by the eye and hand of the most skilled
observer in the course of a year. Fundamental catalogues, constructed
by the old, time-honoured method, will continue to furnish
indispensable starting-points for measurement; and one of especial
excellence was published by Professor Newcomb in 1899;[1579] but the
relative places of the small crowded stars—the sidereal
οι‘ πο′λλοι—will
henceforth be derived from their autographic statements on
the sensitive plate. Even the secondary purpose—that of asteroidal
discovery—served by detailed stellar enumeration, is more surely
attained by photography than by laborious visual comparison. For
planetary movement betrays itself in a comparatively short time by[Pg 416]
turning the imprinted image of the object affected by it from a dot
into a trail.
In the arduous matter of determining star distances progress has
been steady, and bids fair to become rapidly accelerated. Together,
yet independently, Gill and Elkin carried out, at the Cape Observatory
in 1882-83, an investigation of remarkable accuracy into the parallaxes
of nine southern stars. One of these was the famous α Centauri,
the distance of which from the earth was ascertained to be just one-third
greater than Henderson had made it. The parallax of Sirius,
on the other hand, was doubled, or its distance halved; while
Canopus proved to be quite immeasurably remote—a circumstance
which, considering that, among all the stellar multitude, it is outshone
only by the radiant Dog-star, gives a stupendous idea of its real
splendour and dimensions.
Inquiries of this kind were, for some years, successfully pursued
at the observatory of Dunsink, near Dublin. Annual perspective
displacements were by Dr. Brünnow detected in several stars, and
in others remeasured with a care which inspired just confidence.
His parallax for α Lyræ (0·13′) was authentic, though slightly too
large (Elkin’s final results gave π = 0·082′); and the received value
for the parallax of the swiftly travelling star “Groombridge 1,830′
scarcely differs from that arrived at by him in 1871 (π = 0·09′).
His successor as Astronomer-Royal for Ireland, Sir Robert Stawell
Ball (now Lowndean Professor of Astronomy in the University of
Cambridge), has done good service in the same department. For
besides verifying approximately Struve’s parallax of half a second
of arc for 61 Cygni, he refuted, in 1811, by a sweeping search for
(so-called) “large” parallaxes, certain baseless conjectures of comparative
nearness to the earth, in the case of red and temporary
stars.[1580] Of 450 objects thus cursorily examined, only one star of the
seventh magnitude, numbered 1,618 in Groombridge’s Circumpolar
Catalogue, gave signs of measurable vicinity. Similarly, a reconnaissance
among rapidly moving stars lately made by Dr. Chase
with the Yale heliometer[1581] yielded no really large, and only eight
appreciable parallaxes among the 92 subjects of his experiments.
A second campaign in stellar parallax was undertaken by Gill and
Elkin in 1887. But this time the two observers were in opposite
hemispheres. Both used heliometers. Dr. Elkin had charge of the
fine instrument then recently erected in Yale College Observatory;
Sir David Gill employed one of seven inches, just constructed under
his directions, in first-rate style, by the Repsolds of Hamburg.
Dr. Elkin completed in 1888 his share of the more immediate joint[Pg 417]
programme, which consisted in the determination, by direct measurement,
of the average parallax of stars of the first magnitude. It came
out, for the ten northern luminaries, after several revisions, 0·098′,
equivalent to a light-journey of thirty-three years. The deviations
from this average were, indeed, exceedingly wide. Two of the stars,
Betelgeux and α Cygni, gave no certain sign of any perspective
shifting; of the rest, Procyon, with a parallax of 0·334′, proved the
nearest to our system. At the mean distance concluded for these
ten brilliant stars, the sun would show as of only fifth magnitude;
hence it claims a very subordinate rank among the suns of space.
Sir David Gill’s definitive results were published in 1900.[1582] As the
average parallax of the eleven brightest stars in the southern
hemisphere, they gave 0·13′, a value enhanced by the exceptional
proximity of α Centauri. Yet four of these conspicuous objects—Canopus,
Rigel, Spica, and β Crucis—gave no sign of perspective
response to the annual change in our point of view. The list
included eleven fainter stars with notable proper motions, and most
of these proved to have fairly large parallaxes. Among other
valuable contributions to this difficult branch may be instanced
Bruno Peter’s measurements of eleven stars with the Leipzig
heliometer, 1887-92;[1583] Kapteyn’s application of the method by
differences in right ascension to fifteen stars observed on the
meridian 1885-89;[1584] and Flint’s more recent similar determinations
at Madison, Wisconsin.[1585]
The great merit of having rendered photography available for
the sounding of the celestial depths belongs to Professor Pritchard.
The subject of his initial experiment was 61 Cygni. From measurements
of 200 negatives taken in 1886, he derived for that classic
star a parallax of 0·438′, in satisfactory agreement with Ball’s of
0·468′. A detailed examination convinced the Astronomer-Royal
of its superior accuracy to Bessel’s result with the heliometer. The
Savilian Professor carried out his project of determining all second
magnitude stars to the number of about thirty,[1586] conveniently
observable at Oxford, obtaining as the general outcome of the
research an average parallax of 0·056′, for objects of that rank.
But this value, though in itself probable, cannot be accepted as
authoritative, in view of certain inaccuracies in the work adverted
to by Jacoby,[1587] Hermann Davis, and Gill. The method has, nevertheless,[Pg 418]
very large capabilities. Professor Kapteyn showed, in
1889,[1588] the practicability of deriving parallaxes wholesale from
plates exposed at due intervals, and applied his system, in 1900,
with encouraging success, to a group of 248 stars.[1589] The apparent
absence of spurious shiftings justified the proposal to follow up the
completion of the Astrographic Chart with the initiation of a
photographic “Parallax Durchmusterung.”
Observers of double stars are among the most meritorious, and
need to be among the most patient and painstaking workers in
sidereal astronomy. They are scarcely as numerous as could be
wished. Dr. Doberck, distinguished as a computer of stellar orbits,
complained in 1882[1590] that data sufficient for the purpose had not
been collected for above 30 or 40 binaries out of between five and
six hundred certainly or probably within reach. The progress since
made is illustrated by Mr. Gore’s useful Catalogue of Computed
Binaries, including fifty-nine entries, presented to the Royal Irish
Academy, June 9, 1890.[1591] Few have done more towards supplying
the deficiency of materials than the late Baron Ercole Dembowski of
Milan. He devoted the last thirty years of his life, which came
to an end January 19, 1881, to the revision of the Dorpat Catalogue,
and left behind him a store of micrometrical measures as numerous
as they are precise.
Of living observers in this branch, Mr. S. W. Burnham is beyond
question the foremost. While pursuing legal avocations at Chicago,
he diverted his scanty leisure by exploring the skies with a 6-inch
telescope mounted in his back-yard; and had discovered, in May,
1882, one thousand close and mostly very difficult double stars.[1592]
Summoned as chief assistant to the new Lick Observatory in 1888,
he resumed the work of his predilection with the 36-inch and 12-inch
refractors of that establishment. But although devoting most of
his attention to much-needed remeasurements of known pairs, he
incidentally divided no less than 274 stars, the majority of which
lay beyond the resolving power of less keen and effectually aided
eyesight. One of his many interesting discoveries was that of a
minute companion to α Ursæ Majoris (the first Pointer), which
already gives unmistakable signs of orbital movement round the
shining orb it is attached to. Another pair, κ Pegasi, detected in
1880, was found in 1892 to have more than completed a circuit in
the interim.[1593] Its period of a little over eleven years is the shortest[Pg 419]
attributable to a visible binary system, except that of δ Equulei,
provisionally determined by Professor Hussey in 1900 at 5·7 years,[1594]
and indicated by spectroscopic evidence to be of uncommon brevity.[1595]
Burnham’s Catalogue of 1,290 Double Stars, discovered by him from
1871 to 1899,[1596] is a record of unprecedented interest. Nearly all
the 690 pairs included in it, 2′ or less than 2′ apart, must be
physically connected; and they offer a practically unlimited field for
investigation; while the notes, diagrams, and orbits appended
profusely to the various entries, are eminently helpful to students
and computers. The author is continuing his researches at the
Yerkes Observatory, having quitted the Lick establishment in 1892.
The first complete enrolment of southern double stars was made by
Mr. R. T. A. Innes in 1899.[1597] The couples enumerated, twenty-one
per cent. of which are separated by less than one second of arc, are
2,140 in number. They include 305 discovered by himself. Dr. See
gathered a rich harvest of nearly 500 new southern pairs with the
Lowell 24-inch refractor in 1897.[1598] Professor Hough’s discoveries in
more northerly zones amount to 623;[1599] Hussey’s at Lick to 350; and
Aitken’s already to over 300.
There is as yet no certainty that the stars of 61 Cygni form a true
binary combination. Mr. Burnham, indeed, holds them to be in
course of definitive separation; and Professor Hall’s observations
at Washington, 1879 to 1891, although favouring their physical
connection, are far from decisive on the point.[1600] Dr. Wilsing, from
certain anomalous displacements of their photographed images,
concluded in 1893[1601] the presence of an invisible third member of the
system, revolving in a period of twenty-two months; but the effects
noticed by him were probably illusory.
Important series of double-star observations were made by
Perrotin at Nice in 1883-4;[1602] by Hall, with the 26-inch Washington
equatoreal, 1874 to 1891;[1603] by Schiaparelli from 1875 onward; by
Glasenapp, O. Stone, Leavenworth, Seabroke, and many besides.
Finally, Professor Hussey’s revision of the Pulkowa Catalogue[1604] is a
work of the teres atque rotundus kind, which leaves little or nothing
to be desired. The methods employed in double-star determinations
remain, at the beginning of the twentieth century, essentially[Pg 420]
unchanged. The camera has scarcely encroached upon this part of
the micrometer’s domain.[1605]
A research of striking merit into the origin of binary stars was
published in 1892 by Dr. T. J. J. See, in the form of an Inaugural
Dissertation for his doctor’s degree in the University of Berlin.
The main result was to show the powerful effects of tidal friction
in prescribing the course of their development from double nebulæ,
revolving almost in contact, to double suns, far apart, yet inseparable.
The high eccentricities of their eventual orbits were
shown to result necessarily from this mode of action, which must
operate with enormous strength on closely conjoined, nearly equal
masses, such as the rapidly revolving pairs disclosed by the spectroscope.
That these are still in an early stage of their life-history is
probable in itself, and is re-affirmed by the exceedingly small
density indicated for eclipsing stars by the ratio of phase-duration
to period.
Stellar photometry, initiated by the elder Herschel, and provided
with exact methods by his son at the Cape, by Steinheil and Seidel
at Munich, has of late years assumed the importance of a separate
department of astronomical research. Two monumental works on
the subject, compiled on opposite sides of the Atlantic, were thus
appropriately coupled in the bestowal of the Royal Astronomical
Society’s Gold Medal in 1886. Harvard College Observatory led
the way under the able direction of Professor E. C. Pickering.
His photometric catalogue of 4,260 stars,[1606] constructed from nearly
95,000 observations of light-intensity during the years 1879-82,
constitutes a record of incalculable value for the detection and
estimation of stellar variability. It was succeeded in 1885 by
Professor Pritchard’s “Uranometria Nova Oxoniensis,” including
photometric determinations of the magnitude of all naked-eye stars,
from the pole to ten degrees south of the equator to the number of
2,784. The instrument employed was the “wedge photometer,”
which measures brightness by resistance to extinction. A wedge of
neutral-tint glass, accurately divided to scale, is placed in the path
of the stellar rays, when the thickness of it they have power to
traverse furnishes a criterion of their intensity. Professor Pickering’s
“meridian photometer,” on the other hand, is based upon Zöllner’s
principle of equalization effected by a polarising apparatus. After
all, however, as Professor Pritchard observed, “the eye is the real
photometer,” and its judgment can only be valid over a limited
range.[1607] Absolute uniformity, then, in estimates made by various
means, under varying conditions, and by different observers, is not[Pg 421]
to be looked for; and it is satisfactory to find substantial agreement
attainable and attained. Only in an insignificant fraction of the
stars common to the Harvard and Oxford catalogues discordances
are found exceeding one-third of a magnitude; a large proportion
(71 per cent.) agree within one-fourth, a considerable minority (31 per
cent.) within one-tenth of a magnitude.[1608] The Harvard photometry
was extended, on the same scale, to the opposite pole in a catalogue
of the magnitudes of 7,922 southern stars,[1609] founded on Professor
Bailey’s observations in Peru, 1889-91. Measurements still more
comprehensive were subsequently executed at the primary establishment.
With a meridian photometer of augmented power, the
surprising number of 473,216 settings were made during the years
1891-98, nearly all by the indefatigable director himself, and they
afforded materials for a “Photometric Durchmusterung,” published
in 1901, including all stars to 7·5 magnitude north of declination -40°.[1610]
A photometric zone, 20° wide, has for some time been in
course of observation at Potsdam by MM. Müller and Kempf.
The instrument employed by them is constructed on the polarising
principle as adapted by Zöllner.
Photographic photometry has meanwhile risen to an importance
if anything exceeding that of visual photometry. For the usefulness
of the great international star-chart now being prepared would be
gravely compromised by systematic mistakes regarding the magnitudes
of the stars registered upon it. No entirely trustworthy
means of determining them have, however, yet been found. There
is no certainty as to the relative times of exposure needed to get
images of stars representative of successive photometric ranks. All
that can be done is to measure the proportionate diameters of such
images, and to infer, by the application of a law learned from
experience, the varied intensities of light to which they correspond.
The law is, indeed, neither simple nor constant. Different investigators
have arrived at different formulæ, which, being purely
empirical, vary their nature with the conditions of experiment.
Probably the best expedient for overcoming the difficulty is that
devised by Pickering, of simultaneously photographing a star and
its secondary image, reduced in brightness by a known amount.[1611]
The results of its use will be exhibited in a catalogue of 40,000 stars
to the tenth magnitude, one for each square degree of the heavens.
A photographic photometry of all the lucid stars, modelled on the
visual photometry of 1884, is promised from the same copious source
of novelties. The magnitudes of the stars in the Draper Catalogue[Pg 422]
were determined, so to speak, spectrographically. The quantity
measured in all cases was the intensity of the hydrogen line near G.
By the employment of this definite and uniform test, results were
obtained, of special value indeed, but in strong disaccord with those
given by less exclusive determinations.
Thought, meantime, cannot be held aloof from the great subject
upon the future illustration of which so much patient industry is
being expended. Nor are partial glimpses denied to us of relations
fully discoverable, perhaps, only through centuries of toil. Some
important points in cosmical economy have, indeed, become quite
clear within the last fifty years, and scarcely any longer admit of a
difference of opinion. One of these is that of the true status of
nebulæ.
This was virtually settled by Sir J. Herschel’s description in 1847
of the structure of the Magellanic clouds; but it was not until
Whewell, in 1853, and Herbert Spencer, in 1858,[1612] enforced the
conclusions necessarily to be derived therefrom that the conception
of the nebulæ as remote galaxies, which Lord Rosse’s resolution of
many into stellar points had appeared to support, began to withdraw
into the region of discarded and half-forgotten speculations. In the
Nubeculæ, as Whewell insisted,[1613] “there coexist, in a limited compass
and in indiscriminate position, stars, clusters of stars, nebulæ, regular
and irregular, and nebulous streaks and patches. These, then, are
different kinds of things in themselves, not merely different to us.
There are such things as nebulæ side by side with stars and with
clusters of stars. Nebulous matter resolvable occurs close to nebulous
matter irresolvable.”
This argument from coexistence in nearly the same region of
space, reiterated and reinforced with others by Mr. Spencer, was
urged with his accustomed force and freshness by Mr. Proctor. It
is unanswerable. There is no maintaining nebulæ to be simply
remote worlds of stars in the face of an agglomeration like the
Nubecula Major, containing in its (certainly capacious) bosom both
stars and nebulæ. Add the facts that a considerable proportion of
these perplexing objects are gaseous, and that an intimate relation
obviously subsists between the mode of their scattering and the lie
of the Milky Way, and it becomes impossible to resist the conclusion
that both nebular and stellar systems are parts of a single scheme.[1614]
As to the stars themselves, the presumption of their approximate
uniformity in size and brightness has been effectually dissipated.
Differences of distance can no longer be invoked to account for[Pg 423]
dissimilarity in lustre. Minute orbs, altogether invisible without
optical aid, are found to be indefinitely nearer to us than such
radiant objects as Canopus, Betelgeux, or Rigel. Moreover, intensity
of light is perceived to be a very imperfect index to real magnitude.
Brilliant suns are swayed from their course by the attractive power
of massive yet faintly luminous companions, and suffer eclipse from
obscure interpositions. Besides, effective lustre is now known to
depend no less upon the qualities of the investing atmosphere than
upon the extent and radiative power of the stellar surface. Red
stars must be far larger in proportion to the light diffused by them
than white or yellow stars.[1615] There can be no doubt that our sun
would at least double its brightness were the absorption suffered by
its rays to be reduced to the Sirian standard; and, on the other
hand, that it would lose half its present efficiency as a light-source
if the atmosphere partially veiling its splendours were rendered as
dense as that of Aldebaran.
Thus, variety of all kinds is seen to abound in the heavens; and
it must be admitted that the consequent insecurity of all hypotheses
as to the relative distances of individual stars singularly complicates
the question of their allocation in space. Nevertheless, something
has been learnt even on that point; and the tendency of modern
research is, on the whole, strongly confirmatory of the views
expressed by Herschel in 1802. He then no longer regarded the
Milky Way as the mere visual effect of an enormously extended
stratum of stars, but as an actual aggregation, highly irregular in
structure, made up of stellar clouds and groups and nodosities. All
the facts since ascertained fit in with this conception, to which
Proctor added arguments favouring the view, since adopted by
Barnard[1616] and Easton,[1617] that the stars forming the galactic stream are
not only situated more closely together, but are also really, as well
as apparently, of smaller dimensions than the lucid orbs studding
our skies. By the laborious process of isographically charting the
whole of Argelander’s 324,000 stars, he brought out in 1871[1618]
signs of relationship between the distribution of the brighter stars
and the complex branchings of the Milky Way, which has been
stamped as authentic by Newcomb’s recent statistical inquiries.[1619]
There is, besides, a marked condensation of stars, especially in the
southern hemisphere, towards a great circle inclined some twenty
degrees to the galactic plane; and these were supposed by Gould to
form with the sun a subordinate cluster, of which the components[Pg 424]
are seen projected upon the sky as a zone of stellar brilliants.[1620] The
zone has, however, galactic rather than solar affinities, and represents,
perhaps, not a group, but a stream.
The idea is gaining ground that the Milky Way is designed, in its
main outlines, on a spiral pattern, and that its various branches and
sections are consequently situated at very different distances from
ourselves. Proctor gave a preliminarily interpretation of their complexities
on this principle, and Easton of Rotterdam[1621] has renewed
the attempt with better success.
A most suggestive delineation of the Milky Way, completed in
1889, after five years of labour, by Dr. Otto Boedicker, Lord Rosse’s
astronomer at Parsonstown, was published by lithography in 1892.
It showed a curiously intricate structure, composed of dimly
luminous streams, and shreds, and patches, intermixed with dark
gaps and channels. Ramifications from the main trunk ran out
towards the Andromeda nebula and the “Bee-hive” cluster in
Cancer, involved the Pleiades and Hyades, and, winding round the
constellation of Orion, just attained the Sword-handle nebula.
The last delicate touches had scarcely been put to the picture,
when the laborious eye-and-hand method was, in this quarter, as
already in so many others, superseded by a more expeditious
process. Professor Barnard took the first photographs ever secured
of the true Milky Way, July 28, August 1 and 2, 1889, at the
Lick Observatory. Special conditions were required for success;
above all, a wide field and a strong light-grasp, both complied with
through the use of a 6-inch portrait-lens. Even thus, the sensitive
plate needed some hours to pick out the exceedingly faint stars
collected in the galactic clouds. These cannot be photographed
under the nebulous aspect they wear to the eye; the camera takes
note of their real nature, and registers their constituent stars rank
by rank. Hence the difficulty of disclosing them. “In the photographs
made with the 6-inch portrait-lens,” Professor Barnard
wrote, “besides myriads of stars, there are shown, for the first time,
the vast and wonderful cloud-forms, with all their remarkable
structure of lanes, holes, and black gaps, and sprays of stars. They
present to us these forms in all their delicacy and beauty, as no eye
or telescope can ever hope to see them.”[1622] In Plate VI. one of these
strange galactic landscapes is reproduced. It occurs in the Bow of
Sagittarius, not far from the Trifid nebula, where the aggregations
of the Milky Way are more than usually varied and characteristic.
One of their distinctive features comes out with particular prominence.
Region of the Milky Way in Sagittarius—showing a double
black aperture.
Photographed by Professor E. E. Barnard.
[Pg 425]
It will be noticed that the bright mass near the centre of
the plate is tunnelled with dark holes and furrowed by dusky lanes.
Such interruptions recur perpetually in the Milky Way. They
are exemplified on the largest scale in the great rift dividing it into
two branches all the way from Cygnus to Crux; and they are reproduced
in miniature in many clusters.
Mr. H. C. Russell, at Sydney in 1890, successfully imitated Professor
Barnard’s example.[1623] His photographs of the southern Milky
Way have many points of interest. They show the great rift,
black to the eye, yet densely star-strewn to the perception of the
chemical retina; while the “Coal-sack” appears absolutely dark
only in its northern portion. His most remarkable discovery, however,
was that of the spiral character of the two Nubeculæ. With
an effective exposure of four and a half hours, the Greater Cloud came
out as “a complex spiral, with two centres”; while the similar conformation
of its minor companion developed only after eight hours
of persistent actinic action. The revelation is full of significance.
Scarcely less so, although after a different fashion, is the disclosure
on plates exposed by Dr. Max Wolf, with a 5-inch lens, in June,
1891, of a vastly extended nebula, bringing some of the leading
stars in Cygnus into apparently organic connection with the piles of
galactic star-dust likewise involved by it.[1624] Barnard has similarly
found great tracts of the Milky Way to be photographically nebulous,
and the conclusion seems inevitable that we see in it a prodigious
mixed system, resembling that of the Pleiades in point of composition,
though differing widely from it in plan of structure. Of
corroborative testimony, moreover, is the discovery independently
resulting from Gill’s and Pickering’s photographic reviews, that stars
of the first type of spectrum largely prevail in the galactic zone of the
heavens.[1625] With approach to that zone, Kapteyn noticed a steady
growth of actinic intensity relative to visual brightness in the stars
depicted on the Cape Durchmusterung plates.[1626] In other words,
stellar light is, in the Milky Way, bluer than elsewhere. And the
reality of the primitive character hence to be inferred for the entire
structure was, in a manner, certified by Mr. McClean’s observation
that Helium stars—the supposed immediate products of nebulous
matter—crowd towards its medial plane.
The first step towards the unravelment of the tangled web of
stellar movements was taken when Herschel established the reality,[Pg 426]
and indicated the direction of the sun’s journey. But the gradual
shifting backward of the whole of the celestial scenery amid which
we advance accounts for only a part of the observed displacements.
The stars have motions of their own besides those reflected upon
them from ours. All attempts, however, to grasp the general
scheme of these motions have hitherto failed. Yet they have not
remained wholly fruitless. The community of slow movement in
Taurus, upon which Mädler based his famous theory, has proved to
be a fact, and one of very extended significance.
In 1870 Mr. Proctor undertook to chart down the directions and
proportionate amounts of about 1,600 proper motions, as determined
by Messrs. Stone and Main, with the result of bringing to light the
remarkable phenomenon termed by him “star-drift.”[1627] Quite unmistakably,
large groups of stars, otherwise apparently disconnected,
were seen to be in progress together, in the same direction and at
the same rate, across the sky. An example of this kind of unanimity
was alleged by him in the five intermediate stars of the Plough;
and that the agreement in thwartwise motion is no casual one is
practically demonstrated by the concordant radial velocities determined
at Potsdam for four out of the five objects in question. All
of these approach the earth at the rate of about eighteen miles a
second; and the fifth and faintest, δ Ursæ, though not yet measured,
may be held to share their advance. One of them, moreover, ζ Ursæ,
alias Mizar, carries with it three other stars—Alcor, the Arab
“Rider” of the horse, visible to the naked eye, besides a telescopic
and a spectroscopic attendant. So that the group may be regarded
as octuple. It is of vast compass. Dr. Höffler assigned to it in 1897[1628]—although
on grounds more or less hypothetical—a mean parallax
corresponding to a light-journey of 192 years, which would give to
the marching squadron a total extent of at least fourteen times the
distance from the sun to α Centauri, while implying for its brightest
member—ε Ursæ Majoris—the lustre of six hundred suns. The
organising principle of this grand scheme must long remain
mysterious.
It is no solitary example. Particular association, indeed—as was
surmised by Michell far back in the eighteenth century—appears to
be the rule rather than an exception in the sidereal system. Stars
are bound together by twos, by threes, by dozens, by hundreds.
Our own sun is, perhaps, not exempt from this gregarious tendency.
Yet the search for its companions has, up to the present, been[Pg 427]
unavailing. Gould’s cluster[1629] seems remote and intangible; Kapteyn’s
collection of solar stars proved to have been a creation of erroneous
data, and was abolished by his unrelenting industry. Rather, we
appear to have secured a compartment to ourselves for our long
journey through space. A practical certainty has, at any rate, been
gained that whatever aggregation holds the sun as a constituent is
of a far looser build than the Pleiades or Præsepe. Of all such
majestic communities the laws and revolutions remain, as yet,
inaccessible to inquiry; centuries may elapse before even a rudimentary
acquaintance with them begins to develop; while the economy
of the higher order of association, which we must reasonably believe
that they unite to compose, will possibly continue to stimulate and
baffle human curiosity to the end of time.
FOOTNOTES:
[1369] Report Brit. Assoc., 1868, p. 166. Rutherfurd gave a rudimentary sketch of a
classification of the kind in December, 1862, but based on imperfect observation.
See Am. Jour. of Sc., vol. xxxv., p. 77.
[1370] Publicationem, Potsdam, No. 14, 1884, p. 31.
[1371] Von Konkoly once derived from a slow-moving meteor a hydro-carbon
spectrum. A. S. Herschel, Nature, vol. xxiv., p. 507.
[1372] Phil. Trans., vol. cliv., p. 429.
[1373] Am. Jour. of Sc., vol. xix., p. 467.
[1374] Photom. Unters., p. 243.
[1375] Spectre Solaire, p. 38.
[1376] Mr. J. Birmingham, in the Introduction to his Catalogue of Red Stars,
adduced sundry instances of colour-change in a direction the opposite to that
assumed by Zöllner to be the inevitable result of time. Trans. R. Irish Acad.,
vol. xxvi., p. 251. A learned discussion by Dr. T. J. J. See, moreover, enforces
the belief that Sirius was absolutely red eighteen hundred years ago. Astr. and
Astroph., vol. xi., p. 269.
[1377] Phil. Trans., vol. clxiv., p. 492.
[1378] Astr. Nach., No. 2,000.
[1379] Proc. Roy. Soc., vols. xvi., p. 31; xvii., p. 48.
[1380] Annalen der Physik, Bd. xx., p. 155.
[1381] Ibid., p. 153.
[1382] Knowledge, vol. xiv., p. 101.
[1383] Meteoritic Hypothesis, p. 380.
[1384] Phil. Trans., vol. cxci. A., p. 128; Spectra of Southern Stars, p. 3.
[1385] See the author’s System of the Stars, p. 84.
[1386] A designation applied by Sir Norman Lockyer to third-type stars.
[1387] See ante, p. 198.
[1388] Bothkamp Beobachtungen, Heft ii., p. 146.
[1389] Astr. Nach., No. 2,539.
[1390] Ibid., No. 2,548; Observatory, vol. vi., p. 332.
[1391] Month. Not., vol. xlvii., p. 92.
[1392] Publ. Astr. Pac. Soc., vol. i., p. 80; Observatory, vol. xiii., p. 46.
[1393] Lockyer, Proc. Roy. Soc., vol. lvii., p. 173.
[1394] Astr. Nach., No. 3,129.
[1395] Month. Not., vol. lix., p. 505.
[1396] Astr. Nach., No. 2,581.
[1397] Ibid., Nos. 2,651-2.
[1398] Ibid., No. 3,051; Astr. and Astrophysics, vol. xi., p. 25; Bélopolsky, Astr.
Nach., No. 3,129.
[1399] Comptes Rendus, t. lxv., p. 292.
[1400] Copernicus, vol. iii., p. 207.
[1401] System of the Stars, p. 70; Harvard Annals, vol. xxviii., pt. ii., p. 243
(Miss Cannon).
[1402] Potsdam Publ., No. 14, p. 17.
[1403] Proc. Roy. Soc., vol. xlix., p. 33.
[1404] Miss A. J. Cannon, Harvard Annals, vol. xxviii., pt. ii., p. 141.
[1405] Astr. and Astroph., vol. xiii., p. 448.
[1406] Potsdam Publ., No. 2.
[1407] The results of Von Konkoly’s extension of Vogel’s work to 15° of south
declination were published in O Gyalla Beobachtungen, Bd. viii., Th. ii., 1887.
[1408] Astroph. Jour., vols. viii., p. 237; ix., p. 271.
[1409] Ibid., vol. ix., p. 119.
[1410] Phil. Trans., vol. cliv., p. 413. Some preliminary results were embodied in
a “note” communicated to the Royal Society, February 19, 1863 (Proc. Roy.
Soc., vol. xii., p. 444).
[1411] Bothkamp Beob., Heft i., p. 25.
[1412] Astroph. Jour., vol. vi., p. 423.
[1413] Phil. Trans., vol. cliv., p. 429, note.
[1414] Month. Not., vol. xxiii., p. 180.
[1415] Proc. Roy. Soc., vol. xxv., p. 446.
[1416] Phil. Trans., vol. clxxi., p. 669; Atlas of Stellar Spectra, p. 22.
[1417] Astr. Nach., No. 2,301; Monatsb., Berlin, 1879, p. 119; 1880, p. 192.
[1418] Jour. de Physique, t. v., p. 98.
[1419] System of the Stars, p. 39.
[1420] See ante, p. 198.
[1421] Proc. Roy. Soc., vol. xlviii., p. 314.
[1422] Harvard Circulars, Nos. 12, 18; Astroph. Jour., vol. v., p. 92.
[1423] Astroph. Jour., vol. vi., p. 233.
[1424] McClean, Phil. Trans., vol. cxci. A., p. 129.
[1425] Proc. Roy. Soc., vol. lxii., p. 417.
[1426] Ibid., April 27, 1899; Astroph. Jour., vol. x., p. 272.
[1427] Astr. Nach., No. 3,565.
[1428] Ibid., No. 3,583.
[1429] Lunt, Astroph. Jour., vol. xi., p. 262; Proc. Roy. Soc., vol. lxvi., p. 44;
Lockyer, ibid., November 23, 1899; Nature, vol. lxi., p. 263.
[1430] Die Spectralanalyse, p. 314.
[1431] Henry Draper Memorial, First Ann. Report, 1887.
[1432] Mem. Amer. Acad., vol. xi., p. 215.
[1433] Harvard Annals, vol. xxvii.
[1434] Harvard Annals, vol. xxviii., parts i. and ii.
[1435] See ante, p. 201.
[1436] Phil. Trans., vol. clviii., p. 529.
[1437] Schellen, Die Spectralanalyse, Bd. ii., p. 326 (ed. 1883).
[1438] Proc. Roy. Soc., vol. xx., p. 386.
[1439] System of the Stars, p. 199.
[1440] Pickering, Am. Jour. of Sc., vol. xxxix., p. 46; Vogel, Astr. Nach.
No. 3,017.
[1441] Sitzungsberichte, Berlin, May 2, 1901; Astroph. Jour., vol. xiii., p. 324.
[1442] The “relative orbit” of a double star is that described by one round the other
as a fixed point. Micrometrical measures are always thus executed. But in
reality both stars move in opposite directions, and at rates inversely as their
masses round their common centre of gravity.
[1443] Vogel, Astr. Nach., Nos. 3,017, 3,039.
[1444] Huggins, Pres. Address, 1891; Cornu, Sur la Méthode Doppler-Fizeau
p. D. 38.
[1445] Sitzungsb., Berlin, 1890, p. 401; Astr. Nach., No. 2,995.
[1446] Ibid.
[1447] Astroph. Jour., vol. v., p. 1; Newall, Month. Not., vol. lvii., p. 575.
[1448] Bull. de l’Acad. de St. Pétersb., tt. vi., viii.
[1449] Astroph. Jour., vol. x., p. 177; Month. Not., vol. lx., p. 418; Vogel,
Sitzungsb., Berlin, April 19, 1900.
[1450] Month. Not., vol. lx., p. 595.
[1451] Hussey, Astr. Jour., No. 484.
[1452] Astroph. Jour., vols. x, p. 180; xiv., p. 140; Lick Bulletin, No. 4; Bélopolsky,
Astr. Nach., No. 3,637.
[1453] The significance of the name “El Ghoul” leaves little doubt that the Arab
astronomers took note of this star’s variability. E. M. Clerke, Observatory,
vol. xv., p. 271.
[1454] Phil. Trans., vol. lxxiii., p. 484.
[1455] Proc. Amer. Acad., vol. xvi., p. 17; Observatory, vol. iv., p. 116. For a
preliminary essay by T. S. Aldis, see Phil. Mag., vol. xxxix., p. 363, 1870.
[1456] Astr. Nach., No. 2,947.
[1457] Astr. Jour., Nos. 165-6, 255-6, 509. See also Knowledge, vol. xv., p. 186.
[1458] Bauschinger, V. J. S. Astr. Ges., Jahrg. xxix.; but cf. Searle, Harvard
Annals, vol. xxix., p. 223; Boss, Astr. Jour., No. 343.
[1459] Comptes Rendus, t. cxx., p. 125.
[1460] Myers, Astroph. Jour., vol. vii., p. 1; A. W. Roberts, Ibid., vol. xiii., p. 181.
[1461] Proc. R. Irish Ac., July, 1884.
[1462] Ibid., vol. i., p. 97.
[1463] Astr. Jour., Nos. 179, 180.
[1464] Ibid., Nos. 300, 379.
[1465] Astr. Jour., Nos. 491-2.
[1466] System of the Stars, p. 125.
[1467] Proc. Roy. Soc., vol. xv., p. 146.
[1468] Weiss, Astr. Nach., No. 1,590; Espin, Ibid., No. 3,200.
[1469] Comptes Rendus, t. lxxxiii., p. 1172.
[1470] Monatsb., Berlin, 1877, pp. 241, 826.
[1471] Copernicus, vol. ii., p. 101.
[1472] Burnham, Month. Not., vol. lii., p. 457.
[1473] Astr. Nach., No. 2,682.
[1474] A. Hall, Am. Jour. of Sc., vol. xxxi., p. 301.
[1475] Young, Sid. Messenger, vol. iv., p. 282; Hasselberg, Astr. Nach., No. 2,690.
[1476] Report Brit. Assoc., 1885, p. 935.
[1477] Month. Not., vol. xlvii., p. 54.
[1478] Nature, vol. xxxii., p. 522.
[1479] Astr. Nach., Nos. 1,267, 2,715.
[1480] Month. Not., vol. xxi., p. 32.
[1481] Observatory, vol. viii., p. 335.
[1482] Astr. Nach., No. 3,118; Astr. and Astroph., vol. xi., p. 907.
[1483] Cape Results, p. 137.
[1484] Trans. R. Soc. of Edinburgh, vol. xxvii., p. 51; Astr. and Astroph., August,
1892, p. 593.
[1485] Vogel, Astr. Nach., No. 3,079.
[1486] Observatory, vol. xv., p. 287; Seeliger, Astr. Nach., No. 3,118; Astr. and
Astroph., vol. xi., p. 906.
[1487] Ranyard, Knowledge, vol. xv., p. 110.
[1488] Proc. Roy. Soc., vol. li., p. 492.
[1489] Burnham, Month. Not., vol. liii., p. 58.
[1490] Astr. Nach., Nos. 3,118, 3,143.
[1491] Renz, Ibid., Nos. 3,119, 3,238; Huggins, Astr. and Astroph., vol. xiii.,
p. 314.
[1492] Astr. Nach., No. 3,111.
[1493] Bélopolsky, Astr. Nach., No. 3,120.
[1494] Nature, September 15, 1892.
[1495] Astr. Nach., Nos. 3,122, 3,129.
[1496] Ibid., No. 3,133; Astr. and Astroph., vol. xi., p. 715.
[1497] Publ. Astr. Pac. Soc., vol. iv., p. 244.
[1498] Barnard, Astroph. Jour., vol. xiv., p. 152; Campbell, Observatory, vol. xxiv.,
p. 360.
[1499] Pop. Astr., March, 1895, p. 307.
[1500] Harvard Circular, No. 4, December 20, 1895. The first Nova Persei was
spectrographically recorded in 1887.
[1501] Vogel, Sitzungsb., Berlin, April 19, 1900, p. 389.
[1502] Sidgreaves, Observatory, vol. xxiv., p. 191.
[1503] Ibid., Knowledge, vol. xxv., p. 10.
[1504] Lick Bulletin, No. 8.
[1505] Astr. Nach., No. 3,736.
[1506] Astroph. Jour., vol. xiv., p. 167.
[1507] Lick Bulletin, No. 10.
[1508] Astroph. Jour., vols. xiv., p. 293; xv., p. 129.
[1509] Cf. the theories on the subject of M. Wolf, Astr. Nach., Nos. 3,752, 3,753;
Kapteyn, Ibid., No. 3,756; F. W. Very, Ibid., No. 3,771; and W. E. Wilson,
Proc. Roy. Dublin Soc., No. 45, p. 556.
[1510] Phil. Trans., vol. cliv., p. 437.
[1511] Phil. Trans., vol. clviii., p. 540. The true proportion seems to be about
one-tenth (Harvard Annals, vol. xxvi., pt. ii., p. 205), the Tulse Hill working-list
having been formed of specially selected objects.
[1512] Scheiner, Astr. Nach., No. 3,476; Astroph. Jour., vol. vii., p. 231; Campbell,
Ibid., vols. ix., p. 312; x., p. 22.
[1513] Proc. Roy. Soc., vols. xlvi., p. 40; xlviii., p. 202.
[1514] Publ. Astr. Pac. Soc., vol. ii., p. 265; Proc. Roy. Soc., vol. xlix., p. 399.
[1515] Astr. Nach., No 3,549.
[1516] Atlas of Stellar Spectra, p. 125.
[1517] Knowledge, vol. xix., p. 39.
[1518] Astr. Nach., Nos. 1,366, 1,391, 1,689; Chambers, Descriptive Astr. (3rd ed.),
p. 543; Flammarion, L’Univers Sidéral, p. 818.
[1519] Month. Not., vol. li., p. 94.
[1520] Ibid., vol. lix., p. 372.
[1521] Ibid., vol. lx., p. 424.
[1522] Dreyer, Ibid., vol. lii., p. 100.
[1523] Wash. Obs., vol. xxv., App. 1.
[1524] Am. Jour. of Sc., vol. xiv., p. 433; C. Dreyer, Month. Not., vol. xlvii., p. 419.
[1525] Ibid., vol. li., p. 496.
[1526] Reproduced in Knowledge, April, 1893.
[1527] Unless an exception be found in the Pleiades nebulæ, which may be assumed
to share the small apparent movement of the stars they adhere to.
[1528] Abhandl. Akad. der Wiss., Leipzig, 1857, Bd. iii., p. 295.
[1529] Month. Not., vol. lii., p. 31.
[1530] Proc. Roy. Soc., 1874, p. 251.
[1531] Publ. Astr. Pac. Soc., vol. ii., p. 278.
[1532] System of the Stars, p. 257.
[1533] Proc. Roy. Soc., vol. xlix., p. 399.
[1534] Potsdam Publ., Bd. vii., Th. i.
[1535] Astr. Nach., No. 2,714; Schönfeld, V. J. S. Astr. Ges., Jahrg. xxi., p. 58.
[1536] Astroph. Journ., vol. xiii., p. 80.
[1537] Proc. Roy. Soc., vol. xxxiii., p. 425; Report Brit. Assoc., 1882, p. 444. An
impression of the four lower lines in the same spectrum was almost simultaneously
obtained by Dr. Draper. Comptes Rendus, t. xciv., p. 1243.
[1538] Proc. Roy. Soc., vol. xlviii., p. 213.
[1539] Month. Not., vol. xlviii., p. 360.
[1540] Proc. Roy. Soc., vol. xlvi., p. 40; System of the Stars, p. 79.
[1541] Sitzungsb., Berlin, February 13, 1890.
[1542] Wash. Obs., vol. xxv., App. i., p. 226.
[1543] Comptes Rendus, t. xcii., p. 261.
[1544] Month. Not., vol. xliii., p. 255.
[1545] Harvard Annals, vol. xviii., p. 116.
[1546] Sid. Mess., vol. ix., p. 1.
[1547] Knowledge, vol. xv., p. 191.
[1548] Month. Not., vol. xlix., p. 65.
[1549] System of the Stars, p. 269.
[1550] Astr. Nach., Nos. 2,749, 2,754.
[1551] Vogel, Astr. Nach., 2,854.
[1552] Nature, vol. xliii., p. 419.
[1553] L’Astronomie, t. xl., p. 171.
[1554] Astr. Nach., Bände xlvii., p. 1; xlviii., p. 1; xlix., p. 81. Pickering,
Mem. Am. Ac., vol. xi., p. 180.
[1555] Gould on Celestial Photography, Observatory, vol. ii., p. 16.
[1556] Annals N. Y. Acad. of Sciences, vol. vi., p. 239, 1892; Elkin, Publ. Astr.
Pac. Soc., vol. iv., p. 134.
[1557] Trans. Yale Observatory, vol. i., pt. i.
[1558] Astroph. Jour., vol. xiii., p. 56.
[1559] Astr. Nach., No. 2,719.
[1560] Ibid., No. 2,726.
[1561] Ibid., No. 2,730.
[1562] Month. Not., vol. xlvii., p. 24.
[1563] Les Mondes, t. iii., p. 529.
[1564] Mouchez, Comptes Rendus, t. cvi., p. 912.
[1565] Astr. Nach., No. 3,422.
[1566] Ibid., No. 3,441.
[1567] Ibid., Nos. 3,018, 3,032.
[1568] Journ. Brit. Astr. Assoc., vol. ix., p. 133.
[1569] Astr. Nach., No. 3,253.
[1570] Observatory, vol. xxi., pp. 351, 386.
[1571] Reproduced in Astroph. Journ., vol. xi., p. 324.
[1572] Ibid., p. 347.
[1573] Astr. Nach., No. 3,704.
[1574] Sitzungsb. Bayer. Akad., March 23, 1901.
[1575] Annals of the Cape Observatory, vols. iii., iv., v.
[1576] Month. Not., vol. lx., p. 381.
[1577] D. Klumpke, Observatory, vol. xv., p. 305.
[1578] Gilbert, Sid. Mess., vol. i., p. 288.
[1579] Astr. Papers for the Amer. Ephemeris, vol. viii., pt. ii.
[1580] Nature, vol. xxiv., p. 91; Dunsink Observations, pt. v., 1884.
[1581] Elkin, Report for 1891-92, p. 25; Newcomb, The Stars, p. 151.
[1582] Annals of the Cape Observatory, vol. viii., pt. ii. Some of the measures were
made by Messrs. Finlay and de Sitter.
[1583] Astr. Nach., No. 3,483; Observatory, vol. xxi., p. 180.
[1584] Annalen der Sternwarte in Leiden, Bd. vii.
[1585] Report of Harvard Conference in 1898 (Snyder).
[1586] Researches in Stellar Parallax, pt. ii., 1892.
[1587] V. J. S. Astr. Ges., Jahrg., xxviii., p. 117.
[1588] Bulletin de la Carte du Ciel, No. 1, p. 262.
[1589] Publ. of the Astr. Laboratory at Groningen, No. 1.
[1590] Nature, vol. xxvi., p. 177.
[1591] Proc. R. Irish Acad., vol. i., p. 571, ser. iii.
[1592] Mem. R. A. S., vol. xlvii., p. 178.
[1593] Astr. Nach., No. 3,142.
[1594] Publ. Astr. Pac. Soc., No. 76.
[1595] Campbell, Lick Bulletin, No. 4.
[1596] Publ. Yerkes Observatory, vol. i., 1900.
[1597] Annals Cape Observatory, vol. ii., pt. ii.
[1598] Astr. Jour., Nos. 431-2.
[1599] W. J. Hussey, Publ. Astr. Pac. Soc., No. 74.
[1600] Astr. Jour., No. 258.
[1601] Sitzungsberichte, Berlin, October 26, 1893.
[1602] Annales de l’Obs. de Nice, t. ii.
[1603] Washington Observations, 1888, App. i.
[1604] Publ. Lick Observatory, vol. v., 1901.
[1605] T. Lewis, Observatory, vol. xvi., p. 312.
[1606] Harvard Annals, vol. xiv., pt. i., 1884.
[1607] Observatory, vol. viii., p. 309.
[1608] Month. Not., vol. xlvi., p. 277.
[1609] Harvard Annals, vol. xxxiv.
[1610] Ibid., vol. xlv.
[1611] Carte Phot. du Ciel. Réunion du Comité Permanent, Paris, 1891, p. 100.
[1612] Essays (2nd ser.), The Nebular Hypothesis.
[1613] On the Plurality of Worlds, p. 214 (2nd ed.).
[1614] Proctor, Month. Not., vol. xxix., p. 342.
[1615] This remark was first made by J. Michell, Phil. Trans., vol. lvii., p. 25
(1767).
[1616] Pop. Astr., No. 45.
[1617] Astroph. Jour., vol. i., p. 220.
[1618] Month. Not., vols. xxxi., p. 175; xxxii., p. 1.
[1619] The Stars, p. 273.
[1620] System of the Stars, p. 384; Old and New Astronomy, p. 749 (Ranyard).
[1621] Astroph. Jour., vol. xii., p. 156.
[1622] Publ. Astr. Pac. Soc., vol. ii., p. 242.
[1623] Month. Not., vol. li., pp. 40, 97. For reproductions of some of the photographs
in question, see Knowledge, vol. xiv., p. 50.
[1624] Astr. Nach., No. 3,048; Observatory, vol. xiv., p. 301.
[1625] Proc. Roy. Inst., May 29, 1891 (Gill).
[1626] Annals Cape Obs., iii., Introduction, p. 22.
[1627] Proc. Roy. Soc., vol. xviii., p. 169.
[1628] Astr. Nach., No. 3,456; Observatory, vol. xxi., p. 65; Newcomb, The Stars,
p. 80.
[1629] Month. Not., vol. xl., p. 249.
CHAPTER XIII
METHODS OF RESEARCH
Comparing the methods now available for astronomical inquiries
with those in use forty years ago, we are at once struck with the
fact that they have multiplied. The telescope has been supplemented
by the spectroscope and the photographic camera. Now,
this really involves a whole world of change. It means that astronomy
has left the place where she dwelt apart in rapt union with
mathematics, indifferent to all things on earth save only to those
mechanical improvements which should aid her to penetrate further
into the heavens, and has descended into the forum of human knowledge,
at once a suppliant and a patron, alternately invoking help
from and promising it to each of the sciences, and patiently waiting
upon the advances of all. The science of the heavenly bodies has,
in a word, become a branch of terrestrial physics, or rather a higher
kind of integration of all their results. It has, however, this leading
peculiarity, that the materials for the whole of its inquiries are telescopically
furnished. They are such as come very imperfectly, or
not at all, within the cognisance of the unarmed eye.
Spectroscopic and photographic apparatus are simply additions
to the telescope. They do not supersede or render it of less importance.
On the contrary, the efficacy of their action depends
primarily upon the optical qualities of the instruments they are
attached to. Hence the development, to their fullest extent, of the
powers of the telescope is of vital moment to the progress of modern
physical astronomy, while the older mathematical astronomy could
afford to remain comparatively indifferent to it.
The colossal Rosse reflector still marks, as to size, the ne plus ultra
of performance in that line. A mirror four feet in diameter was,
however, sent out to Melbourne by the late Thomas Grubb of Dublin
in 1870. This is mounted in the Cassegrainian manner, so that the
observer looks straight through it towards the object viewed, of
which he really sees a twice-reflected image. The dust-laden atmosphere[Pg 429]
of Melbourne is said to impede very seriously the usefulness
of this originally fine instrument.
It may be doubted whether so large a spectrum will ever again be
constructed. A new material for the mirrors of reflecting telescopes,
proposed by Steinheil in 1856, and independently by Foucault in
1857,[1630] has in a great measure superseded the use of a metallic alloy.
This is glass upon which a thin film of silver has been deposited by
a chemical process originally invented by Liebig. It gives a peculiarly
brilliant reflective surface, throwing back more light than a metallic
mirror of the same area, in the proportion of about sixteen to nine.
Resilvering, too, involves much less risk and trouble than repolishing
a speculum. The first use of this plan on a large scale was in an
instrument of thirty-six inches aperture, finished by Calver for
Dr. Common in 1879. To its excellent qualities turned to account
with rare skill, his triumphs in celestial photography are mainly due.
A more daring experiment was the construction and mounting, by
Dr. Common himself, of a 5-foot reflector. But the first glass
disc ordered from France for the purpose proved radically defective.
When figured, polished, and silvered, towards the close of 1888, it
gave elliptical instead of circular star-images.[1631] A new one had to
be procured, and was ready for astronomical use in 1891. The
satisfactory nature of its performance is vouched for by the observations
made with it upon Jupiter’s new satellite in December, 1892.
This instrument, to which a Newtonian form has been given, had no
rival in respect of light-concentration at the time when it was
built. It has now two—the Paris 50-inch refractor and the Yerkes
5-foot reflector.
It is, however, in the construction of refracting telescopes that the
most conspicuous advances have recently been made. The Harvard
College 15-inch achromatic was mounted and ready for work in June,
1847. A similar instrument had already for some years been in its
place at Pulkowa. It was long before the possibility of surpassing
these masterpieces of German skill presented itself to any optician.
For fifteen years it seemed as if a line had been drawn just
there. It was first transgressed in America. A portrait-painter of
Cambridgeport, Massachusetts, named Alvan Clark, had for some
time amused his leisure with grinding lenses, the singular excellence
of which was discovered in England by Mr. Dawes in 1853.[1632] Seven
years passed, and then an order came from the University of Mississippi
for an object-glass of the unexampled size of eighteen inches.
An experimental glance through it to test its definition resulted,[Pg 430]
as we have seen, in the detection of the companion of Sirius,
January 31, 1862. It never reached its destination in the South.
War troubles supervened, and it was eventually sent to Chicago,
where it served Professor Hough in his investigations of Jupiter, and
Mr. Burnham in his scrutiny of double stars.
The next step was an even longer one, and it was again taken by
a self-taught optician, Thomas Cooke, the son of a shoemaker at
Allerthorpe, in the East Riding of Yorkshire. Mr. Newall of Gateshead
ordered from him in 1863 a 25-inch object-glass. It was
finished early in 1868, but at the cost of shortening the life of its
maker, who died October 19, 1868, before the giant refractor he had
toiled at for five years was completely mounted. This instrument,
the fine qualities of which had long been neutralized by an unfavourable
situation, was presented by Mr. Newall to the University
of Cambridge, a few weeks before his death, April 21, 1889. Under
the care of his son, Mr. Frank Newall, it has proved highly efficient
in the delicate work of measuring stellar radial motions.
Close upon its construction followed that of the Washington
26-inch, for which twenty thousand dollars were paid to Alvan Clark.
The most illustrious point in its career, entered upon in 1873, has
been the discovery of the satellites of Mars. Once known to be
there, these were, indeed, found to be perceptible with very
moderate optical means (Mr. Wentworth Erck saw Deimos with a
7-inch Clark); but the first detection of such minute objects is a
feat of a very different order from their subsequent observation.
Dr. See’s perception with this instrument, in 1899, of Neptune’s
cloud-belts, and his refined series of micrometrical measures of the
various planets, attest the unimpaired excellence of its optical
qualities.
It held the primacy for more than eight years. Then, in
December, 1880, the place of honour had to be yielded to a 27-inch
achromatic, built by Howard Grubb (son and successor of Thomas
Grubb) for the Vienna Observatory. This, in its turn, was surpassed
by two of respectively 29-1/2 and 30 inches, sent by Gautier of Paris
to Nice, and by Alvan Clark to Pulkowa; and an object-glass, three
feet in diameter, was in 1886 successfully turned out by the latter
firm for the Lick Observatory in California. The difficulties, however,
encountered in procuring discs of glass of the size and purity
required for this last venture seemed to indicate that a term to
progress in this direction was not far off. The flint was, indeed, cast
with comparative ease in the workshops of M. Feil at Paris. The
flawless mass weighed 170 kilogrammes, was over 38 inches across,
and cost 10,000 dollars. But with the crown part of the designed
achromatic combination things went less smoothly. The production[Pg 431]
of a perfect disc was only achieved after nineteen failures, involving
a delay of more than two years; and the glass for a third lens,
designed to render the telescope available at pleasure for photographic
purposes, proved to be strained, and consequently went to
pieces in the process of grinding. It has been replaced by one of
33 inches, with which a series of admirable lunar and other photographs
have been taken.
Nor is the difficulty in obtaining suitable material the only
obstacle to increasing the size of refractors. The “secondary
spectrum,” as it is called, also interposes a barrier troublesome to
surmount. True achromatism cannot be obtained with ordinary
flint and crown-glass; and although in lenses of “Jena glass,”
outstanding colour is reduced to about one-sixth its usual amount,
their term of service is fatally abridged by rapid deterioration.
Nevertheless, a 13-inch objective of the new variety was
mounted at Königsberg in 1898; and discs of Jena crown and flint,
23 inches across, were purchased by Brashear at the Chicago Exhibition
of 1893. An achromatic combination of three kinds of glass,
devised by Mr. A. Taylor[1633] for Messrs. Cooke of York, has less serious
drawbacks, but has not yet come into extensive use. Meanwhile, in
giant telescopes affected to the full extent by chromatic aberration,
such as the Lick and Yerkes refractors, the differences of focal length
for the various colours are counted by inches,[1634] and this not through
any lack of skill in the makers, but by the necessity of the case.
Embarrassing consequences follow. Only a small part of the
spectrum of a heavenly body, for instance, can be distinctly seen at
one time; and a focal adjustment of half an inch is required in
passing from the observation of a planetary nebula to that of its
stellar nucleus. A refracting telescope loses, besides, one of its
chief advantages over a reflector when its size is increased beyond
a certain limit. That advantage is the greater luminosity of the
images given by it. Considerably more light is transmitted through
a glass lens than is reflected from an equal metallic surface. But
only so long as both are of moderate dimensions. For the glass
necessarily grows in thickness as its area augments, and consequently
stops a larger percentage of the rays it refracts. So that a point
at length arrives—fixed by the late Dr. Robinson at a diameter a
little short of 3 feet[1635]—where the glass and the metal are, in this
respect, on an equality; while above it the metal has the advantage.
And since silvered glass gives back considerably more light than[Pg 432]
speculum metal, the stage of equalisation with lenses is reached
proportionately sooner where this material is employed.[1636]
The most distinctive faculty of reflectors, however, is that of
bringing rays of all refrangibilities to a focus together. They are
naturally achromatic. None of the beams they collect are thrown
away in colour-fringes, obnoxious both in themselves and as a waste
of the chief object of astrophysicists’ greed—light. Reflectors,
then, are in this respect specially adapted to photographic and
spectrographic use. But they have a countervailing drawback.
The penalties imposed by bigness are for them peculiarly heavy.
Perfect definition becomes with increasing size, more and more
difficult to attain; once attained, it becomes more and more difficult
to keep. For the huge masses of material employed to form great
object-glasses or mirrors tend with every movement to become
deformed by their own weight. Now, the slightest bending of a
mirror is fatal to its performance, the effect being doubled by reflection;
while in a lens alteration of figure is compensated by the
equal and contrary flexures of the opposing surfaces, so that the
emergent beams pursue much the same paths as if the curves of
the refracting medium had remained theoretically perfect. For this
reason work of precision must remain the province of refracting
telescopes, although great reflectors retain the primacy in the
portraiture of the heavenly bodies, as well as in certain branches of
spectroscopy. Professor Hale, accordingly, summarised a valuable
discussion on the subject by asserting[1637] “that the astrophysicist may
properly consider the reflector to be an even more important part
of his instrumental equipment than the refractor.” A new era in
its employment west of the Atlantic opened with the transfer from
Halifax to Mount Hamilton of the Crossley reflector. Its prerogatives
in nebular photography were splendidly indicated in 1899
by Professor Keeler’s exquisite and searching portrayals of the
cloud-worlds of space, and those obtained two years later, with a
similar, though smaller, instrument, by Professor Ritchey of the
Yerkes Observatory, were fully comparable with them. The performances
of the Yerkes 5-foot reflector still belong to the future.
Ambition as regards telescopic power is by no means yet satisfied.
Nor ought it to be. The advance of astrophysical researches of all
kinds depends largely upon light-grasp. For the spectroscopic
examination of stars, for the measurement of their motions in the[Pg 433]
line of sight, for the discovery and study of nebulæ, for stellar and
nebular photography, the cry continually is “more light.” There
is no enterprising head of an observatory but must feel cramped
in his designs if he can command no more than 14 or 15 inches
of aperture, and he aspires to greater instrumental capacity, not
merely with a view to the chances of discovery, but for the steady
prosecution of some legitimate line of inquiry. Thus projects of
telescope-building on a large scale are rife, and some obtain realisation
year by year. Sir Howard Grubb finished in 1893 a 28-inch
achromatic for the Royal Observatory, Greenwich; the Thompson
equatoreal, mounted at the same establishment in 1897, carries on a
single axis a 26-inch photographic refractor and a 30-inch silvered-glass
reflector; the Victoria telescope, inaugurated at the Cape in
1901, comprises a powerful spectrographic apparatus, together with
a chemically corrected 24-inch refractor, the whole being the
munificent gift of Mr. Frank McClean; at Potsdam, at Meudon,
at Paris, at Alleghany, engines for light-concentration have been, or
shortly will be, erected of dimensions which, two generations back,
would have seemed extravagant and impossible.
Perhaps the finest, though not absolutely the greatest, among
them, marked the summit and end of the performances of Alvan G.
Clark, the last survivor of the Cambridgeport firm.
In October, 1892, Mr. Yerkes of Chicago offered an unlimited sum
for the provision of the University of that city with a “superlative”
telescope. And it happened, fortunately, that a pair of glass discs,
nearly 42 inches in diameter, and of perfect quality, were ready
at hand. They had been cast by Mantois for the University of
Southern California, when the erection of a great observatory on
Wilson’s Peak was under consideration. In the Clark workshop
they were combined into a superb objective, brought to perfection
by trials and delicate touches extending over nearly five years.
Then the maker accompanied it to its destination, by the shore of a
far Western Lake Geneva, and died immediately after his return,
June 9, 1897. Nor has the implement of celestial research he just
lived to complete been allowed to “rust unburnished.” Manipulated
by Hale, Burnham, and Barnard, it has done work that would have
been impracticable with less efficient optical aid. Its construction
thus marks a noticeable enlargement of astronomical possibilities,
exemplified—to cite one among many performances—by Barnard’s
success in keeping track of cluster-variables when below the common
limit of visual perception.
With the Lick telescope results have also been achieved testifying
to its unsurpassed excellence. Holden’s and Schaeberle’s views of
planetary nebulæ, Burnham’s and Hussey’s hair’s-breadth star-splitting[Pg 434]
operations, Keeler’s measurements of nebular radial
motion, Barnard’s detections and prolonged pursuit of faint comets,
his discovery of Jupiter’s tiny moon, Campbell’s spectroscopic
determinations—all this could only have been accomplished, even
by an exceptionally able and energetic staff, with the aid of an
instrument of high power and quality. But there was another
condition which should not be overlooked.
The best telescope may be crippled by a bad situation. The
larger it is, indeed, the more helpless is it to cope with atmospheric
troubles. These are the worst plagues of all those that afflict the
astronomer. No mechanical skill avails to neutralise or alleviate
them. They augment with each increase of aperture; they grow
with the magnifying powers applied. The rays from the heavenly
bodies, when they can penetrate the cloud-veils that too often bar
their path, reach us in an enfeebled, scattered, and disturbed condition.
Hence the twinkling of stars, the “boiling” effects at the
edges of sun, moon, and planets; hence distortions of bright, effacements
of feeble telescopic images; hence, too, the paucity of the
results obtained with many powerful light-gathering machines.
No sooner had the Parsonstown telescope been built than it
became obvious that the limit of profitable augmentation of size
had, under climatic conditions at all nearly resembling those
prevailing there, been reached, if not overpassed; and Lord Rosse
himself was foremost to discern the need of pausing to look round
the world for a clearer and stiller air than was to be found within
the bounds of the United Kingdom. With this express object
Mr. Lassell transported his 2-foot Newtonian to Malta in 1852, and
mounted there, in 1860, a similar instrument of fourfold capacity,
with which, in the course of about two years, 600 new nebulæ
were discovered. Professor Piazzi Smyth’s experiences during a
trip to the Peak of Teneriffe in 1856 in search of astronomical
opportunities[1638] gave countenance to the most sanguine hopes of
deliverance, at suitable elevated stations, from some of the oppressive
conditions of low-level star-gazing; yet for a number of years
nothing effectual was done for their realisation. Now, at last,
however, mountain observatories are not only an admitted necessity
but an accomplished fact; and Newton’s long forecast of a time
when astronomers would be compelled, by the developed powers of
their telescopes, to mount high above the “grosser clouds” in order
to use them,[1639] had been justified by the event.
James Lick, the millionaire of San Francisco, had already chosen,
when he died, October 1, 1876, a site for the new observatory, to
the building and endowment of which he had devoted a part of his[Pg 435]
large fortune. The situation of the establishment is exceptional
and splendid. Planted on one of the three peaks of Mount
Hamilton, a crowning summit of the Californian Coast Range, at
an elevation of 4,200 feet above the sea, in a climate scarce rivalled
throughout the world, it commands views both celestial and
terrestrial which the lover of nature and astronomy may alike
rejoice in. Impediments to observation are there found to be most
materially reduced. Professor Holden, who was appointed, in 1885,
president of the University of California and director of the new
observatory affiliated to it, stated that during six or seven months
of the year an unbroken serenity prevails, and that half the remaining
nights are clear.[1640] The power of continuous work thus afforded
is of itself an inestimable advantage; and the high visual excellences
testified to by Mr. Burnham’s discovery, during a two months’ trip
to Mount Hamilton in the autumn of 1879, of forty-two new double
stars with a 6-inch achromatic, gave hopes since fully realised of a
brilliant future for the Lick establishment. Its advantages are
shared, as Professor Holden desired them to be, by the whole
astronomical world.[1641] A sort of appellate jurisdiction was at once
accorded to the great equatoreal, and more than one disputed point
has been satisfactorily settled by recourse to it.
Its performances, considered both as to quality and kind,
are unlikely to be improved upon by merely outbidding it in size,
unless the care expended upon the selection of its site be imitated.
Professor Pickering thus showed his customary prudence in reserving
his efforts to procure a great telescope until Harvard College owned
a dependent observatory where it could be employed to advantage.
This was found by Mr. W. H. Pickering, after many experiments
in Colorado, California, and Peru, at Arequipa, on a slope of the
Andes, 8,000 feet above the sea-level. Here the post provided
for by the “Boyden Fund” was established in 1891, under
ideal meteorological conditions. Temperature preserves a “golden
mean”; the barometer is almost absolutely steady; the yearly
rainfall amounts to no more than three or four inches. No wonder,
then, that the “seeing” there is of the extraordinary excellence
attested by Mr. Pickering’s observations. In the absence of bright
moonlight, he tells us,[1642] eleven Pleiades can always be counted; the
Andromeda nebula appears to the naked eye conspicuously bright,
and larger than the full moon; third magnitude stars have been
followed to their disappearance at the true horizon; the zodiacal
light spans the heavens as a complete arch, the “Gegenschein”[Pg 436]
forming a regular part of the scenery of the heavens. Corresponding
telescopic facilities are enjoyed. The chief instrument at the station,
a 13-inch equatoreal by Clark, shows the fainter parts of the Orion
nebula, photographed at Harvard College in 1887, by which the
dimensions given to it in Bond’s drawing are doubled; stars are at
times seen encircled by half a dozen immovable diffraction rings, up
to twelve of which have been counted round α Centauri; while on
many occasions no available increase of magnifying power availed
to bring out any wavering in the limbs of the planets. Moreover,
the series of fine nights is nearly unbroken from March to
November.
The facilities thus offered for continuous photographic research
rendered feasible Professor Bailey’s amazing discovery of variable
star-clusters. They belong exclusively to the “globular” class, and
the peculiarity is most strikingly apparent in the groups known
as ο Centauri, and Messier 3, 5, and 15. A large number of their
minute components run through perfectly definite cycles of
change in periods usually of a few hours.[1643] Altogether, about 500
“cluster-variables” have been recorded since 1895. It should be
mentioned that Mr. David Packer and Dr. Common discerned, about
1890, some premonitory symptoms of light-fluctuation among the
crowded stars of Messier 5.[1644] With the Bruce telescope, a photographic
doublet 24 inches in diameter, a store of 5,686 negatives was
collected at Arequipa between 1896 and 1901. Some were exposed
directly, others with the intervention of a prism; and all are
available for important purposes of detection or investigation.
Vapours and air-currents do not alone embarrass the use of giant
telescopes. Mechanical difficulties also oppose a formidable barrier
to much further growth in size. But what seems a barrier often
proves to be only a fresh starting-point; and signs are not wanting
that it may be found so in this case. It is possible that the monumental
domes and huge movable tubes of our present observatories
will, in a few decades, be as much things of the past as Huygens’s
“aerial” telescopes. It is certain that the thin edge of the wedge of
innovation has been driven into the old plan of equatoreal mounting.
M. Loewy, the present director of the Paris Observatory, proposed
to Delaunay in 1871 the direction of a telescope on a novel system.
The design seemed feasible, and was adopted; but the death of
Delaunay and the other untoward circumstances of the time interrupted
its execution. Its resumption, after some years, was rendered
possible by M. Bischoffsheim’s gift of 25,000 francs for expenses, and
the coudé or “bent” equatoreal has been, since 1882, one of the
leading instruments at the Paris establishment.[Pg 437]
Its principle is briefly this: The telescope is, as it were, its own
polar axis. The anterior part of the tube is supported at both ends,
and is thus fixed in a direction pointing towards the pole, with only
the power of twisting axially. The posterior section is joined on to
it at right angles, and presents the object-glass, accordingly, to the
celestial equator, in the plane of which it revolves. Stars in any
other part of the heavens have their beams reflected upon the
object-glass by means of a plane rotating mirror placed in front of
it. The observer, meanwhile, is looking steadfastly down the bent
tube towards the invisible southern pole. He would naturally see
nothing whatever were it not that a second plane mirror is fixed at
the “elbow” of the instrument, so as to send the rays which have
traversed the object-glass to his eye. He never needs to move from
his place. He watches the stars, seated in an arm-chair in a warm
room, with as perfect convenience as if he were examining the seeds
of a fungus with a microscope. Nor is this a mere gain of personal
ease. The abolition of hardship includes a vast accession of power.[1645]
Among other advantages of this method of construction are, first,
that of added stability, the motion given to the ordinary equatoreal
being transferred, in part, to an auxiliary mirror. Next, that of
increased focal length. The fixed part of the tube can be made
almost indefinitely long without inconvenience, and with enormous
advantage to the optical qualities of a large instrument. Finally,
the costly and unmanageable cupola is got rid of, a mere shed
serving all purposes of protection required for the “coudé.”
The desirability of some such change as that which M. Loewy
has realised has been felt by others. Professor Pickering sketched,
in 1881, a plan for fixing large refractors in a permanently horizontal
position, and reflecting into them, by means of a shifting mirror, the
objects desired to be observed.[1646] The observations for his photometric
catalogues are, in fact, made with a “broken transit,” in
which the line of sight remains permanently horizontal, whatever
the altitude of the star examined. Sir Howard Grubb, moreover,
set up, in 1882, a kind of siderostat at the Crawford Observatory,
Cork. In a paper read before the Royal Society, January 21, 1884,
he proposed to carry out the principle on a more extended scale;[1647]
and shortly afterwards undertook its application to a telescope
18 inches in aperture for the Armagh Observatory.[1648] The chief
honours, however, remain to the Paris inventor. None of the
prognosticated causes of failure have proved effective. The loss
of light from the double reflection is insignificant. The menaced[Pg 438]
deformation of images is, through the exquisite skill of the
MM. Henry in producing plane mirrors of all but absolute perfection,
quite imperceptible. The definition was admitted to be
singularly good. Sir David Gill stated in 1884 that he had never
measured a double star so easily as he did γ Leonis by its means.[1649]
Sir Norman Lockyer pronounced it to be “one of the instruments
of the future”; and the principle of its construction was immediately
adopted by the directors of the Besançon and Algiers
Observatories, as well as for a 17-inch telescope destined for a
new observatory at Buenos Ayres. At Paris, it has since been
carried out on a larger scale. A “coudé,” of 23-1/2 inches aperture
and 62 feet focal length was in 1890 installed at the National
Observatory, and has served M. Loewy for his ingenious studies on
refraction and aberration—above all, for taking the magnificent
plates of his lunar atlas. The “bent” form is capable of being, but
has not yet been, adapted to reflectors.[1650]
The “cœlostat,” in the form given to it by Professor Turner, has
proved an invaluable adjunct to eclipse-equipments. It consists
essentially of a mirror rotating in forty-eight hours on an axis
in its own plane, and parallel to the earth’s axis. In the field
of a telescope kept rigidly pointed towards such a mirror, stars
appear immovably fixed. The employment of long-focus lenses for
coronal photography is thus facilitated, and the size of the image
is proportional to the length of the focus. Professor Barnard,
accordingly, depicted the totality of 1900 with a horizontal telescope
61-1/2 feet long, fed by a mirror 18 inches across, the diameter of the
moon on his plates being 7 inches. The largest siderostat in the
world is the Paris 50-inch refractor, which formed the chief attraction
of the Palais d’Optique at the Exhibition of 1900. It has a focal
length of nearly 200 feet, and can be used either for photographic
or for visual purposes.
Celestial photography has not reached its grand climacteric; yet
its earliest beginnings already seem centuries behind its present
performances. The details of its gradual yet rapid improvement
are of too technical a nature to find a place in these pages. Suffice
it to say that the “dry-plate” process, with which such wonderful
results have been obtained, appears to have been first made available
by Sir William Huggins in photographing the spectrum of Vega in
1876, and was then successively adopted by Common, Draper, and
Janssen. Nor should Captain Abney’s remarkable extension of the
powers of the camera be left unnoticed. He began his experiments
on the chemical action of red and infra-red rays in 1874, and at
length succeeded in obtaining a substance—the “blue” bromide of[Pg 439]
silver—highly sensitive to these slower vibrations of light. With
its aid he explored a vast, unknown, and for ever invisible region of
the solar spectrum, presenting to the Royal Society, December 5,
1879,[1651] a detailed map of its infra-red portion (wave-lengths 7,600 to
10,750), from which valuable inferences may yet be derived as to
the condition of the various kinds of matter ignited in the solar
atmosphere. Upon plates rendered “orthochromatic” by staining
with alizarine, or other dye-stuffs, the whole visible spectrum can
now be photographed; but those with their maximum of sensitiveness
near G are found preferable, except where the results of light-analysis
are sought to be completely recorded. And since photographic
refractors are corrected for the blue rays, exposures with
them of orthochromatic surfaces would be entirely futile.
The chemical plate has two advantages over the human retina:[1652]
First, it is sensitive to rays which are utterly powerless to produce
any visual effect; next, it can accumulate impression almost indefinitely,
while from the retina they fade after one-tenth part of a
second, leaving it a continually renewed tabula rasa.
It is, accordingly, quite possible to photograph objects so faint as
to be altogether beyond the power of any telescope to reveal—witness
the chemical disclosure of the invisible nebula encircling Nova Persei—and
we may thus eventually learn whether a blank space in the
sky truly represents the end of the stellar universe in that direction,
or whether farther and farther worlds roll and shine beyond, veiled
in the obscurity of immeasurable distance.
Of many ingenious improvements in spectroscopic appliances
the most fundamentally important relate to what are known as
“gratings.” These are very finely striated surfaces, by which light-waves
are brought to interfere, and are thus sifted out, strictly
according to their different lengths, into “normal” spectra. Since
no universally valid measures can be made in any others, their
production is quite indispensable to spectroscopic science. Fraunhofer,
who initiated the study of the diffraction spectrum, used a real
grating of very fine wires: but rulings on glass were adopted by
his successors, and were by Nobert executed with such consummate
skill that a single square inch of surface was made to contain
100,000 hand-drawn lines. Such rare and costly triumphs of art,
however, found their way into very few hands, and practical
availability was first given to this kind of instrument by the inventiveness
and mechanical dexterity of two American investigators.
Both Rutherfurd’s and Rowland’s gratings are machine-ruled, and
reflect instead of transmitting the rays they analyse; but Rowland’s
present to them a very much larger diffractive surface, and consequently[Pg 440]
possess a higher resolving power. The first preliminary to
his improvements was the production, in 1882, of a faultless screw,
those previously in use having been the inevitable source of periodical
errors in striation, giving, in their turn, ghost-lines as subjects of
spectroscopic study.[1653] Their abolition was not one of Rowland’s
least achievements. With his perfected machine a metallic area of
6-1/4 by 4-1/4 inches can be ruled with exquisite accuracy to almost any
degree of fineness; he considered, however, 43,000 lines to the inch
to be the limit of usefulness.[1654] The ruled surface is, moreover, concave,
and hence brings the spectrum to a focus without a telescope. A
slit and an eye-piece are alone needed to view it, and absorption of
light by glass lenses is obviated—an advantage especially sensible
in dealing with the ultra- or infra-visible rays.
The high qualities of Rowland’s great photographic map of the
solar spectrum were thus based upon his previous improvement of
the instrumental means used in its execution. The amount of detail
shown in it is illustrated by the appearance on the negatives of
150 lines between H and K; and many lines depict themselves as
double which, until examined with a concave grating, had passed
for one and indivisible. A corresponding hand-drawing, for which
M. Thollon received in 1886 the Lalande Prize, exhibits, not the
diffractive, but the prismatic spectrum as obtained with bisulphide
of carbon prisms of large dispersive power. About one-third of the
visible gamut of the solar radiations (A to b) is covered by it; it
includes 3,200 lines, and is over ten metres long.[1655] The grating is
an expensive tool in the way of light. Where there is none to
spare, its advantages must be foregone. They could not, accordingly,
be turned to account in stellar spectroscopy until the Lick telescope
was at hand to supply more abundant material for research. By the
use thus made possible of Rowland’s gratings, Professor Keeler was
able to apply enormous dispersion to the rays of stars and nebulæ,
and so to attain a previously unheard-of degree of accuracy in their
measurement. His memorable detection of nebular movement in
line of sight ensued as a consequence. Professor Campbell, his
successor, has since obtained, by the same means, the first satisfactory
photographs of stellar diffraction-spectra.
The means at the disposal of astronomers have not multiplied
faster than the tasks imposed upon them. Looking back to the year
1800, we cannot fail to be astonished at the change. The comparatively
simple and serene science of the heavenly bodies known to our
predecessors, almost perfect so far as it went, incurious of what lay
beyond its grasp, has developed into a body of manifold powers and[Pg 441]
parts, each with its separate mode and means of growth, full of
strong vitality, but animated by a restless and unsatisfied spirit,
haunted by the sense of problems unsolved, and tormented by conscious
impotence to sound the immensities it perpetually confronts.
Knowledge might be said, when the Mécanique Céleste issued from
the press, to be bounded by the solar system; but even the solar
system presented itself under an aspect strangely different from
what it now wears. It consisted of the sun, seven planets, and
twice as many satellites, all circling harmoniously in obedience to a
universal law, by the compensating action of which the indefinite
stability of their mutual relations was secured. The occasional
incursion of a comet, or the periodical presence of a single such
wanderer chained down from escape to outer space by planetary
attraction, availed nothing to impair the symmetry of the majestic
spectacle.
Now, not alone the ascertained limits of the system have been
widened by a thousand millions of miles, with the addition of one
more giant planet and seven satellites to the ancient classes of
its members, but a complexity has been given to its constitution
baffling description or thought. Five hundred circulating planetary
bodies bridge the gap between Jupiter and Mars, the complete
investigation of the movements of any one of which would overtask
the energies of a lifetime. Meteorites, strangers, apparently,
to the fundamental ordering of the solar household, swarm, nevertheless,
by millions in every cranny of its space, returning at regular
intervals like the comets so singularly associated with them, or
sweeping across it with hyperbolic velocities, brought, perhaps, from
some distant star. And each of these cosmical grains of dust has
a theory far more complex than that of Jupiter; it bears within it
the secret of its origin, and fulfils a function in the universe. The
sun itself is no longer a semi-fabulous, fire-girt globe, but the vast
scene of the play of forces as yet imperfectly known to us, offering
a boundless field for the most arduous and inspiring researches.
Among the planets the widest variety in physical habitudes is seen
to prevail, and each is recognised as a world apart, inviting inquiries
which, to be effective, must necessarily be special and detailed. Even
our own moon threatens to break loose from the trammels of calculation,
and commits “errors” which sap the very foundations of the
lunar theory, and suggest the formidable necessity for its complete
revision. Nay, the steadfast earth has forfeited the implicit confidence
placed in it as a time-keeper, and questions relating to the
stability of the earth’s axis and the constancy of the earth’s rate of
rotation are among those which it behoves the future to answer.
Everywhere there is multiformity and change, stimulating a curiosity[Pg 442]
which the rapid development of methods of research offers the
possibility of at least partially gratifying.
Outside the solar system, the problems which demand a practical
solution are virtually infinite in number and extent. And these have
all arisen and crowded upon our thoughts within less than a hundred
years. For sidereal science became a recognised branch of astronomy
only through Herschel’s discovery of the revolutions of double stars
in 1802. Yet already it may be, and has been called, “the astronomy
of the future,” so rapidly has the development of a keen
and universal interest attended and stimulated the growth of power
to investigate this sublime subject. What has been done is little—is
scarcely a beginning; yet it is much in comparison with the total
blank of a century past. And our knowledge will, we are easily
persuaded, appear in turn the merest ignorance to those who
come after us. Yet it is not to be despised, since by it we reach
up groping fingers to touch the hem of the garment of the Most
High.[Pg 443]
APPENDIX
TABLE I
CHRONOLOGY, 1774-1893
| 1774, March 4 | Herschel’s first observation. Subject, the Orion Nebula. |
| 1774 | Sun-spots geometrically proved to be depressions by Wilson. |
| 1774 | First experimental determination of the earth’s mean density by Maskelyne. |
| 1781, March 13 | Discovery of Uranus. |
| 1782 | Herschel’s first Catalogue of Double Stars. |
| 1783 | Herschel’s first investigation of the sun’s movement in space. |
| 1783 | Goodricke’s discovery of Algol’s law of variation. |
| 1784 | Analogy between Mars and the Earth pointed out by Herschel. |
| 1784 | Construction of the Heavens investigated by Herschel’s method of star-gauging. “Cloven-disc” plan of the Milky Way. |
| 1784 | Discovery of binary stars anticipated by Michell. |
| 1786 | Herschel’s first Catalogue of Nebulæ. |
| 1787, Jan. 11 | Discovery by Herschel of two Uranian moons (Oberon and Titania). |
| 1787, Nov. 19 | Acceleration of the moon explained by Laplace. |
| 1789 | Herschel’s second Catalogue of Nebulæ, and classification by age of these objects. |
| 1789 | Completion of Herschel’s forty-foot reflector. |
| 1789, Aug. 28 and Sept. 17 | His discovery with it of the two inner Saturnian satellites. |
| 1789 | Repeating-circle invented by Borda. |
| 1789 | Five-foot circle constructed by Ramsden for Piazzi. |
| 1790 | Maskelyne’s Catalogue of thirty-six fundamental stars. |
| 1791 | Herschel propounds the hypothesis of a fluid constitution for nebulæ. |
| 1792 | Atmospheric refraction in Venus announced by Schröter. |
| 1794 | Rotation-period of Saturn fixed by Herschel at 10h. 16m.[Pg 446] |
| 1795 | Herschel’s theory of the solar constitution. |
| 1796 | Herschel’s first measures of comparative stellar brightness |
| 1796 | Laplace’s Nebular Hypothesis published in Exposition du Système du Monde. |
| 1797 | Publication of Olbers’s method of computing cometary orbits. |
| 1798 | Retrograde motions of Uranian satellites detected by Herschel. |
| 1799 | Publication of first two volumes of Mécanique Céleste. |
| 1799, May 7 | Transit of Mercury observed by Schröter. |
| 1799, Nov. 12 | Star-shower observed by Humboldt at Cumana. |
| 1800 | Monatliche Correspondenz started by Von Zach. |
| 1800 | Invisible heat-rays detected in the solar spectrum by Herschel. |
| 1801, Jan. 1 | Discovery of Ceres by Piazzi. |
| 1801 | Publication of Lalande’s Histoire Céleste. |
| 1801 | Investigation by Herschel of solar emissive variability in connection with spot-development. |
| 1802, March 28 | Discovery of Pallas by Olbers. |
| 1802 | Herschel’s third Catalogue of Nebulæ. |
| 1802 | Herschel’s discovery of binary stars. |
| 1802 | Marks of clustering in the Milky Way noted by Herschel. |
| 1802 | Wollaston records seven dark lines in the solar spectrum. |
| 1802, Nov. 9 | Transit of Mercury observed by Herschel. |
| 1804, Sept. 2 | Transit of Mercury observed by Herschel. |
| 1804 | Foundation of Optical Institute at Munich. |
| 1805 | Herschel’s second determination of the solar apex. |
| 1807, March 29 | Discovery of Vesta by Olbers. |
| 1811 | Herschel’s theory of the development of stars from nebulæ. |
| 1811, Feb. 9 | Death of Maskelyne. Pond appointed to succeed him as Astronomer-Royal. |
| 1811, Sept. 12 | Perihelion passage of great comet. |
| 1814 | Herschel demonstrates the irregular distribution of stars in space. |
| 1815 | Fraunhofer maps 324 dark lines in the solar spectrum. |
| 1818 | Publication of Bessel’s Fundamenta Astronomiæ. |
| 1819 | Recognition by Encke of the first short-period comet. |
| 1819, June 26 | Passage of the earth through the tail of a comet. |
| 1820 | Foundation of the Royal Astronomical Society. |
| 1821 | Foundation of Paramatta Observatory. |
| 1821, September | First number of Astronomische Nachrichten. |
| 1822, May 24 | First calculated return of Encke’s comet. |
| 1822, August 25 | Death of Herschel.[Pg 447] |
| 1823 | Bessel introduces the correction of observations for personal equation. |
| 1823 | Fraunhofer examines the spectra of fixed stars. |
| 1824 | Distance of the sun concluded by Encke to be 95-1/4 million miles. |
| 1824 | Publication of Lohrmann’s Lunar Chart. |
| 1824 | Dorpat refractor mounted equatoreally. |
| 1826 | Commencement of Schwabe’s observations of sun-spots. |
| 1826, Feb. 27 | Biela’s discovery of a comet. |
| 1827 | Orbit of a binary star calculated by Savary. |
| 1829 | Completion of the Royal Observatory at the Cape of Good Hope. |
| 1829 | The Königsberg heliometer mounted. |
| 1830 | Publication of Bessel’s Tabulæ Regiomontanæ. |
| 1832 | Discovery by Brewster of “atmospheric lines” in the solar spectrum. |
| 1833 | Magnetic observatory established at Göttingen. |
| 1833, Nov. 12, 13 | Star-shower visible in North America. |
| 1833 | Completion of Sir J. Herschel’s survey of the northern heavens. |
| 1834, Jan. 16 | Sir J. Herschel’s landing at the Cape. |
| 1835, September | Airy appointed Astronomer-Royal in succession to Pond. |
| 1835, Nov. 16 | Perihelion passage of Halley’s comet. |
| 1837 | Solar movement determined by Argelander. |
| 1837 | Bessel’s application of the heliometer to measurements of stellar parallax. |
| 1837 | Publication of Beer and Mädler’s Der Mond. |
| 1837, Dec. 16 | Outburst of η Carinæ observed by Sir J. Herschel. |
| 1837 | Thermal power of the sun measured by Herschel and Pouillet. |
| 1838 | Parallax of 61 Cygni determined by Bessel. |
| 1839, Jan. 9 | Parallax of α Centauri announced by Henderson. |
| 1839 | Completion of Pulkowa Observatory. |
| 1839 | Solidity of the earth concluded by Hopkins. |
| 1840, March 2 | Death of Olbers. |
| 1840 | First attempt to photograph the moon by J. W. Draper. |
| 1842 | Doppler enounces principle of colour-change by motion. |
| 1842 | Conclusion of Baily’s experiments in weighing the Earth. |
| 1842, July 8 | Total solar eclipse. Corona and prominences observed by Airy, Baily, Arago, and Struve. |
| 1843, Feb. 27 | Perihelion-passage of great comet. |
| 1845, February | Completion of Parsonstown reflector. |
| 1845, April | Discovery with it of spiral nebulæ.[Pg 448] |
| 1845, April 2 | Daguerreotype of the sun taken by Foucault and Fizeau. |
| 1845, Oct. 21 | Place of Neptune assigned by Adams. |
| 1845, Dec. 8 | Discovery of Astræa by Hencke. |
| 1845, Dec. 29 | Duplication of Biela’s comet observed at Yale College. |
| 1846 | Melloni’s detection of heating effects from moonlight. |
| 1846, March 17 | Death of Bessel. |
| 1846, Sept. 23 | Discovery of Neptune by Galle. |
| 1846, Oct. 10 | Neptune’s satellite discovered by Lassell. |
| 1847 | Publication of Sir J. Herschel’s Results of Observations at the Cape of Good Hope. |
| 1847 | Cyclonic theory of sun-spots stated by him. |
| 1848 | J. R. Mayer’s meteoric hypothesis of solar conservation. |
| 1848 | Motion-displacements of Fraunhofer lines adverted to by Fizeau. |
| 1848, April 27 | New Star in Ophiuchus observed by Hind. |
| 1848, Sept. 19 | Simultaneous discovery of Hyperion by Bond and Lassell. |
| 1849 | First experimental determination of the velocity of by Fizeau. |
| 1848, April 27 | New Star in Ophiuchus observed by Hind. |
| 1848, Sept. 19 | Simultaneous discovery of Hyperion by Bond and Lassell. |
| 1849 | First experimental determination of the velocity of light (Fizeau). |
| 1850, July 17 | Vega photographed at Harvard College. |
| 1850, Nov. 15 | Discovery by Bond of Saturn’s dusky ring. |
| 1851 | O. Struve’s first measurements of Saturn’s ring-system |
| 1851, July 28 | Total solar eclipse observed in Sweden. |
| 1851, Oct. 24 | Discovery by Lassell of two inner Uranian satellites. |
| 1851 | Schwabe’s discovery of sun-spot periodicity published by Humboldt. |
| 1852, May 6 | Coincidence of magnetic and sun-spot periods announced by Sabine. |
| 1852, Oct. 11 | Variable nebula in Taurus discovered by Hind. |
| 1852 | Lassell’s two-foot reflector transported to Malta. |
| 1853 | Adams shows Laplace’s explanation of the moon’s acceleration to be incomplete. |
| 1854 | Hansen infers from lunar theory a reduced value for the distance of the sun. |
| 1854 | Helmholtz’s “gravitation theory” of solar energy. |
| 1856 | Piazzi Smyth’s observations on the Peak of Teneriffe. |
| 1857 | Saturn’s rings shown by Clerk Maxwell to be of meteoric formation. |
| 1857, April 27 | Double-star photography initiated at Harvard College. |
| 1858 | Solar photography begun at Kew. |
| 1858, Sept. 30 | Perihelion of Donati’s comet. |
| 1859 | Spectrum analysis established by Kirchhoff and Bunsen. |
| 1859 | Carrington’s discovery of the compound nature of the sun’s rotation. |
| 1859, Sept. 1 | Luminous solar outburst and magnetic storm. |
| 1859, Oct. 19 | Merope nebula discovered by Tempel.[Pg 449] |
| 1859, Dec. 15 | Chemical constitution of the sun described by Kirchhoff. |
| 1860, Feb. 27 | Discovery by Liais of a “double comet.” |
| 1860, May 21 | New star in Scorpio detected by Auwers. |
| 1860, July 18 | Total solar eclipse observed in Spain. Prominences shown by photography to be solar appendages. |
| 1861, June 30 | The earth involved in the tail of a great comet. |
| 1861-1862 | Kirchhoff’s map of the solar spectrum. |
| 1862 | Solar hydrogen-absorption recognised by Ångström. |
| 1862, Jan. 31 | Discovery by Alvan G. Clark of the companion of Sirius. |
| 1862 | Foucault determines the sun’s distance by the velocity of light. |
| 1862 | Opposition of Mars. Determination of solar parallax. |
| 1862 | Completion of Bonner Durchmusterung. |
| 1863 | Secchi’s classification of stellar spectra. |
| 1863 | Foundation of the German Astronomical Society. |
| 1864, March 5 | Rotation period of Mars determined by Kaiser. |
| 1864 | Huggins’s first results in stellar spectrum analysis. |
| 1864, Aug. 5 | Spectroscopic examination of Tempel’s comet by Donati shows it to be composed of glowing gas. |
| 1864, Aug. 29 | Discovery by Huggins of gaseous nebulæ. |
| 1864 | Value of 91,000,000 miles adopted for the sun’s distance. |
| 1864 | Croll’s explanation of glacial epochs. |
| 1864, Nov. 23 | Death of Struve. |
| 1865, Jan. 4 | Spectroscopic observation by Huggins of the occultation of ε Piscium. |
| 1865, Jan. 16 | Faye’s theory of the solar constitution. |
| 1865 | Kew results published. |
| 1865 | Zöllner argues for a high temperature in the great planets. |
| 1866 | Identity of the orbits of the August meteors and of comet 1862 iii. demonstrated by Schiaparelli. |
| 1866 | Delaunay explains lunar acceleration by a lengthening of the day through tidal friction. |
| 1866, March 4 | Spectroscopic study of the sun’s surface by Lockyer. |
| 1866, March 12 | New star in Corona Borealis detected by Birmingham. |
| 1866, October | Schmidt announces the disappearance of the lunar crater Linné. |
| 1866, Nov. 13 | Meteoric shower visible in Europe. |
| 1867 | Period of November meteors determined by Adams. |
| 1867, Aug. 29 | Total solar eclipse. Minimum sun-spot type of corona observed by Grosch at Santiago. |
| 1867 | Discovery of gaseous stars in Cygnus by Wolf and Rayet. |
| 1868, February | Principle of daylight spectroscopic visibility of prominences started by Huggins.[Pg 450] |
| 1868, Aug. 18 | Great Indian eclipse. Spectrum of prominences observed. |
| 1868, Aug. 19 | Janssen’s first daylight view of a prominence. |
| 1868, Oct. 26 | Lockyer and Janssen independently announce their discovery of the spectroscopic method. |
| 1868 | Doppler’s principle applied by Huggins to measure stellar radial movements. |
| 1868 | Publication of Ångström’s map of the normal solar spectrum. |
| 1868 | Spectrum of Winnecke’s comet found by Huggins to agree with that of olefiant gas. |
| 1869, Feb. 11 | Tenuity of chromospheric gases inferred by Lockyer and Frankland. |
| 1869, Feb. 13 | Huggins observes a prominence with an “open slit.” |
| 1869, Aug. 7 | American eclipse. Detection of bright-line coronal spectrum. |
| 1870 | Mounting of Newall’s 25-inch achromatic at Gateshead. |
| 1870 | Proctor indicates the prevalence of drifting movements among the stars. |
| 1870 | A solar prominence photographed by Young. |
| 1870, Dec. 22 | Sicilian eclipse. Young discovers reversing layer. |
| 1871, May 11 | Death of Sir J. Herschel. |
| 1871, June 9 | Line-displacements due to solar rotation detected by Vogel. |
| 1871, Dec. 12 | Total eclipse visible in India. Janssen observes reflected Fraunhofer lines in spectrum of corona. |
| 1872 | Conclusion of a three years’ series of observations on lunar heat by Lord Rosse. |
| 1872 | Spectrum of Vega photographed by H. Draper. |
| 1872 | Faye’s cyclonic hypothesis of sun-spots. |
| 1872 | Young’s solar-spectroscopic observations at Mount Sherman. |
| 1872 | Cornu’s experiments on the velocity of light. |
| 1872, Nov. 27 | Meteoric shower connected with Biela’s comet. |
| 1873 | Determination of mean density of the earth by Cornu and Baille. |
| 1873 | Solar photographic work begun at Greenwich. |
| 1873 | Erection of 26-inch Washington refractor. |
| 1874 | Light-equation redetermined by Glasenapp. |
| 1874 | Vogel’s classification of stellar spectra. |
| 1874, Dec. 8 | Transit of Venus. |
| 1876 | Publication of Neison’s The Moon. |
| 1876, Nov. 24 | New star in Cygnus discovered by Schmidt. |
| 1876 | Spectrum of Vega photographed by Huggins. First use of dry gelatine plates in celestial photography. |
| 1877, May 19 | Klein observes a supposed new lunar crater (Hyginus N.). |
| 1877 | Measurement by Vogel of selective absorption in solar atmosphere.[Pg 451] |
| 1877, Aug. 16-17 | Discovery of two satellites of Mars by Hall at Washington. |
| 1877, Sept. 23 | Death of Leverrier. |
| 1877 | Canals of Mars discovered by Schiaparelli. |
| 1877 | Opposition of Mars observed by Gill at Ascension. Solar parallax deduced = 8·78′. |
| 1878, January | Stationary meteor-radiants described by Denning. |
| 1878 | Publication of Schmidt’s Charte der Gebirge des Mondes. |
| 1878 | First observations of Great Red Spot on Jupiter. |
| 1878 | Conclusion of Newcomb’s researches on the lunar theory. |
| 1878, May 6 | Transit of Mercury. |
| 1878 | Foundation of Selenographical Society. |
| 1878, July 29 | Total eclipse visible in America. Vast equatoreal extension of the corona. |
| 1878, October | Completion of Potsdam Astrophysical Observatory. |
| 1878, Dec. 12 | Lockyer’s theory of celestial dissociation communicated to the Royal Society. |
| 1879 | Michelson’s experiments on the velocity of light. |
| 1879 | Publication of Gould’s Uranometria Argentina. |
| 1879, November | Observations of the spectra of sun-spots begun at South Kensington. |
| 1879, Dec. 5 | Abney’s map of the infra-red solar spectrum presented to the Royal Society. |
| 1879, Dec. 18 | Ultra-violet spectra of white stars described by Huggins. |
| 1879, Dec. 18 | Communication of G. H. Darwin’s researches into the early history of the moon. |
| 1880, Jan. 31 | Discovery at Cordoba of a great southern comet. |
| 1880 | Conditions of Algol’s eclipses determined by Pickering. |
| 1880 | Pickering computes mass-brightness of binary stars. |
| 1880, Sept. 30 | Draper’s photograph of the Orion nebula. |
| 1880 | The bolometer invented by Langley. |
| 1881, Jan. 20 | Communication of G. H. Darwin’s researches into the effects of tidal friction on the evolution of the solar system. |
| 1881 | Langley’s observations of atmospheric absorption on Mount Whitney. |
| 1881, June 16 | Perihelion of Tebbutt’s comet. |
| 1881, June 24 | Its spectrum photographed by Huggins. |
| 1881, June | Photographs of Tebbutt’s comet by Janssen and Draper. |
| 1881, Aug. 15 | Retirement of Sir George Airy. Succeeded by Christie. |
| 1881, Aug. 22 | Perihelion of Schaeberle’s comet. |
| 1881 | Publication of Stone’s Cape Catalogue for 1880. |
| 1881 | Struve’s second measures of Saturn’s ring-system.[Pg 452] |
| 1882 | Newcomb’s determination of the velocity of light. Resulting solar parallax = 8·79′. |
| 1882 | Correction by Nyrén of Struve’s constant of aberration. |
| 1882, March 7 | Spectrum of Orion nebula photographed by Huggins. |
| 1882, May 17 | Total solar eclipse observed at Sohag in Egypt. |
| 1882, May 27 | Sodium-rays observed at Dunecht in spectrum of Comet Wells. |
| 1882, June 10 | Perihelion of Comet Wells. |
| 1882, Sept. 17 | Perihelion of Great Comet. Daylight detection by Common. Transit observed at the Cape. |
| 1882, Sept. 18 | Iron lines identified in spectrum by Copeland and J. G. Lohse. |
| 1882, September | Photographs of comet taken at the Cape Observatory, showing a background crowded with stars. |
| 1882, Dec. 6 | Transit of Venus. |
| 1882 | Duplication of Martian canals observed by Schiaparelli. |
| 1882 | Completion by Loewy at Paris of first equatoreal Coudé. |
| 1882 | Rigidity of the earth concluded from tidal observations by G. H. Darwin. |
| 1882 | Experiments by Huggins on photographing the corona without an eclipse. |
| 1882 | Publication of Holden’s Monograph of the Orion Nebula. |
| 1883, Jan. 30 | Orion Nebula photographed by Common. |
| 1883, May 6 | Caroline Island eclipse. |
| 1883, June 1 | Great comet of 1882 observed from Cordoba at a distance from the earth of 470 million miles. |
| 1883 | Parallaxes of nine southern stars measured by Gill and Elkin. |
| 1883 | Catalogue of the spectra of 4,051 stars by Vogel. |
| 1884, Jan. 25 | Return to perihelion of Pons’s comet. |
| 1884 | Photometric Catalogue by Pickering of 4,260 stars. |
| 1884 | Publication of Gore’s Catalogue of Variable Stars. |
| 1884 | Publication of Faye’s Origine du Monde. |
| 1884, Oct. 4 | Eclipse of the moon. Heat-phases measured by Boeddicker at Parsonstown. |
| 1884 | Dunér’s Catalogue of Stars with Banded Spectra. |
| 1884 | Backlund’s researches into the movements of Encke’s comet. |
| 1885, February | Langley measures the lunar heat-spectrum. |
| 1885 | Publication of Uranometria Nova Oxoniensis. |
| 1885, Aug. 17 | New star in Andromeda nebula discerned by Gully. |
| 1885, Sept. 5 | Thollon’s drawing of the solar spectrum presented to the Paris Academy. |
| 1885, Sept. 9 | Solar eclipse visible in New Zealand.[Pg 453] |
| 1885, Nov. 16 | Photographic discovery by Paul and Prosper Henry of a nebula in the Pleiades. |
| 1885, Nov. 27 | Shower of Biela meteors. |
| 1885 | Thirty-inch achromatic mounted at Pulkowa. |
| 1885 | Publication of Rowland’s photographic map of the normal solar spectrum. |
| 1885 | Bakhuyzen’s determination of the rotation period of Mars. |
| 1885 | Stellar photographs by Paul and Prosper Henry. |
| 1886, Jan. 26 | Spectra of forty Pleiades simultaneously photographed at Harvard College. |
| 1886, Feb. 5 | First visual observation of the Maia nebula with Pulkowa 30-inch refractor. |
| 1886, March | Photographs by the Henrys of the Pleiades, showing 2,326 stars with nebulæ intermixed. |
| 1886, May | Photographic investigations of stellar parallax undertaken by Pritchard. |
| 1886, May 6 | Periodical changes in spectra of sun-spots announced by Pritchard. |
| 1886, June 4 | An international Photographic Congress proposed by Gill. |
| 1886, Aug. 29 | Total eclipse of the sun observed at Grenada. |
| 1886, Oct. 1 | Roberts’s photograph showing annular structure of the Andromeda nebula. |
| 1886, Dec. 8 | Roberts’s photograph of the Pleiades nebulosities. |
| 1886 | Solar heat-spectrum extended by Langley to below five microns. |
| 1886, Dec. 28 | Detection by Copeland of helium-ray in spectrum of the Orion nebula. |
| 1886 | Thirty-inch refractor mounted at Nice. |
| 1886 | Publication of Argentine General Catalogue. |
| 1886 | Completion of Auwers’s reduction of Bradley’s observations. |
| 1886 | Draper Memorial photographic work begun at Harvard College. |
| 1886 | Photographic detection at Harvard College of bright hydrogen lines in spectra of variables (Mira Ceti and U Orionis). |
| 1887, Jan. 18 | Discovery by Thome at Cordoba of a great comet belonging to the same group as the comet of 1882. |
| 1887 | Publication of Lockyer’s Chemistry of the Sun. |
| 1887, April 16 | Meeting at Paris of the International Astrophotographic Congress. |
| 1887 | Heliometric triangulation of the Pleiades by Elkin. |
| 1887 | L. Struve’s investigation of the sun’s motion, and redetermination of the constant of precession. |
| 1887 | Von Konkoly’s extension to 15° S. Dec. of Vogel’s spectroscopic Catalogue. |
| 1887 | Auwers’s investigation of the solar diameter. |
| 1887 | Publication of Schiaparelli’s Measures of Double Stars (1875-85).[Pg 454] |
| 1887, April 8 | Death of Thollon at Nice. |
| 1887, Aug. 19 | Total eclipse of the sun. Shadow-path crossed Russia. Observations marred by bad weather. |
| 1887, November | Langley’s researches on the temperature of the moon. |
| 1887, Nov. 17 | Lockyer’s Researches on Meteorites communicated to the Royal Society. |
| 1887 | Completion of 36-inch Lick refractor. |
| 1888 | Küstner’s detection of variations in the latitude of Berlin brought before the International Geodetic Association. |
| 1888 | Chandler’s first Catalogue of Variable Stars. |
| 1888 | Mean parallax of northern first magnitude stars determined by Elkin. |
| 1888 | Publication of Dreyer’s New General Catalogue of 7,844 nebulæ. |
| 1888 | Vogel’s first spectrographic determinations of stellar radial motion. |
| 1888 | Carbon absorption recognised in solar spectrum by Trowbridge and Hutchins. |
| 1888, Jan. 28 | Total eclipse of the moon. Heat-phases measured at Parsonstown. |
| 1888, Feb. 5 | Remarkable photograph of the Orion nebula spectrum taken at Tulse Hill. |
| 1888, June 1 | Activity of the Lick Observatory begun. |
| 1888 | Completion of Dr. Common’s 5-foot reflector. |
| 1888 | Heliometric measures of Iris for solar parallax at the Cape, Newhaven (U.S.A.), and Leipsic. |
| 1888 | Loewy describes a comparative method of determining constant of aberration. |
| 1888 | Presentation of the Dunecht instrumental outfit to the nation by Lord Crawford. Copeland succeeds Piazzi Smyth as Astronomer-Royal for Scotland. |
| 1888, Sept. 12 | Death of R. A. Proctor. |
| 1889 | Photograph of the Orion nebula taken by W. H. Pickering, showing it to be the nucleus of a vast spiral. |
| 1889 | Discovery at a Harvard College of the first-known spectroscopic doubles, ζ Ursæ Majoris and β Aurigæ. |
| 1889 | Eclipses of Algol demonstrated spectrographically by Vogel. |
| 1889 | Completion of photographic work for the Southern Durchmusterung. |
| 1889 | Boeddicker’s drawing of the Milky Way. |
| 1889 | Draper Memorial photographs of southern star-spectra taken in Peru. |
| 1889 | Pernter’s experiments on scintillation from the Sonnblick.[Pg 455] |
| 1889 | H. Struve’s researches on Saturn’s satellites. |
| 1889 | Harkness’s investigation of the masses of Mercury, Venus, and the Earth. |
| 1889 | Heliometric measures of Victoria and Sappho at the Cape. |
| 1889, Jan. 1 | Total solar eclipse visible in California. |
| 1889, Feb. 7 | Foundation of the Astronomical Society of the Pacific. |
| 1889, March | Investigation by Sir William and Lady Huggins of the spectrum of the Orion nebula. |
| 1889, July-Aug. | First photographs of the Milky Way taken by Barnard. |
| 1889, August 2 | Observation by Barnard of four companions to Brooks’s comet. |
| 1889, Nov. 1 | Passage of Japetus behind Saturn’s dusky ring observed by Barnard. |
| 1889, December | Schiaparelli announces synchronous rotation and revolution of Mercury. |
| 1889, Dec. 22 | Total eclipse of the sun visible in Guiana. Death of Father Perry, December 27. |
| 1889 | Spectrum of Uranus investigated visually by Keeler, photographically by Huggins. |
| 1890 | Long-exposure photographs of ring-nebula in Lyra. |
| 1890 | Determinations of the solar translation by L. Boss and O. Stumpe. |
| 1890 | Schiaparelli finds for Venus an identical period of rotation and revolution. |
| 1890 | Publication of Thollon’s map of the solar spectrum. |
| 1890 | Bigelow’s mathematical theory of coronal structures. |
| 1890 | Foundation of the British Astronomical Association. |
| 1890 | Measurements by Keeler at Lick of nebular radial movements. |
| 1890 | Janssen’s ascent of Mont Blanc, by which he ascertained the purely terrestrial origin of the oxygen-absorption in the solar spectrum. |
| 1890 | Newcomb’s discussion of the transits of Venus of 1761 and 1769. |
| 1890 | Spiral structure of Magellanic Clouds displayed in photographs taken by H. C. Russell of Sydney. |
| 1890 | Publication of the Draper Catalogue of Stellar Spectra. |
| 1890, April 24 | Spica announced by Vogel to be a spectroscopic binary. |
| 1890, June | Gore’s Catalogue of computed Binaries. |
| 1890, November | Study by Sir William and Lady Huggins of the spectra of Wolf and Rayet’s stars in Cygnus. |
| 1890, November | Discovery by Barnard of a close nebulous companion to Merope in the Pleiades. |
| 1890, November | McClean Spectrographs of the High and Low Sun. |
| 1891 | Capture-theory of comets developed by Callandreau, Tisserand, and Newton. |
| 1891 | Dunér’s spectroscopic researches on the sun’s rotation.[Pg 456] |
| 1891 | Preponderance of Sirian stars in the Milky Way concluded by Pickering, Gill, and Kapteyn. |
| 1891 | Detection by Mrs. Fleming of spectral variations corresponding to light-changes in β Lyræ. |
| 1891 | Establishment of the Harvard College Station at Arequipa in Peru (height 8,000 feet). |
| 1891 | Variations of latitude investigated by Chandler. |
| 1891 | Prominence-photography set on foot by Hale at Chicago and Deslandres at Paris. |
| 1891 | Schmidt’s Theory of Refraction in the Sun. |
| 1891, April | Meeting at Paris of the Permanent Committee for the Photographic Charting of the Heavens. |
| 1891, May 9 | Transit of Mercury. |
| 1891, Aug. 19 | Presidential Address by Huggins at the Cardiff Meeting of the British Association. |
| 1891, Dec. 10 | Nova Aurigæ photographed at Harvard College. |
| 1891, Dec. 20 | Photographic maximum of Nova Aurigæ. |
| 1891, Dec. 22 | First photographic discovery of a minor planet by Max Wolf at Heidelberg. |
| 1892 | Commencement of international photographic charting work. |
| 1892 | Photographic determination by Scheiner of 833 stars in the Hercules Cluster (M 13). |
| 1892 | Publication of Vogel’s spectrographic determinations for fifty-one stars. |
| 1892 | Publication of Pritchard’s photographic parallaxes. |
| 1892, Jan. 2 | Death of Sir George Airy. |
| 1892, Jan. 21 | Death of Professor Adams. |
| 1892, Feb. 1 | Announcement by Anderson of the outburst of a new star in Auriga. |
| 1892, Feb. 5 | Appearance of the largest sun-spot ever photographed at Greenwich. |
| 1892, March | Photograph of Argo nebula taken by Gill in twelve hours. |
| 1892, March 6 | Discovery of a bright comet by Swift. |
| 1892, June 29 | Death of Admiral Mouchez. Succeeded by Tisserand as director of the National Observatory, Paris. |
| 1892, Aug. 4 | Favourable Opposition of Mars. |
| 1892, Aug. 17 | Rediscovery at Lick of Nova Aurigæ. |
| 1892, Sept. 9 | Discovery by Barnard of Jupiter’s inner satellite. |
| 1892, Oct. 12 | First photographic discovery of a comet by Barnard. |
| 1892, Nov. 6 | Discovery of Holmes’s comet. |
| 1892, Nov. 23 | Shower of Andromede meteors visible in America. |
| 1892 | Poynting’s Determination of the Earth’s Mean Density. |
| 1892 | Dunér’s Investigation of the System of Υ Cygni. |
| 1892 | Photographic investigation by Deslandres of the spectra of prominences. |
| 1892 | Photographs of the sun with faculæ and chromospheric surroundings taken by Hale with a single exposure. |
| 1892 | Investigation by T. J. J. See of the ancient colour of Sirius.[Pg 457] |
| 1892 | Publication of T. J. J. See’s Thesis on the Evolution of Binary Systems. |
| 1892 | Chandler’s theory of Algol’s inequalities. |
| 1892 | Nebula in Cygnus photographically discovered by Max Wolf. |
| 1893, Jan. 28 | Kapteyn’s investigation of the structure of the universe. |
| 1893, March 10 | Gill announces his results from the Opposition of Victoria, among them a solar parallax = 8·809′. |
| 1893, April 16 | Total solar eclipse observed in South America and West Africa. |
| 1893 | Publication of Kruger’s Catalog der Farbigen Sterne. |
| 1893 | Conclusion of Boys’s series of Experiments on the Density of the Earth. |
| 1893 | Publication of Cordoba Durchmusterung, vol. i. |
| 1893 | Fabry shows comets to be dependents of the Solar System. |
| 1893 | Publication of Easton’s Voie Lactée. |
| 1893 | Campbell detects bright Hα in γ Argûs and Alcyone. |
| 1893 | Nova Normæ photographed July 10; discovered on plates, October 26. |
| 1893, May 28 | Death of Professor Pritchard. |
| 1893, July 27 | Installation of 28-inch refractor at the Royal Observatory, Greenwich. |
| 1893, December | Exterior nebulosities of Pleiades photographed by Barnard. |
| 1893, Dec. 6 | Death of Rudolf Wolf. |
| 1894, January | Sun-spot maximum. |
| 1894 | Publication of Potsdam Photometric Durchmusterung, part i. |
| 1894 | Publication of Roberts’s Celestial Photographs, vol. i. |
| 1894 | Wilson and Gray’s determination of the sun’s temperature. |
| 1894 | Barnard’s micrometric measures of asteroids. |
| 1894 | McClean’s gift of an astrophysical outfit to the Cape Observatory. |
| 1894 | Establishment of the Lowell Observatory at Flagstaff, Arizona. |
| 1894 | Taylor’s triple achromatic objective described. |
| 1894, April 3 | Discovery of Gale’s Comet. |
| 1894 | Sampson’s investigation of the sun’s rotation. |
| 1894, Oct. 20 | Favourable opposition of Mars. |
| 1894, Nov. 11 | Transit of Mercury. |
| 1894, December | Howlett impugns the Wilsonian theory of sun-spots. |
| 1894, Dec. 14 | Death of A. Cowper Ranyard. |
| 1895 | Publication of Newcomb’s Astronomical Constants.[Pg 458] |
| 1895 | Bailey’s Photometric Catalogue of 7,922 Southern Stars. |
| 1895 | Bailey’s photographic discovery of variable star clusters. |
| 1895 | Publication of E. W. Brown’s Lunar Theory. |
| 1895 | Tisserand’s theory of the inequalities of Algol. |
| 1895 | Stratonoff’s determination of the sun’s rotation from photographs of faculæ. |
| 1895 | Binary character of η Aquilæ spectroscopically recognised by Bélopolsky. |
| 1895 | Presentation of the Crossley reflector to the Lick Observatory. |
| 1895, March 23 | Great nebula in Ophiuchus discovered photographically by Barnard. |
| 1895, March 25 | Ramsay’s capture of Helium. |
| 1895, April | Constitution of Saturn’s rings spectrographically demonstrated by Keeler. |
| 1895 | Binary character of δ Cephei spectroscopically detected by Bélopolsky. |
| 1895, June 11 | Death of Daniel Kirkwood. |
| 1895, July 7 | Death of F. W. G. Spörer. |
| 1895, October | Nova Carinæ spectrographically discovered by Mrs. Fleming. |
| 1895, Dec. 12 | Nova Centauri spectrographically discovered by Mrs. Fleming. |
| 1895, Dec. 28 | Death of John Russell Hind. |
| 1896 | Gill’s Report on the Geodetic Survey of South Africa. |
| 1896 | Appearance of Loewy’s Photographic Atlas of the Moon, part i. |
| 1896, January | Fessenden’s electrostatic theory of comets. |
| 1896 | Chandler’s Third Catalogue of Variable Stars. |
| 1896 | Publication of Lick Observatory Photographic Atlas of the Moon, part i. |
| 1896, February | Effects of pressure on wave-length described by Humphreys and Mohler. |
| 1896, April 5 | Opening of new Scottish Royal Observatory on Blackford Hill, Edinburgh. |
| 1896, April | Pickering’s photometric determinations of light curves of variable stars. |
| 1896 | One of the stars of Castor spectroscopically resolved into two by Bélopolsky. |
| 1896, May | Third Astrographic Chart Conference at Paris. |
| 1896, Aug. 9 | Total eclipse of the sun visible in Novaya Zemlya. Reversing layer photographed by Shackleton. |
| 1896, Aug. 30 | Death of Hubert A. Newton. |
| 1896, Sept. 18 | Death of Hippolyte Fizeau. |
| 1896, Oct. 20 | Death of F. Tisserand. Succeeded by Maurice Loewy. |
| 1896, Nov. 13 | Detection by Schaeberle of Procyon’s missing satellite. |
| 1896, Nov. 26 | Death of Benjamin Apthorp Gould.[Pg 459] |
| 1896, November | Second series of hydrogen-lines discovered by Pickering in stellar spectra. |
| 1896, December | Zeeman’s discovery of spectral modifications through magnetic influence. |
| 1896, December | Oxygen-absorption identified in the sun by Runge and Paschen. |
| 1896 | Study of lunar formations by Loewy and Puiseux. |
| 1896 | Mounting of the Mills spectrograph at the Lick Observatory. |
| 1897 | Installation at Greenwich of the Thompson 26-inch photographic refractor. |
| 1897 | Publication of Miss Maury’s Discussion of the Photographed Spectra of 681 Stars. |
| 1897 | Callandreau’s researches on cometary disaggregation. |
| 1897 | Braun’s determination of the earth’s mean density. |
| 1897 | Tenuity of calcium vapour in chromosphere demonstrated spectroscopically by Sir William and Lady Huggins. |
| 1897 | Completion at the Cape Observatory of McClean’s spectrographic survey of the heavens. |
| 1897 | Twenty-one Wolf-Rayet stars found by Mrs. Fleming in Magellanic Cloud. |
| 1897 | Percival Lowell’s New Observations on the Planet Mercury presented to the American Academy. |
| 1897, April 8 | McClean recognises oxygen-absorption in helium stars. |
| 1897, May 9 | Death of E. J. Stone, Radcliffe Observer. |
| 1897, June 10 | Death of Alvan G. Clark. |
| 1897, June 18 | Spectrum of a meteor photographed at Arequipa. |
| 1897, Oct. 21 | Inauguration of the Yerkes Observatory. |
| 1897 | Rabourdin’s photographs of nebulæ with the Meudon reflector. |
| 1897 | Dr. See’s discoveries of Southern double stars with the Lowell 24-inch refractor. |
| 1898, Jan. 22 | Total eclipse of the sun visible in India. |
| 1898, February | Binary character of ζ Geminorum ascertained spectroscopically by Bélopolsky. |
| 1898 | Star with proper motion of nearly 9′ discovered by Innes and Kapteyn from the Cape Durchmusterung plates. |
| 1898, March 8 | Nova Sagittarii photographed on Draper Memorial plates. |
| 1898, June 20 | Opening of Grand-ducal Observatory at Königsstuhl, Heidelberg. |
| 1898 | Keeler succeeds Holden as Director of the Lick Observatory. |
| 1898 | Bruno Peter’s results in stellar parallax. |
| 1898 | Lewis Swift’s discoveries of nebulæ at Echo California. |
| 1898 | Hale’s photographic investigation of carbon stars.[Pg 460] |
| 1898, Aug. 14 | Discovery of Eros by Witt. |
| 1898 | Flint’s investigations of stellar parallax by meridian differences. |
| 1898 | Easton’s spiral theory of the Milky Way. |
| 1898 | Seeliger’s research on star distribution. |
| 1898, October | Multiple hydrogen-bands observed by Campbell in Mira Ceti. |
| 1898, November | Orbit of a Leonid meteor photographically determined by Elkin. |
| 1899 | Publication of Potsdam Photometric Durchmusterung, part ii. |
| 1899 | Innes’s Reference Catalogue of Southern Double Stars. |
| 1899 | Keeler’s photographs of nebulæ with the Crossley reflector and generalization of their spiral character. |
| 1899, January | Spectrum of Andromeda nebula photographed by Scheiner. |
| 1899, April | Photographic discovery of Nova Aquilæ by Mrs. Fleming. |
| 1899, Aug. 26 | Installation of 31-inch photographic refractor at Potsdam. |
| 1899 | Campbell’s detection of Polaris as spectroscopically triple. |
| 1899, October | Duplicate discovery by Campbell and Newall of Capella as a spectroscopic binary. |
| 1899, Nov. 15 | Failure of the Leonids. Deflection of the stream predicted by Johnstone Stoney and Downing. |
| 1899, December | Publication of Sir William and Lady Huggins’s Atlas of Representative Stellar Spectra. |
| 1899 | Thirty-two-inch photographic refractor mounted at Meudon. |
| 1899 | Issue of first volume of Potsdam measures of international catalogue plates. |
| 1900, Jan. 27 | Kapteyn’s determination of the apex of solar motion. |
| 1900 | Chase’s measures for parallax of swiftly-moving stars. |
| 1900 | Publication of Gill’s Researches on Stellar Parallax. |
| 1900 | Kapteyn proposes a method for a stellar parallax Durchmusterung, and gives specimen results for 248 stars. |
| 1900 | Burnham’s general catalogue of 1,290 double stars. |
| 1900 | Publication of the concluding volume of the Cape Photographic Durchmusterung. |
| 1900, May 28 | Spanish-American total eclipse of the sun. |
| 1900, July | International Conference at Paris. Co-operation arranged of fifty-eight observatories in measures of Eros for solar parallax. |
| 1900 | Horizontal refractor, of 50 inches aperture, 197 feet focus, installed in Paris Exhibition.[Pg 461] |
| 1900, Aug. 12 | Death of Professor Keeler. Succeeded by Campbell in direction of Lick Observatory. |
| 1900, November | Opposition of Eros. |
| 1900 | Publication of Roberts’s Celestial Photographs, vol. ii. |
| 1900 | Complete publication of Langley’s researches on infra-red spectrum. |
| 1900 | Printing begun of Paris section of International Photographic Catalogue. |
| 1901, Feb. 22 | Nova Persei discovered by Anderson. |
| 1901, February | Variability of Eros announced by Oppolzer. |
| 1901, April 23 | Apparition of a great comet at the Cape. |
| 1901 | Publication of Pickering’s Photometric Durchmusterung. |
| 1901 | Miss Cannon’s discussion of the spectra of 1,122 Southern stars. |
| 1901 | Kapteyn’s investigation of mean stellar parallax. |
| 1901 | Campbell’s determination of the sun’s velocity. |
| 1901 | Porter’s research on the solar motion in space. |
| 1901 | Bigelow’s magnetic theory of the solar corona. |
| 1901 | Hussey’s measurements of the Pulkowa double stars. |
| 1901 | Radial velocities of the components of δ Equulei measured at Lick. |
| 1901, April 16 | Death of Henry A. Rowland. |
| 1901, June | Nebular spectrum derived from Nova Persei. |
| 1901, Aug. 23 | Nebula near Nova Persei photographed by Max Wolf. |
| 1901, Sept. 20 | The same exhibited in spiral form on a plate taken by Ritchey at the Yerkes Observatory. |
| 1901, Nov. 8 | Photograph taken by Perrine with the Crossley reflector showed nebula in course of rapid change. |
| 1901, Sept. 19 | Unveiling of the McClean “Victoria” telescope at the Royal Observatory, Cape of Good Hope. |
| 1901 | Sun-spot minimum. |
TABLE II
CHEMICAL ELEMENTS IN THE SUN (ROWLAND, 1891).
Arranged according to the number of their representative Lines in the
Solar Spectrum.
| Iron (2000+). | Neodymium. | Cadmium. |
| Nickel. | Lanthanum. | Rhodium. |
| Titanium. | Yttrium. | Erbium. |
| Manganese. | Niobium. | Zinc. |
| Chromium. | Molybdenum. | Copper (2). |
| Cobalt. | Palladium. | Silver (2). |
| Carbon (200+). | Magnesium (20+). | Glucinum (2). |
| Vanadium. | Sodium (11). | Germanium. |
| Zirconium. | Silicon. | Tin. |
| Cerium. | Strontium. | Lead (1). |
| Calcium (75+). | Barium. | Potassium (1). |
| Scandium. | Aluminium (4). |
TABLE III
EPOCHS OF SUN-SPOT MAXIMUM AND MINIMUM FROM 1610 TO 1901.
| Minima. | Maxima. | Minima. | Maxima. | Minima. | Maxima. | ||
| 1610·8 | 1615·5 | 1712·0 | 1718·2 | 1810·6 | 1816·4 | ||
| 1619·0 | 1626·0 | 1723·5 | 1727·5 | 1823·3 | 1829·9 | ||
| 1634·0 | 1639·5 | 1734·0 | 1738·7 | 1833·9 | 1837·2 | ||
| 1645·0 | 1649·0 | 1745·0 | 1750·3 | 1843·5 | 1848·1 | ||
| 1655·0 | 1660·0 | 1755·2 | 1761·5 | 1856·0 | 1860·1 | ||
| 1666·0 | 1675·0 | 1766·5 | 1769·7 | 1867·2 | 1870·6 | ||
| 1679·5 | 1685·0 | 1775·5 | 1778·4 | 1878·9 | 1884·0 | ||
| 1689·5 | 1693·0 | 1784·7 | 1788·1 | 1890·2 | 1894·0 | ||
| 1698·9 | 1705·5 | 1798·3 | 1804·2 | 1901·9 |
TABLE IV.
MOVEMENTS OF SUN AND STARS.
1. Translation of Solar System.
| Apex of Movement. | Authority. | Date. | |
| R. A. | Dec. | ||
| 277° 30′ | + 35° | Newcomb | 1898 |
| 273° 36′ | + 29° 30′ | Kapteyn | 1901 |
| 279° | + 46° | Porter | 1901 |
| 275° | + 45° | Boss | 1901 |
| 277° 30′ | + 20° | Campbell (from stellar spectroscopic measures) | 1902 |
| Velocity=12·4 miles per second (Campbell). | |||
2. Stellar Velocities.
| Name of Star. | Rate. | Direction. | Remarks. |
| Miles per Sec. | |||
| δ Leporis | 58 | Receding | Campbell, 1901 |
| η Cephei | 54 | Approaching | ” 1899 |
| θ Canis Majoris | 60 | Receding | ” 1901 |
| ι Pegasi | 47 | Approaching | ” “ |
| μ Sagittarii | 47 | Approaching | ” “ |
| ε Andromedæ | 52 | Approaching | ” “ |
| ζ Herculis | 44 | Approaching | Bélopolsky, 1893 |
| 61 Cygni | 34 | Approaching | ” “ |
| μ Cassiopeiæ | 60 | Approaching | Campbell, 1901 |
| 1830 Groombridge | 59 | Approaching | ” “ |
| Arcturus | 4·3 | Approaching | Keeler, 1890 |
| Arcturus | 278 | Tangential | Accepting Elkin’s parallax of 0·024′ |
| 1830 Groombridge | 150 | Tangential | Parallax = 0·14′ |
| μ Cassiopeiæ | 113 | Tangential | Parallax = 0·10′ (Peter) |
| Z. C. 5h 243 | 82 | Tangential | Parallax = 0·312′ (Gill) |
| Lacaille, 2,957 | 78 | Tangential | Parallax = 0·064′ (Gill) |
| Lacaille, 9,352 | 73 | Tangential | Parallax = 0·283′ (Gill) |
| o2, Eridani | 72 | Tangential | Parallax = 0·166′ (Gill) |
| ε Eridani | 61 | Tangential | Parallax = 0·149′ (Gill) |
TABLE V.
LIST OF GREAT TELESCOPES.
1. Reflectors–A. Metallic Specula.
| Locality. | Aperture in Inches. | Focal Length in Feet. | Constructor. | Remarks. |
| Birr Castle, Parsonstown, Ireland | 72 | 54 | Third Earl of Rosse, 1845 | Newtonian. |
| Melbourne Observatory | 48 | 28 | T. Grubb, 1870 | Cassegrain. |
| Birr Castle | 36 | — | Third Earl of Rosse, 1839 | Newtonian. Remounted equatoreally 1876. |
| Royal Observatory Greenwich | 24 | 20 | William Lassell, 1846 | Newtonian. Presented by the Missess Lassell to the Royal Observatory |
B. Silvered Glass Mirrors. | ||||
| Ealing, near London | 60 | 27 | A. A. Common, 1891 | Newtonian. |
| Yerkes Observatory | 60 | 25 | G. W. Richey, 1902 | Can be employed at choice as a Coudé or a Cassegrain. |
| National Observatory, Paris | 48 | — | Martin, 1875 | Newtonian. Remodelled for spectrographic work by Deslandres in 1892 |
| Meudon Observatory | 39 | 9·7 | ||
| Lick Observatory | 36 | 17·5 | Calver, 1879 | Mounted by Common at Ealing in 1879. Sold by him to Crossley, 1885. Presented by Crossley to the Lick Observatory, 1895. |
| Toulouse Observatory | 32·5 | 16·2 | Brothers Henry | |
| Marseilles Observatory | 31·5 | — | Foucault | |
| Royal Observatory, Greenwich | 30 | — | Cassegrain. Mounted as a counterpoise to the Thompson equatoreal.[Pg 465] | |
| Westgate-on-Sea | 30 | — | Common, 1889 | The property of Sir Norman Lockyer. |
| Harvard College Observatory | 28 | — | H. Draper, 1870 | Mounted for spectrographic work,1887. |
| Royal Observatory, Edinburgh | 24 | — | T. Grubb, 1872 | |
| Daramona, Ireland | 24 | 10·5 | Sir H. Grubb, 1881 | Remounted 1891. Owned by Mr. W. E.Wilson. |
| Yerkes Observatory | 23·5 | 7·7 | Ritchey, 1901 | Ritchey, Cassegrain, with an equivalent focal length of 38 feet. |
| Harvard College Observatory | 20 | — | Common, 1890 | |
| Crowborough, Sussex | 20 | 8·2 | Sir H. Grubb, 1885 | Mounted with a 7-inch refractor. |
2. Refractors. | ||||
| Palais de l’Optique, Paris | 49·2 | 197 | Gautier, 1900 | Mounted as a siderostat in connection with a plane mirror 79 inches across. |
| Yerkes Observatory | 40 | 62 | Alvan G. Clark, 1897 | |
| Lick Observatory | 36 | 57·8 | A. Clark and Sons, 1888 | For photographic purposes a correcting lens is available, of 33 inches aperture, 47·8 feet focus. |
| Meudon Observatory | 32·5 | 55·2 | Henrys and Gautier, 1891 | Mounted with a photographic refractor of 24·4 inches aperture. |
| Astrophysical Observatory, Potsdam | 31·5 | 39·4 | Steinheil and Repsold, 1899 | Photographic. Mounted with a visual refractor 20 inches in aperture. |
| Bischoffsheim Observatory, Nice | 30·3 | 52·6 | Henrys and Gautier, 1886 | Visual. Mounted on Mont Gros, 1,100 feet above sea level. |
| Imperial Observatory, Pulkowa | 30 | 42 | A. Clark and Sons, 1885 | Visual. Mounted by Repshold. |
| National Observatory, Paris | 28·9 | — | Martin | |
| Royal Observatory, Greenwich | 28 | 28 | Sir H. Grubb, 1894 | Visual and photographic. Mounted by Ransome and Simms.[Pg 466] |
| University Observatory, Vienna | 27 | 34 | Sir H. Grubb, 1881 | Visual. |
| Royal Observatory, Greenwich | 26 | 26 | Sir H. Grubb, 1897 | The Thompson photographic equatoreal. |
| Naval Observatory, Washington | 26 | 29 | A. Clark and Sons, 1873 | |
| Leander McCormick Observatory, Virginia | 26 | 32·5 | A. Clark and Sons, 1881 | |
| Cambridge University Observatory | 25 | — | T. Cooke and Sons. 1870 | Presented to the University in 1889 by Mr. R. S. Newall. |
| Meudon Observatory | 24·4 | 52·2 | Henrys and Gautier, 1891 | Photographic. Mounted with a visual 32·5-inch refractor. |
| Harvard College Observatory | 24 | 11·3 | A. Clark and Sons, 1893 | Photographic doublet. The gift of Miss Bruce. Transfered in 1896 to Arequipa, Peru. |
| Royal Observatory, Cape of Good Hope | 24 | 22·6 | Sir H. Grubb, 1898 | Photographic. The gift of Mr. McClean. Mounted with an 18-inch visual refractor. |
| Lowell Observatory, Flagstaff, Arizona | 24 | 31 | Alvan G. Clark, 1896 | Visual. First mounted near the city of Mexico. Installed at Flagstaff, 1897. |
| National Observatory, Paris | 23·6 | 59 | Henrys and Gautier, 1891 | Visual and photographic. Mounted as an equatoreal Coudé. |
| Halsted Observatory, Princeton, N.J. | 23 | 32 | A. Clark and Sons, 1883 | |
| City Observatory, Edinburgh | 22 | 30 | — | Mounted as a visual equatoreal on the Calton Hill, 1898. |
| Etna Observatory | 21·8 | — | Merz, 1897 | |
| Buckingham Observatory | 21·2 | — | Buckingham and Wragge | |
| Porro Observatory, Turin | 20·5 | — | Porro | [Pg 467] |
| Chamberlin Observatory, Colorado | 20 | 28 | Alvan G. Clark and Saegmüller, 1894 | Visual. Fitted with a reversible crown lens for photography. |
| Manila Observatory | 20 | — | Merz and Saegmüller, 1894 | Visual. Provided with a photographic correcting lens. |
| Strasburg Observatory | 19·2 | 23 | Merz and Repsold, 1880 | |
| Brera Observatory, Milan | 19·1 | 23 | Merz and Repsold | |
| Dearborn Observatory, Illinois | 18·5 | 27 | A. Clark and Sons, 1862 | Mounted 1864 |
| National Observatory, La Plata | 18·1 | 29·5 | Henrys and Gautier, 1890 | Coudé Mount. Visual. |
| Lowell Observatory, Flagstaff, Arizona | 18 | 26·3 | Brashear, 1894 | Mounted with a 12-inch Clark refractor as counterpoise. |
| Van der Zee Observatory, Buffalo, N.Y. | 18 | — | Fitz | Dismounted. |
| Bischoffsheim Observatory, Nice | 16·5 | 26·2 | Henrys and Gautier, 1889 | Coudé Mount. Visual. |
| University Observatory, Vienna | 16·5 | 29·5 | Henrys and Gautier, 1890 | Coudé Mount. Visual. |
| Jesuit Observatory, Zi-ka-Wei | 16·5 | 22·5 | Henrys and Gautier, 1897 | Photographic. Mounted with a visual refractor of equal aperture. |
| Goodsell Observatory, Northfield, Minnesota | 16·2 | — | Brashear, 1891 | |
| Warner Observatory, Rochester, N.Y. | 16 | 22 | A. Clark and Sons, 1891 | |
| Grand-Ducal Observatory, Königsstuhl, Heidelberg | 16 | 6·6 | Brashear and Grubb, 1900 | A twin photographic doublet. The gift of Miss Bruce. Mounted with a visual 10-inch refractor by Pauly. |
| Meudon Observatory | 15·7 | 5·3 | ||
| Washburn Observatory, Wisconsin | 15·6 | 20·3 | A. Clark and Sons, 1879 | |
| Teramo Observatory, Italy | 15·5 | — | T. Cooke and Sons, 1885 | Formerly the property of Mr. Wigglesworth.[Pg 468] |
| Royal Observatory, Edinburgh | 15·1 | — | T. Grubb, 1872 | Presented by Lord Crawford. |
| Madrid Observatory | 15 | – | Merz | |
| Tulse Hill Observatory | 15 | 15 | Sir H. Grubb, 1870 | Lent by the Royal Society to Sir William Huggins. Mounted with an 18-inch Cassegrain reflector. |
| National Observatory, Paris | 15 | 29 | Lerebours | |
| Harvard College Observatory | 15 | 22 | Merz, 1847 | |
| National Observatory, Rio de Janeiro | 15 | — | ||
| Tacubaya Observatory, Mexico | 15 | 15 | Sir H. Grubb, 1880 | |
| Stonyhurst College Observatory | 15 | 15 | Sir H. Grubb, 1893 | |
| Brera Observatory, Milan | 15 | — | ||
| University of Mississippi | 15 | 15 | Sir H. Grubb, 1893 | Visual. Mounted with a photographic 9-inch refractor. |
| Imperial Observatory, Pulkowa | 15 | 22·5 | Merz and Mahler, 1840 | |
| Maidenhead Observatory | 15 | — | Sir H. Grubb, 1893 | The property of Mr. Dunn. Mounted with a twin photographic refractor. |
| Odessa Observatory | 14·9 | — | Merz, 1881 | |
| Bischoffsheim Observatory, Nice | 14·9 | 23 | Henrys and Gautier | |
| Brussels Observatory | 14·9 | 20 | Merz and Cooke, 1877 | |
| Observatory of Bordeaux | 14·9 | 22·4 | Merz and Gautier, 1880 | |
| Observatory of Lisbon | 14·9 | — | Merz and Mahler | [Pg 469] |
TABLE VI.
List of Observatories employed in the Construction of the
Photographic Chart and Catalogue of the Heavens.
All are provided with 13-inch photographic, coupled with 11-inch visual
refractors:
| Name of Observatory. | Constructors of Instruments. | |
| Optical Part. | Mechanical Part. | |
| Paris | Henrys | Gautier |
| Algiers | ,, | ,, |
| Bordeaux | ,, | ,, |
| San Fernando (Spain) | ,, | ,, |
| Vatican | ,, | ,, |
| Cordoba | ,, | ,, |
| Montevideo | ,, | ,, |
| Perth, Western Australia | ,, | ,, |
| Helsingfors | ,, | Repsold |
| Potsdam | Steinheil | ,, |
| Catania | ,, | Salmoiraghi |
| Greenwich | Sir H. Grubb | Sir H. Grubb |
| Oxford | ,, | ,, |
| The Cape | ,, | ,, |
| Melbourne | ,, | ,, |
| Sydney | ,, | ,, |
| Tacubaya (Mexico) | ,, | ,, |
FOOTNOTES:
[1630] Comptes Rendus, t. xliv., p. 339.
[1631] A. A. Common, Memoirs R. Astr. Soc., vol. i., p. 118.
[1632] Newcomb, Pop. Astr., p. 137.
[1633] Month. Not., vol. liv., p. 67.
[1634] Keeler, Publ. Astr. Pac. Soc., vol. ii., p. 160.
[1635] H. Grubb, Trans. Roy. Dub. Soc., vol. i. (new ser.), p. 2.
[1636] Hale, nevertheless (Astroph. Jour., vol. v., p. 128), considers that refractors
preserve their superiority of visual light-grasp over Newtonian reflectors up to
an aperture of 52-1/2, while equalisation is reached for the photographic rays at
34 inches.
[1637] Astroph. Jour., vol. v., p. 130.
[1638] Phil. Trans., vol. cxlviii., p. 465.
[1639] Optics, p. 107 (2nd ed., 1719).
[1640] Observatory, vol. viii., p. 85.
[1641] Holden on Celestial Photography, Overland Monthly, Nov., 1886.
[1642] Observatory, vol. xv., p. 283.
[1643] Bailey, Astroph. Jour., vol. x., p. 255.
[1644] Harvard Circulars, Nos. 2, 18, 24, 33;
[1645] Loewy, Bull. Astr., t. i., p. 286; Nature, vol. xxix., p. 36.
[1646] Nature, vol. xxiv., p. 389.
[1647] Ibid., vol. xxix., p. 470.
[1648] Trans. R. Dublin Soc., vol. iii., p. 61.
[1649] Observatory, vol. vii., p. 167.
[1650] Loewy, Bull. Astr., t. i., p. 265.
[1651] Phil. Trans., vol. clxxi., p. 653.
[1652] Janssen, L’Astronomie, t. ii., p. 121.
[1653] Rev. A. L. Cortie, Astr. and Astrophysics, vol. xi., p. 400.
[1654] Phil. Mag., vol. xiii., 1882, p. 469.
[1655] Bull. Astr., t. iii., p. 331.
INDEX
Abbe, Cleveland, corona of 1878 176 177
Aberdour, Lord, solar chromosphere, 68
Aberration, discovered by Bradley, 3, 15;
cause of, 31, 231
investigations of, 241 438
Abney, daylight coronal photographs, 179;
infra-red photography, 210 223 438
Absorption, terrestrial atmospheric, 134 211 214-216 225;
solar, 134-136 172 213 221 222 225 277
correlative with emission, 135 136 140
Adams, discovery of Neptune, 79-82;
lunar acceleration, 271
orbit of November meteors, 331
Airy, solar translation, 39;
observations during eclipses, 62, 64, 70
Astronomer-Royal, 79
search for Neptune, 80, 81
corona of 1851 175
solar parallax, 227 236
transit of Venus, 233
Mercurian halo, 235
lunar atmosphere, 264
Aitken, double star discoveries, 419
Albedo, of Mercury, 246;
of Venus, 255
of Mars, 283
of minor planets, 288
of Jupiter, 290
of Saturn, 303
of Uranus, 304
Alexander, spiral nebulæ, 118;
observation during eclipse, 245
Algol, variability of light, 10, 390;
eclipses, 390
nature of system, 391
Altitude and azimuth instrument, 120 note, 121
Amici, comet of 1843 103
Anderson, discovery of Nova Aurigæ, 396;
of Nova Persei, 400
Andrews, conditions of liquefaction, 151
Ångström, C. J., Optical Researches, 138;
spark spectrum, 139
nature of photosphere, 152
solar spectroscopy, 210 212
hydrogen in sun, 211
temperature of stars, 375
Ångström, K., infra-red solar spectrum, 210;
solar constant, 225
Arago, eclipse of 1842 62, 64, 65;
prominences, 69
polarization in comets, 103
magnetic relations of auroræ, 130
nature of photosphere, 151
meteor-systems, 329
Arai, photographs of corona of 1887 185
Arcturus, spectrum, 373 383;
radial movement, 387
Argelander, Bonn Durchmusterung, 32, 423;
solar motion, 39
centre of Milky Way, 40
comet of 1811 100
Aristotle, description of a comet, 350
Arrhenius, light-pressure theory of comets, 348
Asten, movements of Encke’s comet, 94
Asteroids, so designated by Herschel, 75
Astronomical physics, 7,141 142
Astronomical Society founded, 6;
Herschel its first President, 14
Astronomy, classification, 1;
popularity and progress, 5
in United States, 6
in Germany, 28
practical reform, 32
of the invisible, 42
physical, 141
Atmosphere, solar, 94, 182 192 221 225;
of Venus, 236 239 253 254
of Mercury, 246-248
of the moon, 263 264
of Mars, 276
of minor planets, 288
Auroræ, periodicity, 129 162;
excited by meteors, 335
Auwers, reduction of Bradley’s observations, 39;
system of Procyon, 42
opposition of Victoria, 238
solar parallax, 240
new star in Scorpio, 395
Babinet, nebular hypothesis, 314
Backlund, movements of Encke’s comet, 94, 360
[Pg 472]
Baden-Powell, Sir George, eclipse expedition, 188
Bailey, nebulosity round Pleiades, 411;
stellar photometric observations, 421
discovery of variable clusters, 436
Baily, early life and career, 59-61;
observations of eclipses, 61-64
density of the earth, 60, 261
Bakhuyzen, rotation of Mars, 275
Ball, Sir Robert, parallaxes of stars, 36 note, 416;
contacts in transits, 239
Barnard, micrometrical measures of Neptune, 84;
of minor planets, 288
of Saturn’s rings, 301
photographs of solar corona, 186 190
transit of Mercury, 245
halo round Venus, 254
surface of Mars, 280
ellipticity of Jupiter’s first satellite, 292
of Uranus, 304
discovery of inner Jovian satellite, 293 434
red spot on Jupiter, 296
eclipse of Japetus, 300
attendants on comet of 1882 363
on Brooks’s comet, 366 367
Swift’s comet, 368
photographic discovery of a comet, 369
observations of Nova Aurigæ, 398 399
Hind’s variable nebula, 403
exterior Pleiades nebulosities, 411
galactic stars, 423
photographs of Milky Way, 424 425
cluster variables, 433
horizontal telescope, 438
Bartlett, photograph of a partial eclipse, 166
Baxendell, meteors of 1866 331
Becker, drawings of solar spectrum, 211
Beckett, Sir E. (Lord Grimthorpe), value of solar parallax, 232
Beer and Mädler, surveys of lunar surface, 265 267;
studies of Mars, 275
Bélopolsky, coronal photographs, 185;
theory of corona, 191
rotation of Venus, 252
of Jupiter, 297
spectroscopic determinations of Saturn’s rings, 300
spectrum of γ Cassiopeiæ, 378
system of Castor, 389 391
detection of variable stars as spectroscopic binaries, 391
Berberich, mass of asteroids, 287;
orbit of Holmes’s comet, 337
Berkowski, daguerrotype of eclipsed sun, 166
Bessel, biographical sketch, 28-30;
reduction of Bradley’s observations, 32
parallax of 61 Cygni, 36
disturbed motion of Sirius and Procyon, 41
trans-Uranian planet, 79
Halley’s comet, 102
theory of instrumental errors, 122
personal equation, 123
rotation of Mercury, 246
lunar atmosphere, 263
cometary emanations, 325 345
multiple tails, 347
comet of 1807 352
Betelgeux, remoteness, 37, 417;
spectrum, 373 381 383 384
radial movement, 387
Bianchini, rotation of Venus, 250
Biela, discovery of a comet, 95
Bigelow, magnetic and solar disturbances, 161;
theory of corona, 191
Bigourdan, eclipse of 1893 187;
velocity of comet of 1882 364
Birmingham, colours of stars, 375 note;
discovery of T Coronæ, 393
Birt, rotation of a sun-spot, 144;
Selenographical Society, 266
Bischoffsheim, Coudé telescope, 436
Black Ligament, 235
Bode, popular writings, 5;
solar constitution, 57
missing planet, 72, 73
Boeddicker, heat-phases during lunar eclipses, 269 270;
drawings of Jupiter, 296
of the Milky Way, 424
Boehm, solar observations, 146 148
Boguslawski, centre of sidereal revolutions, 41;
observation of Halley’s comet, 102
Bolometer, principle of construction, 222
Bond, G. P., his father’s successor, 86;
light of Jupiter, 289
Saturn’s rings, 298
Donati’s comet, 324 325
Andromeda nebula, 409
double-star photography, 409
Bond, W. C., observation of Neptune’s satellite, 84;
discovery of Hyperion, 85
of Saturn’s dusky ring, 86
resolution of nebulæ, 119
celestial photography, 153 409
satellite-transit on Jupiter, 291
Borda, repeating circle, 121
Boss, solar translation, 40;
observations on comets, 352 356
Bossert, proper motions of stars, 415
Bouguer, solar atmospheric absorption, 221
Boulliaud, period of Mira, 10
Bouvard, tables of Uranus, 78;
Encke’s comet, 90
Boys, radio-micrometer, 220;
density of the earth, 261
Bradley, discoveries of aberration and nutation, 3;
solar translation, 10
star-distances, 10, 16
observation
[Pg 473]
observation
on Castor, 17
instruments, 28, 120
observations reduced by Bessel and Auwers, 32, 39
Brahe, Tycho, star of 1572 24
Brandes, observations of meteors, 327 334;
Braun, prominence photography, 197;
density of the earth, 261
Brayley, meteoric origin of planets, 311
Brédikhine, theory of cometary appendages, 100 348;
repulsive forces, 346 347
chemical differences, 347 348
formative types, 351 352 355 363 369
structure of chromosphere, 199
red spot on Jupiter, 294
Andromede meteors, 337
stationary radiants, 341
spectrum of Coggia’s comet, 343
Bremiker, star maps, 81
Brenner, rotation of Venus, 252
Brester, Théorie du Soleil, 152
Brewster, diffraction theory of corona, 67;
telluric lines in solar spectrum, 134
absorption spectra, 136
Brinkley, ostensible stellar parallaxes, 33
Brisbane, establishment of Paramatta Observatory, 6, 90
Brooks, fragment of 1882 comet, 363;
cometary discoveries, 365 366
Brünnow, stellar parallaxes, 113 416
Bruno, Giordano, motion of stars, 9
Buffham, rotation of Uranus, 303
Buffon, internal heat of Jupiter, 289
Bunsen, discovery of spectrum analysis, 132
Burchell, magnitude of η Carinæ, 48
Burnham, stellar orbits, 46;
coronal photographs, 186
measures of Nova Aurigæ, 399
of planetary nebulæ, 404
discoveries of double stars, 418 430 433 435
catalogue, 419
system of 61 Cygni, 419
Burton, canals of Mars, 279;
rotation of Jupiter’s satellites, 292
Calandrelli, stellar parallaxes, 33
Callandreau, capture theory of comets, 98
Campani, Saturn’s dusky ring, 86
Campbell, Lieutenant, polarisation of corona, 170
Campbell, Professor, stellar radial velocities, 39, 406 434;
flash spectrum, 189
spectroscopic observations of Saturn’s rings, 300
Wolf-Rayet stars, 380
spectroscopic binaries, 389
Nova Aurigæ, 398
translation of solar system, 406
stellar diffraction-spectra, 440
Canals of Mars, 278-280
Cannon, Miss A. J., spectrographic researches, 386
Canopus, remoteness, 37, 417;
spectrum, 416
Capella, spectrum, 373 383 384;
a spectroscopic binary, 389
Carbon, material of photosphere, 152;
absorption by, in sun, 212
in stars, 374
Carbonelle, origin of meteorites, 340
Carinæ, η, light variation, 48, 49;
spectrum, 379
Carrington, astronomical career, 144 145;
sun-spot observations, 146
solar rotation, 147
spot-distribution, 148
luminous outburst on sun, 159 160
Jovian and sun-spot periods, 163
origin of comets, 370
Cassini, Domenico, discoveries of Saturnian satellites, 84;
of division in ring, 85
solar rotation period, 146
solar parallax, 228
rotation of Venus, 250
of Mars, 274
of Jupiter, 290 295
satellite of Venus, 256
satellite-transit on Jupiter, 291
Cassini, J. J., stellar proper motions, 10;
sun-spots on limb, 54
theory of corona, 66
rotation of Venus, 250
structure of Saturn’s rings, 299
Ceres, discovery, 73, 74;
diameter, 75, 288
Chacornac, observation of sun-spot, 156;
star-maps, 284 413
variable nebula, 403
Challis, search for Neptune, 81, 82;
duplication of Biela’s comet, 96
Charlois, discoveries of minor planets, 283
Charroppin, coronal photographs, 186
Chase, photographic discovery of a comet, 338;
stellar parallaxes, 416
Chladni, origin of meteors, 327 332
Christie, Mercurian halo, 245
Chromosphere, early indications, 68;
distinct recognition, 69, 70, 167
depth, 174 175 200
metallic injections, 195
eruptive character, 199
spectrum, 200
Clark, Alvan, large refractors, 114 429 430 436
Clark, Alvan G., discovery of Sirian companion, 42, 430;
40-inch refractor, 433
Clarke, Colonel, figure of the earth, 262
Clarke, F. W., celestial dissociation, 206
Clausen, period of 1843 comet, 105;
cometary systems, 362
[Pg 474]
Clerihew, secondary tail of 1843 comet, 103
Clusters, variable stars in, 436
Coggia, discovery of a comet, 343
Comet, Halley’s, return in 1759 4, 88;
orbit computed by Bessel, 29
capture by Neptune, 98, 365
return in 1835 101-103 345
type of tail, 346 352
of 1843 7,103-105
type of tail, 346 352
relationships, 349-351
Newton’s, 88, 364
Encke’s, 90
changes of volume, 92
of brightness, 95
acceleration, 93, 94
capture by Mercury, 99
Winnecke’s, 94, 342
Biela’s, 95-97, 333
Brorsen’s, 97
Vico’s, 97, 367
Faye’s, 98
of 1811 99-101 346
of 1807 100 347 352
of 1819 101 103
Lexell’s, 106 367
Tewfik, 178 358 362 369
Donati’s, 323-325 347 348
of 1861 326 327 346
Perseid, 327 332
Leonid, 327 332 333 343
Klinkerfues’s, 335
Holmes’s, 337 343 369
Coggia’s, 343 346 347
of 1901 343
of 1880 349 351
Aristotle’s, 350
Tebbutt’s, 352-355
Schaeberle’s, 355 356
Wells’s, 356 357
of September, 1882 358-361 362-364
Thome’s, 361
Pons-Brooks, 365 366
Sawerthal’s, 366
Brooks’s, of 1889 366 367
Swift’s, 368
Cometary tails, repulsive action upon, 100 103 104 346-348;
coruscations in, 105
three types, 346-348 355 363
multiple, 347 348 351 352 355 363 368
Comets, subject to gravitation, 88;
of short period, 92, 93
translucency, 95, 105 106 353
small masses, 96, 106
capture by planets, 98, 306 367
changes of volume, 102 365 369
polarisation of light, 103 354 355
refractive inertness, 106 353
relations to meteor-systems, 327 332-336
disintegration, 333 339 362 363
spectra, 342-344 354 355 362-364
luminous by electricity, 344 355 357
systems, 353 355 357 362 365
origin, 369-371
Common, reflectors for eclipse photography, 187;
Jupiter’s inner satellite, 293
detection of great comet near the sun, 358
its five nuclei, 362
photographs of Andromeda nebula, 395
of Orion nebula and Jupiter, 407 408
great reflectors, 412 429
cluster variables, 436
Common, Miss, drawing of eclipsed sun, 187
Comstock, lunar atmosphere, 264
Comte, celestial chemistry, 140;
astronomy, 142
Cooke, 25-inch refractor, 430
Copeland, comets of 1843 and 1880 349;
spectrum of comet of 1882 364
of γ Cassiopeiæ, 378
of Nova Andromedæ, 395; of Orion nebula, 407
discoveries of gaseous stars, 379
Nova Aurigæ, 396 398
Copernicus, stellar parallax, 16
Cornu, telluric lines in solar spectrum, 202;
velocities in prominences, 205
ultra-violet solar spectrum, 210 215
velocity of light, 232 note, 241
spectrum of hydrogen, 383
of Nova Cygni, 393
Cornu and Bailie, density of the earth, 261
Corona of 1842 62-64, 67;
early records and theories, 65-67
photographs, 166 173 178 181 185-190
spectrum, 170 173 178 188 190 193
varying types, 174-176 193
of 1877 175-177
of 1882 177
of 1869 183
of 1886 185
of 1889 185-187
of 1893 188
of 1898 189
of 1900 189
of 1901 190
daylight photography of, 179-180
glare theory, 182
mechanical theory, 191
electro-magnetic theories, 191 192
Cortie, movements in sun-spots, 157;
their spectral changes, 208
Cotes, corona of 1715 176
Croll, secular changes of climate, 259 260;
derivation of solar energy, 313
Crookes, chemical elements, 210
Crova, solar constant, 225
Cusa, solar constitution, 57
Cysatus, Orion nebula, 21;
comet of 1618 362
Damoiseau, theory of Halley’s comet, 101
D’Arrest, orbits of minor planets, 285;
Andromede meteors, 334
ages of stars, 375
variable nebulæ, 403
measures of nebulæ, 404
Darwin, G. H., rigidity of the earth, 258;
Saturn’s ring system, 301
origin of the moon, 316-318
development of solar system, 318 319 322
solar tidal friction, 319
Daubrée, falls of aerolites, 339
Davidson, satellite-transit on Jupiter, 292
Davis, stellar parallaxes, 417
Dawes, prominences in 1851 70;
Saturn’s dusky ring, 86
a star
[Pg 475]
behind a comet, 106
solar observations, 143 164
observations and drawings of Mars, 276 278 280
satellite-transits on Jupiter, 291 292
De Ball, markings on Mercury, 248
Delambre, Greenwich observations, 3;
solar rotation, 146
light-equation, 231
De la Roche, Newton’s law of cooling, 217
De la Rue, celestial photography, 152 153 268;
solar investigations, 154
expedition to Spain, 166 167
De la Tour, experiments on liquefaction, 151
Delaunay, tidal friction, 271 272;
Coudé telescope, 436
Delisle, diffraction theory of corona, 67;
transits of Venus, 233 239
Dembowski, double star measurements, 418
Denning, observations of Mercury, 246 247;
mountain on Venus, 253
rotation of Jupiter, 290
red spot, 295
periodicity of markings, 297
rotation of Saturn, 302
meteors of 1885 336
of 1892 338
stationary radiants, 341
Denza, meteors of 1872 334
Derham, theory of sun-spots, 53;
ashen light on Venus, 255
Deslandres, eclipse expedition, 187;
rotation of corona, 188
prominence photography, 198
hydrogen spectrum in prominences, 198 383
photographs of Jupiter, 297
radial movements of Saturn’s rings, 300
helium absorption in stars, 376
stellar radial velocities, 406
Diffraction, corona explained by, 67, 70, 181;
spectrum, 139 210 223 439
Dissociation in the sun, 152 206-210;
in space, 312
Doberck, orbits of double stars, 38, 418
Dollond, discovery of achromatic telescope, 4,112
Donati, discovery of comet, 323;
spectra of comets, 342
of stars, 372
Doppler, effect of motion on light, 200
Douglass, observations of Jupiter’s satellites, 292
Downing, perturbations of the Leonids, 338
Draper, H., ultra-violet spectrum, 210;
oxygen in sun, 213
photographs of the moon, 268
of Jupiter’s spectrum, 291
of Tebbutt’s comet, 354
of spectrum of Vega, 382
of Orion nebula 407
Draper, J. W., lunar photographs, 152;
distribution of energy in spectrum, 223 note
Draper Memorial, 384-386
Dreyer, Catalogue of Nebulæ, 50
Dulong and Petit, law of radiation, 217 219
Dunér, spectra of sun-spots, 156;
spectroscopic measurement of solar rotation, 203
spectroscopic star catalogue, 381
Dunkin, solar translation, 39
Duponchel, sun-spot period, 163
Durchmusterung, Bonn, 33, 412;
Cape photographic, 412
parallax, 418
photometric, 421
Dyson, coronal photographs, 190
Earth, mean density, 60, 261;
knowledge regarding, 257
rigidity, 257 259
variation of latitude, 258 259
figure, 261 262
effects of tidal friction, 271-273
bodily tides, 316
primitive disruption, 317
Easton, structure of Milky Way, 423 424
Ebert, coronoidal discharges, 192
Eclipse, solar, of 1836 61;
of 1842 62-65, 67, 69
of 1851 69, 70, 166
of 1860 166 167
of 1868 167-170
of 1869 170
of 1870 171
of 1871 173
of 1878 174-177
of 1882 177 178
of 1883 180 181
of 1885 183
of 1886 184
of 1887 185
of 1889 185-187
of 1893 187 188
of 1896 188
of 1898 189
of 1900 189 190
of 1901 190
Eclipses, lunar, heat-phases during, 269 270
Eclipses, solar, importance, 59;
ancient, 60, 273
classification, 61
results, 192 193
Eddie, comet of 1880 349;
of 1882 363
Edison, tasimeter, 177
Egoroff, telluric lines in solar spectrum, 211 214
Elements, chemical, dissociation in sun, 206 209 210
Elkin, star parallaxes, 37, 416 417;
photography of meteors, 338
transit of great comet, 358 360
secondary tail, 363
triangulation of the Pleiades, 410
Elliot, opinions regarding the sun, 57
Elvins, red spot on Jupiter, 296
Encke, star maps, 78;
calculation of short-period comet, 90
resisting medium, 93
distance of the sun, 230 232
period of Pons’s comet, 365
[Pg 476]
Engelmann, rotation of Jupiter’s satellites, 292
Ericsson, solar temperature, 218
Erman, meteoric rings, 330
Eros, measures of, for solar parallax, 238;
discovery, 284
variability, 285
Ertborn, mountain in Venus, 253
Espin, spectra of variable stars, 379;
stars with banded spectra, 381
Nova Aurigæ, 397 398
Euler, resisting medium, 93
Evershed, eclipse photographs, 189 200
Evolution, of solar system, 308 309 313-316 322;
of earth-moon system, 316-318
of stellar systems, 420
Fabricius, David, discovery of Mira Ceti, 10
Fabricius, John, detection of sun-spots, 52
Faculæ, relation to spots, 53, 155 158;
solar rotation from, 155
photographed, 197 198 377
Faye, nature of prominences, 70, 166;
discovery of a comet, 98
cyclonic theory of sun-spots, 144 157
solar constitution, 150-152
maximum of 1883 163
velocities in prominences, 205
distance of the sun, 240
planetary evolution, 314 315 321
Feilitsch, solar appendages, 70
Fényi, solar observations, 184 204
Ferrel, tidal friction, 272
Ferrer, nature of corona, 67;
prominences, 69
Fessenden, electrical theory of comets, 348
Finlay, transit of great comet, 358 360
Fizeau, daguerrotype of the sun, 153;
Doppler’s principle, 201
velocity of light, 232
Flammarion, canals of Mars, 280;
trans-Neptunian planet, 306
Flamsteed, solar constitution, 57;
distance, 228
Flaugergues, detection of 1811 comet, 99;
transit of Mercury, 244
Fleming, Mrs., spectrum of β Lyræ, 379;
preparation of Draper Catalogue, 386
discoveries of new stars, 399
Flint, star-parallaxes, 417
Fontana, mountains of Venus, 252;
satellite, 256
spots on Mars, 274
Forbes, George, trans-Neptunian planets, 306 307
Forbes, James D., spectrum of annularly eclipsed sun, 134;
solar constant, 225
Foucault, spectrum of voltaic arc, 137;
photograph of the sun, 153
velocity of light, 232 240
silvered glass reflectors, 429
Fraunhofer, early accident, 33;
improvement of refractors, 34
clockwork motion, 121
spectra of flames, 131
of sun and stars, 133 134 372
objective prism, 385
diffraction gratings, 439
Fraunhofer lines, mapped, 133 136;
origin, 135-137 171 172
reflected in coronal spectrum, 170 173 181
in cometary spectra, 354 357
shifted by radial motion, 201
Freycinet, distribution of minor planets, 287
Fritz, auroral periodicity, 162
Frost, solar heat radiation, 222
Galileo, descriptive astronomy, 2;
double-star method of parallaxes, 16
discovery of sun-spots, 52
solar rotation, 146
planets and sun-spots, 163
darkening at sun’s edge, 221
Galle, discovery of Neptune, 81, 82;
Saturn’s dusky ring, 86
distance of the sun, 237
path of Andromede meteors, 334
Galloway, solar translation, 39
Gambart, discovery of comet, 95
Gauss, orbits of minor planets, 74;
Theoria Motus, 77
magnetic observations, 126 127
cometary orbits, 370
Gautier, sun-spot and magnetic periods, 126 128;
sun-spots and weather, 129
German Astronomical Society, 6,414
Gill, star-parallaxes, 37, 42, 416 417;
expedition to Ascension, 237
distance of the sun, 237 238 240
constant of aberration, 241
arc measurements, 261 262
comet of 1882 359 412
oxygen-absorption in stars, 384
photograph of Argo nebula, 404
Cape Durchmusterung, 412
photographic celestial survey, 413
actinic intensity of galactic stars, 425
Coudé telescope, 438
Gladstone, J. H., spectrum analysis, 134 136
Glaisher, occultation by Halley’s comet, 106
Glasenapp, coronal photographs, 185;
light equation, 231 241
double star measures, 419
Glass, optical, excise duty on, 112 115;
Guinand’s, 113 114
Jena, 431
Gledhill, spot on Jupiter, 294
[Pg 477]
Goldschmidt, nebulæ in the Pleiades, 411
Goodricke, periodicity of Algol, 390
Gore, catalogue of variable stars, 391;
of computed binaries, 418
Gothard, bright-line stellar spectra, 378 379;
spectrum of Nova Aurigæ, 398
photographs of nebulæ, 409
Gould, variation of latitude, 258;
photograph of Mars, 281
comets of 1807 and 1881 349 352
luminous instability of stars, 392
photographic measures of the Pleiades, 410
Uranometria Argentina, 415
solar cluster, 423 426
Graham, discovery of Metis, 77
Grant, solar envelope, 70, 167;
transit phenomena, 254
Green, observation of Mars, 280
Greenwich observations, 3, 27, 32
Gregory, David, achromatic lenses, 112 note
Gregory, James, double star method of parallaxes, 16;
reflecting telescopes, 109
Groombridge, star catalogue, 31
Grosch, corona of 1867 176
Grubb, Sir Howard, photographic reflector, 409;
great refractors, 430 433
siderostat, 437
Grubb, Thomas, Melbourne reflector, 110 note, 428
Gruithuisen, snow-caps of Venus, 255;
lunar inhabitants, 265
Gully, detection of Nova Andromedæ, 394
Guthrie, nebulous glow round Venus, 253
Hadley, Saturn’s dusky ring, 86;
reflecting telescope, 109
Haerdtl, Winnecke’s comet, 94
Hale, luminous outburst on sun, 161;
daylight coronal photography, 179
spectrum of prominences, 195 198
prominence photography, 197 198
photographs of faculæ, 198 377
carbon in chromosphere, 200
bright lines in fourth-type stars, 381
reflectors and refractors, 432
Hall, Asaph, parallax of the sun, 241;
discovery of Martian satellites, 282
rotation of Saturn, 302
double star measurements, 419
Hall, Chester More, invention of achromatic telescope, 112
Hall, Maxwell, rotation of Neptune, 305
Halley, stellar proper motions, 9;
composition of nebulæ, 22
observation of η Carinæ, 48
eclipse of 1715 66, 68
predicted return of comet, 88
magnetic theory of auroræ, 130
transits of Venus, 233
lunar acceleration, 271
origin of meteors, 327
Halm, magnetic relations of latitude variation, 259
Hansen, solar parallax from lunar theory, 230
Hansky, coronal photographs, 188 189
Harding, discovery of Juno, 75;
celestial atlas, 77
Harkness, spectrum of corona, 170;
corona of 1878 175
shadow of the moon in solar eclipses, 182
light equation, 231
distance of the sun, 237 240 241 242
Harriot, observations on Halley’s comet, 29
Hartley, gallium in the sun, 200 213
Hartwig, Nova Andromedæ, 394
Hasselberg, metallic spectra, 211;
spectra of comets, 342 357
of Nova Andromedæ, 395
Hastings, composition of photosphere, 152;
observations at Caroline Island, 181
Saturn’s dusky ring, 299
Hegel, number of the planets, 73
Heis, radiant of Andromedes, 334
Heliometer, 34, 234 237 238 240
Helium, a constituent of prominences, 194 195 199;
no absorption by, in solar spectrum, 213
absorptive action in first-type stars, 376
bright in gaseous stars, 377 378 380
in Orion nebula, 407
Helmholtz, gravitational theory of sun-heat, 311-313
Hencke, discoveries of minor planets, 76
Henderson, parallax of α Centauri, 36, 416;
observation of chromosphere, 68
Henry, Paul and Prosper, lunar twilight, 264;
markings on Uranus, 303
photograph of Saturn, 408
photographs of nebulæ in the Pleiades, 410 411
stellar photography, 413
plane mirrors, 438
Herrick and Bradley, duplication of Biela’s comet, 96
Herschel, Alexander S., cometary and meteoric orbits, 332
Herschel, Caroline, her brother’s assistant, 12;
observation of Encke’s comet, 90
Herschel, Colonel, spectrum of prominences, 168;
of reversing layer, 172
of corona, 174
[Pg 478]
Herschel, Sir John, life and work, 45-50;
Magellanic clouds, 47, 422
sun-spots, 58, 59, 144
solar flames, 68
anticipation of Neptune’s discovery, 81
status of Hyperion, 85
Biela’s comet, 95
Halley’s, 102
comet of 1843 103
sixth star in “trapezium,” 113
grinding of specula, 116
spectrum analysis, 136
solar photography, 145 154
solar constitution, 151
shadow round eclipsed sun, 182
actinometrical experiments, 216
solar heat, 217
climate and eccentricity, 259
lunar atmosphere, 263
surface of Mars, 276
Andromeda nebula, 396
observations of nebulæ, 404
double nebulæ, 412
Herschel, Sir William, discovery of Uranus, 5;
founder of sidereal astronomy, 9
biographical sketch, 11-14
sun’s motion in space, 15, 39, 425
revolutions of double stars, 18, 442
structure of Milky Way, 19-21, 423
nature of nebulæ, 21-26, 401
results of his observations, 25
centre of sidereal system, 40
theory of the sun, 54-56, 70
asteroids, 75
discoveries of Saturnian and Uranian satellites, 84, 87, 110
comet of 1811 99
reflecting telescopes, 109-111
sun-spots and weather, 129
transit of Mercury, 244
refraction in Venus, 252
lunar volcanoes, 266
terrestrial affinity of Mars, 274
Jovian trade-winds, 289
rotation of Jupiter’s satellites, 292
ring of Saturn, 298
rotation of Saturn, 302
origin of comets, 369
stellar photometry, 420
Herz, comets’ tails, 348
Hevelius, “Mira” Ceti, 10;
contraction of comets, 92
granular structure of comet, 362
Higgs, photographs of solar spectrum, 211 214
Hind, solar flames, 70;
Iris and Flora discovered by, 77
distortion of Biela’s comet, 96
transit of a comet, 101
earth in a comet’s tail, 326
comets of 1843 and 1880 349
Schmidt’s comet, 363
new star, 392
variable nebula, 403
Hirn, solar temperature, 220;
resistance in space, 348
Hodgson, outburst on the sun, 160
Hoeffler, star-drift in Ursa Major, 426
Hoek, cometary systems, 362
Holden, Uranian satellites, 87;
eclipse expedition, 180
coronal extensions, 186
solar rotation, 203
transit of Mercury, 245
intra-Mercurian planets, 250
drawing of Venus, 252
lunar photographs, 268
canals on Mars, 279
surface of Mars, 281
transits of Jupiter’s satellites, 292
markings on Uranus, 304
disintegration of comet, 362
colours of double stars, 374
Nova Aurigæ, 398
Orion and Trifid nebulæ, 403 404
director of Lick Observatory, 435
Holden and Schaeberle, observations of nebulæ, 433
Holmes, discovery of a comet, 337
Homann, solar translation, 406
Hooke, solar translation, 10;
stellar parallax, 16
repulsive action on comets, 102 note
automatic movement of telescopes, 120
spots on Mars, 274 275
Hopkins, solidity of the earth, 257
Horrebow, sun-spot periodicity, 125;
satellite of Venus, 256
Hough, G. W., red spot on Jupiter, 295 430;
observations of double stars, 419
Houzeau, solar parallax, 240
Howlett, sun-spot observations, 155
Hubbard, period of comet of 1843 105 351
Huggins, Sir William, spectroscopic observations of prominences, 170 195
hydrogen spectrum in stars, 178 198
daylight coronal photography, 178 179
repulsive action in corona, 191
stellar motions in line of sight, 201 386 387
transit of Mercury, 245
occultation of a star, 263
snowcaps on Mars, 276
spectrum of Mars, 277
of Jupiter, 290
Jovian markings and sun-spots, 297
spectrum of Uranus, 304
of comets, 342 343
photographs, 354 357
stellar spectroscopy, 373
colours of stars, 374
classification of star spectra, 376
photographs, 382 383 438
stellar chemistry, 381 382
spectra of new stars, 393 395
theory of Nova Aurigæ, 397
spectra of nebulæ, 401 402 407
nebular radial movement, 405
Huggins, Sir William and Lady, photograph of Uranian spectrum, 305;
spectra of Wolf-Rayet stars, 380
ultra-violet spectrum of Sirius, 383
nitrogen in stars, 384
spectrum of Nova Aurigæ, 396-398
of Andromeda nebula, 403
of Orion nebula, 407
[Pg 479]
Humboldt, sun-spot period, 126;
magnetic observations, 127
meteoric shower, 329
Hussey, T. J., search for Neptune, 79
Hussey, W. J., cloud effects on Mars, 281;
cometary appendages, 369
period of δ Equulei, 419
discoveries of double stars, 419 433
Huygens, stellar parallax, 16;
Orion nebula, 22
discovery of Titan, 84
Saturn’s ring, 85, 301
spot on Mars, 275
Hydrogen, a constituent of prominences, 168 195 199;
spectrum, 178 198 383 384
absorption in stars, 198 373 381-383
in sun, 211
theoretical material of comets’ tails, 347
emissions in stars, 377-380 384 393 397
in nebulæ, 402 407
Innes, Southern double stars, 419
Jacoby, measurement of Rutherfurd’s plates, 410;
Pritchard’s parallax work, 417
Janssen, photographs of the sun, 165;
spectroscopic observations of prominences, 168 169
escape from Paris in a balloon, 171
coronal spectrum, 173 181
coronal photographs, 181
rarefaction of chromospheric gases, 182
oxygen absorption in solar spectrum, 214
transit of 1874 234
spectrum of Venus, 254
of Saturn, 303
photographs of Tebbutt’s comet, 353 354
of Orion nebula, 407
Japetus, eclipse of, 300;
variability in light, 302
Jewell, solar spectroscopy, 200 211
Joule, heat and motion, 309
Jupiter, mass corrected, 77, 92;
conjectured influence on sun-spot development, 163
physical condition, 289 290
spectrum, 290 291
satellite-transits, 291 292
discovery of inner satellite, 293
red spot, 293-296
photographs, 297 408
periodicity of markings, 297
Kaiser, rotation of Mars, 275;
map of Mars, 278
Kammermann, observation of Maia nebula, 410
Kant, status of nebulæ, 14;
Sirius the central sun, 40
planetary intervals, 71
tidal friction, 272
condition of Jupiter, 289
cosmogony, 308
Kapteyn, solar translation, 40;
Cape Durchmusterung, 412
stellar parallaxes, 417 418
actinic intensity of galactic stars, 425
solar cluster, 426
Kayser and Runge, spectroscopic investigations, 211 213
Keeler, red spot on Jupiter, 296;
spectroscopic determination of movements in Saturn’s rings, 300
spectrum of Uranus, 304
of third type stars, 382
of nebulæ, 402
photographs of nebulæ, 403 411 412 432
nebular radial movements, 405 434 440
grating spectroscope, 440
Kepler, star of 1604 25;
solar corona, 66
missing planets, 71
cometary decay, 91, 339
comet of 1618 96
physical astronomy, 141
Kiaer, comets’ tails, 348
Kirchhoff, foundation of spectrum analysis, 132 135-137 372;
map of solar spectrum, 137
solar constitution, 149 151 172
Kirkwood, distribution of minor planets, 286;
grouped orbits, 287
divisions in Saturn’s rings, 301 302
origin of planets, 314
their mode of rotation, 321
comets and meteors, 333 339
Kleiber, Perseid radiants, 341
Klinkerfues, comet predicted by, 335 339;
apparitions of Southern comet, 350
tidal theory of new stars, 397
Knobel, cloud effects on Mars, 281
Konkoly, spectrum of γ Cassiopeiæ, 378;
spectroscopic survey, 381 note
Kreil, lunar magnetic action, 130
Kreutz, period of 1843 comet, 105;
orbit of 1861 comet, 327
period of great September comet, 361
cause of disintegration, 363
eclipse-comet of 1882 362
Krüger, segmentation of great comet, 362
Küstner, variation of latitude, 258
Kunowsky, spots on Mars, 275
Lacaille, southern nebulæ, 22;
η Carinæ, 48
Lagrange, theory of solar system, 2;
planetary disruption, 76
Lahire, diffraction theory of corona, 67;
distance of the sun, 228
mountains of Venus, 252
Lalande, popularisation of astronomy, 5;
revolving stars, 18
Histoire Céleste, 31, 415
nature of sun-spots, 53
observations of Neptune, 83
Lambert, solar motion, 10;
construction
[Pg 480]
construction of the universe, 14, 40
missing planets, 71
Lamont, magnetic period, 127 128
Lamp, ashen light on Venus, 256
Langdon, mountains of Venus, 253
Langley, solar granules, 165;
corona of 1878 176
spectroscopic effects of solar rotation, 202
infra-red spectrum, 210 223 224
experiments at Pittsburg, 221
bolometer, 222
distribution of energy in spectrum, 224 225
atmospheric absorption, 224 225 276
solar constant, 225
lunar heat-spectrum, 269
temperature of lunar surface, 270
age of the sun, 312
Laplace, lunar acceleration, 2,271;
Système du Monde, 5
nebular hypothesis, 25, 308 309 313 314 322
stability of Saturn’s rings, 85, 298
solar atmosphere, 94, 221
Lexell’s comet, 106 367
solar distance by lunar theory, 230
origin of meteors, 328
of comets, 370
Lassell, discovery of Neptune’s satellite, 83;
of Hyperion, 85
Saturn’s dusky ring, 86
observations at Malta, 87, 434
reflectors, 114
equatoreal mounting, 121
Latitude, variation of, 258 259
Laugier, period of 1843 comet, 105;
solar rotation, 146
Le Chatelier, temperature of the sun, 219
Lescarbault, pseudo-discovery of Vulcan, 248;
halo round Venus, 254
Lespiault, orbits of minor planets, 285
Le Sueur, spectrum of Jupiter, 291
Leverrier, discovery of Neptune, 80-82;
Lexell’s comet, 98, 367
distance of the sun, 230 240
revolutions of Mercury, 248
supposed transits of Vulcan, 249
mass of asteroids, 287
orbit of November meteors, 332
Perseids and Leonids, 333
Lexell, comet of 1770 98, 106 367
Liais, supposed transit of Vulcan, 249;
comet of 1861 326
division of a comet, 339
Librations, of Mercury, 247;
of Venus, 251
of the moon, 266
Lick, foundation of observatory, 434
Light, velocity, 38, 232 241;
extinction in space, 45
refrangibility changed by movement, 201
Ligondès, development of solar system, 316
Lindsay, Lord, expedition to Mauritius, 234
Line of sight, movements in, 201 386;
of solar limbs, 202 203
in prominences, 204 208
of stars, 201 386 387
binaries detected by, 387-391
Listing, dimensions of the globe, 262
Littrow, chromosphere, 70;
sun-spot periodicity, 126
Liveing and Dewar, carbon in the sun, 212
Lockyer, solar spectroscopy, 156 212;
theory of sun-spots, 159 163
daylight observations of prominences, 169 194 204 205
eclipse of 1870 171
slitless spectroscope, 173
corona of 1878 175
glare theory of corona, 182
eclipse of 1886 184
chromospheric spectrum, 195
classification of prominences, 196
their radial movements, 204
celestial dissociation, 206-210
chemistry of sun-spots, 207
spots on Mars, 275
meteoritic hypothesis, 376 402
equatoreal Coudé, 438
Loewy, constant of aberration, 241 438;
lunar photographs, 268
director of Paris Observatory, 414
equatoreal Coudé, 436 437
Lohrmann, lunar chart, 265;
Linné, 267
Lohse, J. G., spectrum of great comet, 364
Lohse, O., daylight coronal photography, 178 note;
spectral investigations, 211
twilight on Venus, 256
red spot on Jupiter, 294
periodicity of Jupiter’s markings, 297
motion of Sirius, 386
spectrum of Nova Cygni, 393
Louville, nature of corona, 67;
chromosphere, 68
Lowell, rotation of Mercury, 248;
of Venus, 252
markings on Venus, 255
observations of Mars, 280 281
satellites, 283
Lyman, atmosphere of Venus, 254
McClean, photographs of solar spectrum, 211 215;
helium stars, 377
oxygen stars, 384
equipment of Cape Observatory, 433
Macdonnell, luminous ring round Venus, 254
Maclaurin, eclipse of 1737 65
Maclear, Admiral, observations during eclipses, 172 182
Maclear, Sir Thomas, maximum of η Carinæ, 49;
observation of Halley’s comet, 102
Mädler, central sun, 41;
observations
[Pg 481]
observations
of Venus, 253
lunar rills, 263
aspect of Linné, 267
common proper motions, 426
Magellanic clouds, 47, 422;
spiral character, 425
Magnetism, terrestrial, international observations, 126;
periodicity, 127 128
solar relations, 128 160 161 163 205
lunar influence, 130
Mann, last observation of Donati’s comet, 325
Maraldi, solar corona, 67;
rotation of Mars, 274
satellite-transits on Jupiter, 291
spot on Jupiter, 295
Marius, Andromeda nebula, 21;
sun-spots, 52
Mars, oppositions, 228;
solar parallax from, 228 237 240
polar spots, 274 276 277 280 281
permanent markings, 274-276
rotation, 274 275
atmosphere, 276 277
climate, 277 278
canals, 278-281
photographs, 281
satellites, 282 283 314 320 321
Marth, revolutions of Neptune’s satellite, 305
Maskelyne, components of Castor, 18;
Astronomer-Royal, 27
experiment at Schehallien, 261
comets and meteors, 332
Maunder, photographs of corona of 1886 185;
comparative massiveness of stars, 375
constitution of nebulæ, 403
Maunder, Mrs., coronal photographs, 189 190
Maury, director of Naval Observatory, 7;
duplication of Biela’s comet, 96
Maury, Miss A. C., spectrographic investigations, 386;
discoveries of spectroscopic binaries, 387 388
Maxwell, J. Clerk, structure of Saturn’s rings, 298 300
Mayer, C., star satellites, 17
Mayer, Julius R., tidal friction, 272;
meteoric sustentation of sun’s heat, 310
Mayer, Tobias, stellar motions, 10;
solar translation, 15
repeating circle, 122
solar distance, 230
satellite of Venus, 256
lunar surface, 263
Mazapil meteorite, 340
Meldrum, sun-spots and cyclones, 164
Melloni, lunar heat, 269
Melvill, spectra of flames, 131
Mercury, mass, 92;
luminous phenomena during transits, 244 245
spectrum, 245
mountainous conformation, 246 247
rotation, 247 248
theory of movements, 248 250
Mersenne, reflecting telescope, 108
Messier, catalogue of nebulæ, 22
Meteoric hypothesis of solar sustentation, 310;
of planetary formation, 311
Meteoritic hypothesis of cosmical constitution, 376 402
Meteors, origin, 327 328;
relations to comets, 327 332-334 340
Leonids, 328-334 338
Perseids, 329 332 333 341
Andromedes, 334-338
stationary radiants, 341
Meunier, canals of Mars, 280
Meyer, divisions of Saturn’s rings, 302;
comet of 1880 351
cometary refraction, 353
comet Tewfik, 362
Michell, double stars, 17;
torsion balance, 261
star systems, 426
Michelson, velocity of light, 241
Milky Way, grindstone theory, 14;
clustering power, 20, 26
structure, 20, 41, 45, 47, 423-425
centre of gravity, 40, 41
frequented by Wolf-Rayet, temporary, and helium stars, 380 399 425
by gaseous nebulæ, 402
drawings and photographs, 424 425
Miller, W. A., spectrum analysis, 132 136 137;
stellar chemistry, 373
Mira, light changes, 10;
spectrum, 374 379
Mitchel, lectures at Cincinnati, 6
Mitchell, photograph of reversing layer, 190
Möller, theory of Faye’s comet, 98
Mohn, origin of comets, 370
Moll, transit of Mercury, 245
Monck, Perseid meteors, 341;
new stars, 395
Moon, acceleration, 2,271 272;
magnetic influence, 130
photographs, 152 153 268
solar parallax from disturbed motion, 230 240
study of surface, 263
atmosphere, 263-265
charts, 265-267
librations, 266
superficial changes, 267 268
thermal radiations, 269 270
rotation, 272
tables, 272 273
origin, 316-318
Morinus, celestial chemistry, 140
Morstadt, Andromede meteors, 332
Mouchez, photographic survey of the heavens, 413;
death, 414
Müller, phases of Mercury, 246;
of minor planets, 288
albedo of Mars, 283
of Jupiter, 290
of Saturn, 303
variability of Neptune, 305
of Pons’s comet, 366
stellar photometry, 421
Munich, Optical Institute, 28, 34
Myer, solar eclipse, 183
[Pg 482]
Myer, solar eclipse, 183
Nasmyth, Lassell’s reflector, 83;
solar willow-leaves, 164
comparative lustre of Mercury and Venus, 255
condition of Jupiter, 289
Nasmyth and Carpenter, The Moon, 265
Nebula, Andromeda, early observations, 21;
new star in, 394 395
photographs, 395 409
structure, 396
spectrum, 402 403
visibility at Arequipa, 435
Nebula, Orion, observed by Herschel, 12;
mentioned by Cysatus, 21
apparent resolvability, 119
suspected variability, 403
radial movement, 405
spectrum, 407
photographs, 407 408 436
Nebulæ, first discoveries, 22;
catalogues, 22, 46, 50, 412
distribution, 23, 48, 422
composition, 24, 47, 401 402
resolution, 47, 117 119
double, 48, 412
spiral, 118 410 412
new stars in, 394-396 399 401
spectra, 401-403 407
variability, 403 404
radial movements, 405
photographs, 407-409 425
Nebular hypothesis, Herschel’s, 24, 25;
Laplace’s, 25, 308 309 322
objections, 313-315
Neison, atmosphere of Venus, 254;
rills on the moon, 263
The Moon, 265
Neptune, discovery, 78-83;
satellite, 83, 305
density, 84
comets captured by, 98, 306 365
mode of rotation, 305 313 315 322
Newall, F., duplicity of Capella, 389;
stellar radial motions, 430
Newall, R. S., 25-inch refractor, 430
Newcomb, runaway stars, 39;
solar translation, 40
origin of minor planets, 76
telescopic powers, 119
corona of 1878 176
of 1869 183
distance of the sun, 231-233
velocity of light, 241
variation of latitude, 259
lunar atmosphere, 263
lunar theory, 272 273
disturbance of Neptune’s satellite, 305
formation of planets, 314
star catalogue, 415
structure of Milky Way, 423
Newton, H. A., capture of comets by planets, 98;
falls of aerolites, 311
November meteors, 330 331
meteors of 1885 336 337
orbits of aerolites, 340
Newton, Sir Isaac, founder of theoretical astronomy, 1,141;
comets subject to gravitation, 88
first speculum, 109
solar radiations, 216
law of cooling, 217-219
telescopes and atmosphere, 434
Niesten, volume of asteroids, 287;
red spot on Jupiter, 293
Nobert, diffraction gratings, 439
Noble, observations of Mercury, 246;
secondary tail of comet, 355
Nolan, origin of the moon, 317;
period of Phobos, 320
Norton, expulsion theory of solar appendages, 193 note;
comets’ tails, 345 347
Nova Aurigæ, 396-399
Nutation, discovered by Bradley, 3, 15;
a uranographical correction, 31
Nyrén, constant of aberration, 241
Observatory, Greenwich, 3, 27, 433;
Cape of Good Hope, 6, 36, 433
Paramatta, 6, 90
Harvard College, 7, 85
Königsberg, 30
Dorpat, 43
Pulkowa, 44
Palermo, 72
Berlin, 90
Anclam, 149
Potsdam, 149
Kew, 153
Arequipa, 264 435 436
Yerkes, 433
Lick, 435
Occultations of stars by comets, 95, 105 106;
by the moon, 263
by Mars, 276
of Jupiter by the moon, 264
Olbers, Bessel’s first patron, 29, 30;
discoveries of minor planets, 74, 75
origin by explosion, 75, 76
career, 89, 90
Biela’s comet, 95
comet of 1811 99
electrical theory of comets, 100 104 324 347
multiple tails, 100
comet of 1819 101
cometary coruscations, 105
November meteors, 329
Olmsted, radiant of Leonids, 328;
orbit, 329
Oppenheim, calculation of Schmidt’s comet, 363
Oppolzer, E. von, theory of sun-spots, 159;
variability of Eros, 285
Oppolzer, Th. von, Winnecke’s comet, 94;
comet of 1843 350
Oxygen in sun, 213-215;
telluric absorption, 214
in stars, 384
Packer, variable stars in cluster, 436
Palisa, search for Vulcan, 181 250;
discoveries of minor planets, 283
Pallas, discovery, 74;
inclination of orbit, 75, 286
diameter, 75, 287 288
Pape, Donati’s comet, 345
Parallax, annual, of stars, 10, 16, 33, 36, 416-418;
horizontal, of sun, 227
Encke’s result, 230 232
[Pg 483]
improved values from oppositions of Mars, 231 237
from light velocity, 231 232 241
from recent transits, 236 240
from observations of minor planets, 238 239
general result, 242
Paris Catalogue of Stars, 415
Paschen, oxygen in sun, 215;
solar temperature, 220
Pastorff, drawings of the sun, 101
Peirce, structure of Saturn’s rings, 298
Perrine, eclipse photographs, 190;
nature of corona, 191
observation
of Holmes’s comet, 369
nebula round Nova Persei, 401
Perrotin, rotation of Venus, 252;
markings on, 255
canals of Mars, 279
clouds on Mars, 281
striation of Saturn’s rings, 299
rotation and compression of Uranus, 303 304
changes of Pons’s comet, 366
Maia nebula, 410
measures of double stars, 419
Perry, eclipse of December, 1889 187
Peter, star-parallaxes, 417
Peters, C. A. F.,
parallax of 61 Cygni, 36
disturbed motion of Sirius, 42
Peters, C. F. W., orbit of Leonid meteors and comet, 332
Peters, C. H. F., sun-spot observations, 147 148;
discoveries of minor planets, 283
star maps, 284 415
Peytal, description of chromosphere, 69
Phobos, rapid revolution, 282 283 314;
tidal relations, 320 321
Photography, solar, 145 153 154 165;
of corona, 166 173 175 178 181 185-190
without an eclipse, 178-180
of prominences, 167 197 198
of coronal spectrum, 171 188 190
of prominence-spectrum, 195 198
of arc-spectrum, 206 211
of solar spectrum, 210 211 215 439 440
of Uranian spectrum, 305
of cometary spectra, 354 357
of stellar and nebular spectra, 382-384 396 398 400 407
lunar, 152 153 268
detection of comets by, 178 338 369
of asteroids, 284
of new stars, 399
use of, in transits of Venus, 234 236 240
Mars depicted by, 277 281
Jupiter, 297 408
comets, 353 354 368 412
nebulæ, 395 401 407-409 411 425
Milky Way, 424 425
star-charting by, 413 414
star-parallaxes by, 417
rapid improvement, 438
Photometry, stellar, 49, 420 421;
of planetary phases, 245 288
of Saturn’s rings, 299
photographic, 421
Photosphere, named by Schröter, 55;
structure, 151 152 164 165
Piazzi, star catalogues, 31;
parallaxes, 33
motion of 61 Cygni, 35
birth and training, 72
5-foot circle, 72, 121
discovery of Ceres, 73, 74
Picard, Saturn’s dark ring, 86;
sun’s distance, 228
Pickering, E. C., photometric measures of Martian satellites, 282;
of minor planets, 287
variability of Japetus, 302
of Neptune, 305
meteoric photography, 339
gaseous stars, 379
hydrogen spectrum in stars 383
spectrographic results, 385
eclipses of Algol, 390
photographic celestial surveys, 399
star density in Pleiades, 411
photometric catalogues, 420 421
photographic photometry, 421
white stars in Milky Way, 425
climate of Arequipa, 435
horizontal telescope, 437
Pickering, W. H., corona of 1886 185;
coronal photographs, January 1, 1889 186
lunar twilight, 264
lunar volcanic action, 267
melting of snow on Mars, 277
Martian snowfall, 281
Jupiter’s satellites, 292
photographs of comets, 368
of Orion nebula, 408
observatory at Arequipa, 435
Pingré, phenomena of comets, 92, 96
Planets, influence on sun-spots, 163;
periods and distances, 228
intra-Mercurian, 248-250
inferior and superior, 288
trans-Neptunian, 306 307
origin, 309 313
relative ages, 314 315
Planets, minor, existence inferred, 71, 72;
discoveries, 73-75, 77, 283 284
solar parallax from, 237-239
distribution of orbits, 286 287
collective volume, 287
atmospheres, 288
Plantade, halo round Mercury, 244
Pleiades, community of movement near, 41;
photographed spectra, 385
measurements, 410
photographs, 410 411
nebulæ, 410 411
Plücker, hydrogen in sun, 212
Plummer, solar translation, 39;
Encke’s comet, 99
Plutarch, solar corona, 65
Pogson, prominence spectrum, 168;
reversing layer, 172
discovery of a comet, 335 339
new star in cluster, 395
[Pg 484]
new star in cluster, 395
Pond, errors of Greenwich quadrant, 28;
controversy with Brinkley, 33
Pons, discoveries of comets, 90, 94, 365
Pontécoulant, return of Halley’s comet, 101
Poor, C. Lane, calculation of Lexell’s comet, 367
Porter, solar translation, 40
Pouillet, solar constant, 216 225;
temperature of the sun, 217
of space, 270
Poynting, mean density of the earth, 261
Prince, glow round Venus, 253
Pritchard, parallax of β Aurigæ, 388;
photographic determinations of stellar parallax, 417
photometric catalogue, 420
Pritchett, corona of January, 1889 186;
red spot on Jupiter, 294
Proctor, glare theory of corona, 182;
speed of ejections from sun, 205
transit of Venus, 233
distance of sun, 236
atmosphere of Venus, 254
rotation of Mars, 275
map and canals of Mars, 278 279
condition of great planets, 289
Nova Andromedæ, 403
status of nebulæ, 422 423
structure of Milky Way, 424
star drift, 426
Procyon, satellite, 42; parallax, 417
Prominences, observed in 1842 63, 64, 68;
described by Vassenius, 68
observed in 1851 70
photographed during eclipse, 167 188 190
without eclipse, 197 198
spectrum, 168 178 194 195 198 199
spectroscopic method of observing, 168-170 194-196
white, 183 184
chemistry, 195 199
classification, 196
distribution, 199
movements in, 204-206
heat of development, 220
Quetelet, periodicity of August meteors, 329
Ranyard, drawing of sun-spot, 101;
coronal types, 175 185
lunar atmosphere, 265
Jupiter’s markings, 297
meteors from fixed radiants, 341
cometary trains, 348
tenuity of nebulæ, 409
Rayet, spectrum of prominences, 168 170
Reduction of observations, 31;
Bessel’s improvements, 32, 122
Baily’s, 60
Refraction, atmospheric, 31;
effects looked for in comets, 106 353
Cytherean, 235 253 254
lunar 263 264
Reichenbach, foundation of Optical Institute, 28, 34, 122
Repsold, astronomical circles, 41, 122;
Cape heliometer, 416
Respighi, slitless spectroscope, 173;
prominences and chromosphere, 194 196 199
solar uprushes, 205
spectrum of γ Argûs, 380
Reversing layer, detected, 171 172;
photographed, 172 189
depth, 173
Riccioli, secondary light of Venus, 255
Riccò, trials with coronagraph, 180;
distribution of prominences, 199
spectrum of Venus, 254
spot on Jupiter, 294
spectrum of great comet, 364
Richer, distance of the sun, 228
Ristenpart, solar translation, 40
Ritchey, nebula round Nova Persei, 401;
photographs of nebulæ, 432
Ritter, development of stars, 375
Roberts, A. W., southern variables, 392
Roberts, Isaac, search for ultra-Neptunian planet, 306;
photographs of Orion nebula, 408
of Andromeda nebula, 409
of the Pleiades, 411
Roberval, structure of Saturn’s rings, 299
Robinson, reflectors and refractors, 431
Roche, inner limit of satellite-formation, 301;
modification of nebular hypothesis, 321
Römer, star places, 10;
invention of equatoreal and transit instrument, 120
of altazimuth, 121
velocity of light, 231
satellite transit on Jupiter, 291
Rosenberger, return of Halley’s comet, 101
Rosetti, temperature of the sun, 219
Rosse, third Earl of, biographical sketch, 114;
great specula, 115-117
discovery of spiral nebulæ, 118
resolution of nebulæ, 119
climate and telescopes, 434
Rosse, fourth Earl of, experiments on lunar heat, 269
Rost, nature of sun-spots, 54
Roszel, mass of asteroids, 287
Rowland, photographic maps of solar spectrum, 210 440;
elements in run, 213
concave gratings, 439 440
Rümker, observation of Encke’s comet, 90
Russell, H. C., red spot on Jupiter, 295;
change in Argo nebula, 404
photographs of Nubeculæ, 425
[Pg 485]
photographs of Nubeculæ, 425
Russell, H. N., atmosphere of Venus, 254
Rutherfurd, lunar photography, 268;
star spectra, 372
photographs of the Pleiades, 410
diffraction gratings, 439
Sabine, magnetic and sun-spot periods, 127 128 130
Safarik, secondary light of Venus, 256;
compression of Uranus, 304
Satellites, discoveries, 110 282 293;
transits, 291 292
variability, 292 302
origin, 309 318
Saturn, low specific gravity, 298;
rotation, 302
spectrum, 303
Saturn’s rings, first disclosure, 85;
dusky ring, 86
stability, 298 300
meteoric constitution, 300
eventual dispersal, 301
Savary, orbits of double stars, 46
Savélieff, solar radiation, 164 225
Sawerthal, discovery of a comet, 366
Schaeberle, discovery of Procyon’s satellite, 42;
coronal photographs, 187 188
theory of corona, 191
meteoric photography, 339
discovery of a comet, 355
Schaeberle and Campbell, observations of Jupiter’s satellites, 292
Scheiner, Father, nature of sun-spots, 52, 54;
equatoreal instrument, 120 note
solar rotation, 146
darkening of sun’s limb, 221
Schiener, Dr. J., photospheric structure, 165;
spectrographic researches, 384 405
spectrum of Andromeda nebula, 403
stars and nebulæ in Orion, 407
Schiaparelli, rotation of Mercury, 247;
of Venus, 251 252
spots on Mars, 275
snow-cap, 277
canals, 278-280
compression of Uranus, 304
comets and meteors, 327 331 332 338
anomalous tail of great comet, 364
Pons’s comet, 365
origin of comets, 370
measures of double stars, 419
Schmidt, A., circular refraction in sun, 159
Schmidt, J., sun-spot period, 126;
lunar rills, 263
lunar maps, 265
disappearance of Linné, 267
cometary appendages, 363
new stars, 393
Schönfeld, extension of Bonn Durchmusterung, 412 414
Schrader, construction of reflectors, 243
Schröter, a follower of Herschel, 5;
motions of sun-spots, 146
biographical sketch, 243 244
observations on Mercury, 244 246 247
on Venus, 250-253 255
on the moon, 263
a lunar city, 265
Linné, 267
spots on Mars, 275
Jovian markings, 290
Schülen, perspective effects in sun-spots, 54
Schuster, photographs of corona, 178 185;
spectra of oxygen, 214
Schwabe, sun-spot periodicity, 125 126
Secchi, chromosphere, 70;
Biela’s comet, 97
cyclonic movements in sun-spots, 144
distribution, 148
profundity, 154
nature, 156 158
constitution of photosphere, 151
eclipse observations, 166 167
reversing layer, 171
observations of prominences, 194 196 199
absence of helium absorption, 213
temperature of the sun, 218
solar atmospheric absorption, 221
Martian canals, 279
spectrum of Uranus, 304
of Coggia’s comet, 343
stellar spectral researches, 372 373
carbon stars, 372 381
gaseous stars, 377
See, stellar orbits, 42, 46;
measures of Neptune, 84
measures of Uranus, 304
belts of Neptune, 306
colour of Sirius, 375 note
southern double stars, 419
evolution of stellar systems, 420
Seeliger, photometry of Saturn’s rings, 299;
rationale of new stars, 396
Seidel, stellar photometry, 420
Sherman, spectrum of Nova Andromedæ, 395
Short, reflectors, 4,109 115 121;
chromosphere, 68
satellite of Venus, 256
striation of Saturn’s rings, 299
Sidereal science, foundation, 9,442;
condition in 1785 10
progress, 50
Sidgreaves, spots and faculæ, 159
Siemens, regenerative theory of the sun, 312
Simony, photographs of ultra-violet spectrum, 215
Sirius, a binary star, 41;
mass, 42
parallax, 42, 416
spectrum, 133 373 383
former redness, 375 note
radial movement, 386 387
Smyth, Admiral, Donati’s comet, 324
Smyth, Piazzi, oxygen spectrum, 215;
lunar radiations, 269
expedition to Teneriffe, 434
Solar spectrum, fixed lines in, 133-135;
maps, 133 136 206 210 211 224 440
distribution of energy, 222 223
[Pg 486]
Solar system,
translation through space, 15, 39, 40, 406
development, 308 309 313-316 322
complexity, 441
Soret, solar temperature, 218
South, observations of double stars, 45;
12-inch lens, 113
Rosse reflector, 117
occultation by Mars, 276
Spectroscopic binaries, 387-391
Spectrum analysis, defined, 130;
first experiments, 131 132
applied to the sun, 133-135 156
to the stars, 133 372 373
Kirchhoff’s theorem, 135
elementary principles, 139 140
effects on science, 141 142
radial motion determined by, 201 386
investigations of comets by, 342 343
of new stars, 393 399
of nebulæ, 401-403
Spencer, position of nebulæ, 422
Spitaler, attendants on Brooks’s comet, 366
Spitta, transits of Jupiter’s satellites, 292
Spörer, solar rotation, 148 149;
chromosphere, 199 200
Stannyan, early notice of chromosphere, 68
Star catalogues, 28, 31, 32, 60, 414 415;
spectroscopic, 381 385 386
photographic, 412-414
photometric, 420 421
Star-drift, 426
Star-maps, 77, 78, 81, 284 413 415;
photographic, 413 414
Stars, movements, 9, 10, 35, 39, 415 426;
radial, 386 387 406
comparative brightness, 13, 49, 50, 420 421
distances, 35-37, 416-418
chemistry, 372 381 382
spectroscopic orders, 373
colours, 374
development, 375-377
actual magnitudes, 422
gregarious, 426
Stars, double, physical connection surmised, 17;
proved, 18, 442
masses, 38, 42
catalogues, 43, 45, 47, 50, 418 419
orbits, 46, 418
discoveries, 43, 46, 47, 418 419 435
photographs, 409
evolution, 420
Stars, gaseous, 377-380
Stars, variable, early discoveries, 9;
η Carinæ, 48, 49, 379
sun-spot analogy, 128 392
spectra, 379
Algol class, 390 391
catalogues, 391 392
Stefan, law of cooling, 219
Steinheil, stellar photometry, 420;
silvered glass reflectors, 429
Stewart, Balfour, Kirchhoff’s principle, 135 note;
solar investigations, 154 155
Stewart, Matthew, solar distance by lunar theory, 230
Stokes, prevision of spectrum analysis, 138
Stone, E. J., reversal of Fraunhofer spectrum, 172;
distance of the sun, 231 232 236
transit of Venus, 240
Cape catalogue, 415
proper motions, 426
Stone, O., star catalogues, 415;
measures of double stars, 419
Stoney, carbon in photosphere, 152;
dynamical theory of planetary atmospheres, 288
perturbations of Leonids, 338
status of red stars, 375
Stratonoff, star counts in Pleiades, 411
Stroobant, satellite of Venus, 256
Struve, F. G. W., stellar parallax, 35;
career and investigations, 43-45
occultation by Halley’s comet, 106
Russo-Scandinavian arc, 261 262
Struve, Ludwig, solar translation, 40
Struve, Otto, parallax of η Cassiopeiæ, 38;
solar velocity, 40
his father’s successor at Pulkowa, 45
eclipse of 1842 62, 64
Neptune’s satellite, 84
research on Saturn’s rings, 300 301
variable nebula, 403
Stumpe, solar translation, 40
Sun, Herschel’s theory, 54-57, 70, 149;
atmospheric circulation, 58, 59
chemical composition, 135 211-213
mode of rotation, 146 147
Kirchhoff’s theory, 149
Faye’s, 150-152
convection currents in, 150 152 165
dissociation, 152 206-210
luminous outbursts, 159-161
explosions, 205
heat emission, 216 217 221 222 225 226
temperature, 217-220 226
problem of distance, 227
results from transits, 230 232 236 240
from oppositions of Mars, 231 237
from light-velocity, 232 241
from measurements of minor planets, 238
concluded value, 242
maintenance of heat supply, 310-313
past and future duration, 312
Sun-spots, speculations regarding, 52, 53;
Wilson’s demonstration, 53, 154
distribution, 53, 58, 148
cyclonic aspect, 58, 144 157 158
periodicity, 126 128 162 163
magnetic relations, 127 160 161
meteorological, 129 164
auroral, 129 130 160 162
photographs, 145 154
level, 155
spectra, 156 207 208
volcanic hypothesis, 158
Lockyer’s rationale, 159
planetary influence, 163
relation to Jovian markings, 297
[Pg 487]
relation to Jovian markings, 297
Swan, chromosphere, 70;
sodium line, 132
Swift, E., discovery of a comet, 368
Swift, L., fallacious glimpse of Vulcan, 181 250;
discovery of a comet, 368
Tacchini, eclipse of 1883 181;
white prominences, 184
prominences and chromosphere, 199 200
spectrum of Venus, 254
Talbot, Fox, spectrum analysis, 131;
spectroscopic method of determining stellar orbits, 387
Tarde, nature of sun-spots, 52
Taylor, eclipse expedition, 187;
spectrum of Uranus, 305
achromatic lenses, 431
Tebbutt, comets discovered by, 326 352;
comet of 1882 359
Telescopes, achromatic, 112 431 432
Telescopes, equatoreal, 84, 120 121
Telescopes, reflecting, Short’s, 4,109 115 121;
Herschel’s, 12, 109-111
Lassell’s, 83, 114 121
varieties of construction, 109 110
Rosse’s, 115-119 434
Common’s, 407 412 429
Telescopes, refracting, Fraunhofer’s, 34, 35, 121;
Clark’s, 114 429 430 433 436
Grubb’s, 430 433
with bent and horizontal mountings, 436-438
Tempel, red spot on Jupiter, 294;
comet discoveries, 327
cometary observations, 352 362
Andromeda nebula, 394
discovery of Merope nebula, 410
Temperature, of the sun, 217-220 226;
of the moon, 269 270
of space, 270
on Mars, 277
Tennant, eclipse observations, 168 169 174
Terby, surface of Mars, 278 279 281;
secondary tail of comet, 355
Thalén, basic lines, 207;
map of solar spectrum, 210
solar elements, 212
Thollon, line-displacements by motion, 202 364;
atlas of solar spectrum, 211 440
lunar atmospheric absorption, 264
Thome, comet discovered by, 361
Thomson, Sir William (Lord Kelvin), solar chemistry, 138;
magnetic influence of the sun, 161
tidal strains, 257
rotation of the earth, 273
dynamical theory of solar heat, 311 312
Thraen, period of Wells’s comet, 357
Tidal friction, effects on moon’s rotation, 271 272 318;
month lengthened by, 316 318
influence on planets, 319-322
on development of binary systems, 420
Tietjen, asteroidal orbits, 284
Tisserand, capture of comets, 98;
lunar acceleration, 273
revolutions of Neptune’s satellite, 305
stationary radiants, 341
perturbations of Algol, 391
director of Paris Observatory, 414
Titius, law of planetary intervals, 71, 72, 85
Todd, eclipse of 1887 185;
solar distance, 236 241
trans-Neptunian planet, 306
Tornaghi, halo round Venus, 254
Transit instrument, 120
Trépied, reversal of Fraunhofer spectrum, 172
Troughton, method of graduation, 122
Trouvelot, veiled spots, 148;
chromosphere in 1878 175
intra-Mercurian planets, 181 250
observations of prominences, 184 196 204
of Mercury, 245 247
rotation of Venus, 252
red spot on Jupiter, 296
Trowbridge and Hutchins, carbon in sun, 212
Tschermak, origin of meteorites, 339
Tupman, transit expedition, 235;
results, 236
Turner, polariscopic coronal photography, 189;
employment of cœlostat, 190 438
stationary radiants, 341
Ulloa, eclipse of 1778 69
United States, observatories founded in, 6,7
Uranus, discovery, 5, 74, 111;
unexplained disturbances, 78, 79, 307
satellites, 87, 303
equatoreal markings, 303 304
spectrum, 304 305
retrograde rotation, 313 315 322
Valerius, darkening of sun’s limb, 221
Vassenius, description of prominences, 68
Venus, transits, 4,229 232;
of 1874 233-236
of 1882 239 240
atmosphere, 236 253 254
mountains, 252 253
spectrum, 254
albedo, 255
ashen light, 255 256
pseudo-satellite, 256
effects upon, of solar tidal friction, 320
Very, temperature of sun, 220;
lunar heat, 270
Vesta, discovery, 75, 76;
diameter, 287
spectrum, 288
Vicaire, solar temperature, 218
Vico, comet discovered by, 97;
rotation
[Pg 488]
rotation
of Venus, 251
Cytherean mountain, 253
Violle, solar temperature, 218 219;
solar constant, 225
Vogel, H. C., solar rotation, 202;
solar atmospheric absorption, 222 224
spectrum of Mercury, 245
of Venus, 255
of Vesta, 288
of Jupiter, 290
of Jupiter’s satellites, 293
of Uranus, 304
rotation of Venus, 252
ashen light, 256
intrinsic light of Jupiter, 291
cometary spectra, 342 343 355 357
carbon in stars, 374; stellar development, 375 376
spectrum of Γ Cassiopeiæ, 378
of Nova Cygni, 393
of Nova Andromedæ, 395
spectroscopic star catalogue, 381
radial motion of Sirius, 386
period of Mizar, 388
eclipses of Algol, 390
components of Nova Aurigæ, 397
spectrographic determinations of radial motion, 405 406
Vogel, H. W., spectrum of hydrogen, 206 note, 383
Vulcan, existence predicted, 248;
pseudo-discoveries, 249 250
Wadsworth, coronal photography, 189
Ward, Nova Andromedæ, 394
Waterston, solar temperature, 218;
meteoric infalls, 311
Watson, fallacious observations of Vulcan, 181 250;
asteroidal discoveries, 284
Webb, comet of 1861 326
Weber, Baily’s Beads, 62;
illusory transit of Vulcan, 249
Weinek, study of lunar photographs, 268
Weiss, comets and meteors, 332 334
Wells, comet discovered by, 356
Wesley, drawings of corona, 175
Wheatstone, spectrum of electric arc, 132;
method of ascertaining light-velocity, 232
Whewell, stars and nebulæ, 422
Williams, A. Stanley, canals of Mars, 279;
markings on Jupiter, 295 297; rotation, 296
Nova Persei, 400
Wilsing, solar rotation from faculæ, 155;
density of the earth, 261
system of, 61 Cygni, 419
Wilson, Alexander, perspective effects in sun-spots, 53, 154
Wilson, H. C., red spot on Jupiter, 295;
compression of Uranus, 304
exterior nebulosities of Pleiades, 411
Wilson, W. E., solar temperature, 220 222;
ultra-Neptunian planets, 306
Winnecke, comet discovered by, 94;
distance of the sun, 231
Donati’s comet, 324 347
Wisniewski, last glimpse of 1811 comet, 99
Witt, discovery of Eros, 284
Wolf, C., objections to Faye’s cosmogony, 315;
origin of Phobos, 321
Wolf, Max, photographic discoveries of minor planets, 283 284;
Nova Andromedæ, 394
Nova Aurigæ, 396
nebula near Nova Persei, 401
photographic nebular survey, 412
galactic nebulosity, 425
Wolf, R., sun-spot and magnetic periodicity, 128 162 163;
analogy of variable stars, 128 392
auroræ, 129
suspicious transits, 249
Wollaston, ratio of moonlight to sunlight, 49;
flame spectra, 131
lines in solar spectrum, 133
Woods, coronal photography, 179 180;
Cape Durchmusterung, 412
Wrangel, auroræ and meteors, 335
Wright, G. F., Ice Age in North America, 260
Wright, Thomas, theory of Milky Way, 14;
structure of Saturn’s rings, 299
Wright, W. H., polarisation of cometary light, 355;
spectrum of nebulæ, 400
Yerkes, donation of a telescope, 433
Young, Miss Anne, nebular hypothesis, 314
Young, C. A., spectrum of sun-spots, 156;
origin, 158
spectrum of corona, 170 177
detection of reversing layer, 171 172
prominences and chromosphere, 194-196 200
photograph of a prominence, 197
spectroscopic measurement of sun’s rotation, 202
solar cyclones and explosions, 204 205
basic lines, 207
spectrum of Venus, 254
red spot on Jupiter, 294
observations of Uranus, 303 304
Andromedes of 1892 337
spectrum of Tebbutt’s comet, 355
of Nova Andromedæ, 395
Young, Thomas, absorption spectra, 136
Zach, Baron von, promotion of astronomy, 5, 6, 28;
Baily’s Beads, 62
search for missing planet, 72
rediscovery of Ceres, 74
use of a heliostat, 120
Zantedeschi, lines in solar spectrum, 134;
lunar radiation, 269
[Pg 489]
lunar radiation, 269
Zenger, observations on Venus, 253 255
Zenker, cometary tails, 348
Zezioli, observation of Andromedes, 334
Zodiacal light, relation to medium of space, 94;
to solar corona, 176; meteoric constitution, 310
Zöllner, electrical theory of comets, 99, 344 346 347;
solar constitution, 158
observations of prominences, 194 196
reversion spectroscope, 202
solar temperature, 220
Mercurian phases, 245
albedo of Venus, 255
of Jupiter, 290
of Saturn, 303
of Uranus, 304
condition of Venus, 256
of great planets, 289
Jovian markings, 297
ages of stars, 375
polarising photometer, 420 421
THE END
BILLING AND SONS, LTD., PRINTERS, GUILDFORD[Pg 490]
THE SYSTEM OF
THE STARS
BY
AGNES M. CLERKE
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TRANSCRIBER’S NOTES:
| Original | |||
| page | line | Original text left as is (sic) | |
| 072 | 13 | The search for it, through confessedly scarcely | |
| 196 | 24 | The first description are tranquil | |
| Original | |||
| page | line | Original text | Replaced with |
| 009 | 11 | byeways | byways |
| 024 | 46 | concentation | concentration |
| 043 | 37 | Is appears from | It appears from |
| 062 | 37 | appearances seem by him | appearances seen by him |
| 082 | 3 | forgotton | forgotten |
| 092 | footnote 1 | 11/9647000 | 1/9647000 (confirmed by looking up reference quoted) |
| 093 | 7 | phenenoma | phenomena |
| 100 | 17 | Bredikhin | Brédikhine |
| 131 | 13 | identifiying | identifying |
| 140 | 40 | terrestial | terrestrial |
| 143 | 25 | appearence | appearance |
| 149 | 27 | bloodvessel | blood vessel |
| 152 | 12 | Angström | Ångström |
| 169 | 3 | undimished | undiminished |
| 171 | 42 | sympton | symptom |
| 172 | 18 | familar | familiar |
| 173 | 42 | photograpic | photographic |
| 182 | 37 | by which i structure | by which its structure |
| 199 | 37 | Bredikhine | Brédikhine |
| 220 | 26 | stata | strata |
| 246 | 30-31 | of its orbit 24 hours 53 seconds. | of its orbit in 24 hours 53 seconds |
| 260 | 13 | garden at its seasons | garden as its seasons |
| 284 | 21 | throngh | through |
| 284 | 13 | oparator | operator |
| 376 | 42 | promptly recognised. in a | promptly recognised in a |
| 377 | footnote 3 | applie | applied |
| 395 | 42 | the gaseous fields o | the gaseous fields of |
| 423 | 35 | relatiouship | relationship |
| 434 | footnote 2 | Optice | Optics |
| 436 | 42 | Its resumption, ofter some years | Its resumption, after some years |
| 436 | footnote 1 | (two references given, within a single footnote. In the text footnote 1 used twice) | (split into two footnotes, and corrected references in the text) |
| 450 | 27 | 1862 Conclusion of a | 1872 Conclusion of a |
| 454 | 40 | spectographically | spectrographically |
| 454 | 18 | spectographic | spectrographic |
| 456 | 4 | Lyrae | Lyræ |
| 488 | index | Wolf, R., sun-spot and magnetic periodicity, 128, 164, 162; | Wolf, R., sun-spot and magnetic periodicity, 128, 162, 163; |