_THE
CONCISE
KNOWLEDGE
LIBRARY_
ASTRONOMY
[Illustration:
Photograph of the Moon taken at Paris by MM. Loewy and Puiseux with
the great Coudé Equatorial, February 14, 1894.
]
THE CONCISE KNOWLEDGE LIBRARY
ASTRONOMY
❧
BY
AGNES M. CLERKE
AUTHOR OF A POPULAR HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY
A. FOWLER, A.R.C.S., F.R.A.S.
DEMONSTRATOR TO THE ROYAL COLLEGE OF SCIENCE
J. ELLARD GORE, F.R.A.S., M.R.I.A.
AUTHOR OF THE SCENERY OF THE HEAVENS, THE WORLDS OF SPACE, ETC.
[Illustration: Black-and-white publisher’s emblem with a tree above a
shield featuring stylized letters, flanked by scrolls reading ‘Inter
Folia Fructus.’]
NEW YORK
D. APPLETON AND COMPANY
1898
_Authorized Edition._
PREFACE
This work aims to present in concise form a popular synopsis of
Astronomical Knowledge to date.
In Section I., Miss Agnes M. Clerke, author of “A Popular History of
Astronomy during the Nineteenth Century,” gives a brief historical
sketch of the science from Hipparchus to the present time. In Section
II., an attempt is made by Mr. A. Fowler, A.R.C.S., F.R.A.S.,
Demonstrator of Astronomical Physics to the Royal College of Science, to
briefly outline the general principles of spherical and gravitational
Astronomy, and to describe the instrumental means now at the command of
observers in the various branches of Astronomical research. The author
begs to record his indebtedness to Loomis’ “Treatise on Astronomy,” and
Young’s “General Astronomy,” which have been frequently consulted,
especially for memorial data; to Mr. W. Shackleton, for assistance in
proof-reading; and to Mr. C. P. Butter, for valuable help in preparing
the diagrams. Section III., contributed by Miss Agnes M. Clerke, deals
with the Solar System; and Section IV., written by Mr. J. Ellard Gore,
F.R.A.S., M.R.I.A., treats of the Sidereal Heavens.
The work is illustrated by a large number of diagrams and other
illustrations, prepared expressly for its pages, as well as by a number
of photographic and other reproductions of photographs and drawings made
by distinguished astronomers in Europe and America. In this connexion
numerous acknowledgments are due.
The Editor begs to express his sense of indebtedness to the following
astronomers and publishers, for kind permission to reproduce original
photographs and drawings from their works:
To M. Loewy, Director de l’Observatoire, Paris, for permission to
reproduce the photograph of the Moon, which forms the frontispiece of
this volume; to Professor Edward S. Holden, Director of the Lick
Observatory, for permission to reproduce drawings and photographs of the
Observatory at Nice, p. 2; the Thirty-six Inch Reflector of Lick
Observatory, p. 40; the Meridian Circle of the Paris Observatory, p.
203; the Spectroscope adapted to the eye end of the Lick Telescope, p.
221; and Jupiter showing the Red Spot, p. 322; to Dr. Isaac Roberts, for
permission to reproduce his photograph of the photographic telescope
used by him, p. 196; to Messrs. Trichnor & Co., of Berlin, for
permission to reproduce two illustrations of Donati’s Comet, pp. 228 and
363; and one of Sun-spots and Magnetic Variations, p. 246—all from
Langley’s “New Astronomy”; to Messrs. Witherby & Co., for permission to
reproduce the photograph of a Sun-spot by Janssen, p. 243; the
photograph of Jupiter, p. 328; the photographs of Swift’s Comet, pp. 374
and 375, Brooks’ Comet, p. 381, and the Milky Way, p. 557, from
_Knowledge_; to Messrs. Taylor & Francis, for the diagram of curves
showing the development of Sun-spots, p. 257; to Professor George E.
Hale, of Kenwood Observatory, Chicago, for his illustrations of Eruptive
Prominences photographed at Kenwood, March 24th, 1896, pp. 264 and 265,
reproduced from the _Astrophysical Journal_; to the Council of the Royal
Society, for the illustration of the Eclipsed Sun, p. 267, reproduced
from “Philosophical Transactions”; to Professor Barnard, for the
photographs of the Corona, reproduced on p. 269; and the drawings of the
Transit of Jupiter’s Satellite, on p. 330, reproduced from the _Monthly
Notices_ of the Royal Astronomical Society; the Eye of Mars, p. 302; and
of Saturn and his Rings, p. 335; to the editor of the _Astronomische
Nachrichten_, for the map of Mercury, by Schiaparelli, reproduced on p.
276; to the editor of _Nature_, for the drawing of Venus by Mascari, on
p. 280; to Messrs. George Philip & Son, for the map of the Moon, given
on p. 291, from Fowler’s “Telescopic Astronomy”; to Messrs. Longman,
Green & Co., for the Chart of Mars, p. 300, and the diagram of the
Apparent Orbit of the Companion of Sirius, p. 439, from Proctor’s “Old
and New Astronomy”; to Professor W. W. Payne of Goodsell Observatory,
for the use of the drawing of the Oases of Mars, p. 304, from “Popular
Astronomy”; and the photograph of Holmes’ Comet, p. 378, from the same
work; to Messrs. A. & C. Black, for the illustrations of the Great Comet
of September, 1882, p. 361, reproduced from Miss Clerke’s “History of
Astronomy”; to Messrs. Crosby, Lockwood & Co., for permission to
reproduce the illustration of the Nebula in Andromeda 31 Messier, p.
398, from the frontispiece of Mr. J. E. Gore’s “Visible Universe”; and
also for the same authority, regarding the diagrams, showing the Stars
visible in the Northern Hemisphere, p. 401; the Stars visible in the
Southern Hemisphere, p. 403; the drawing showing the position of the
Solar Apex, according to different computers, p. 429; and the
photographs of the Spiral Nebula 51 Messier, p. 533; and the Milky Way
in Sagittarius, p. 555, all from the same work; to Messrs. A. D. Innes &
Co., for the use of the diagram, Apparent Orbit of Zeta Herculis, p.
436; Triple Stars, p. 451; and for permission to reproduce the
photographs, 37 Messier, p. 505; the star cluster, Omega Centauri, p.
512; the Nebulæ of Orion, p. 521; and the Magellanic Clouds, p. 537,
from “The Worlds of Space”; and to Messrs. Sutton & Co., for the use of
the diagram of the Apparent Orbit of 70 Ophiuchi, p. 443; the
photographs of the Double Cluster of Perseus, p. 503; the Star Cluster
in Gemini, p. 504; the Star Cluster in Hercules, p. 507; and the drawing
of the Trifid Nebula, Sagittarius, p. 525, from “The Scenery of the
Heavens”; and the drawing of the Temporary Star of 1572, p. 481, from
“Planetary and Stellar Studies,” both published by them.
A. H. M.
CONTENTS
SECTION I.—HISTORY By Agnes M. Clerke.
Chap. Page
I. FROM HIPPARCHUS TO LAPLACE 3
II. A CENTURY OF PROGRESS 21
SECTION II.—GEOMETRICAL ASTRONOMY AND ASTRONOMICAL INSTRUMENTS.
By A. Fowler, A.R.C.S., F.R.A.S.
I. THE EARTH AND ITS ROTATION 41
II. THE EARTH’S REVOLUTION ROUND THE SUN 55
III. HOW THE POSITIONS OF THE HEAVENLY BODIES ARE DEFINED 65
IV. THE EARTH’S ORBIT 72
V. MEAN SOLAR TIME 78
VI. THE MOVEMENTS OF THE MOON 87
VII. MOVEMENTS OF PLANETS, SATELLITES, AND COMETS 98
VIII. ECLIPSES AND OCCULTATIONS 110
IX. HOW TO FIND OUR SITUATION ON THE EARTH 122
X. THE EXACT SIZE AND SHAPE OF THE EARTH 129
XI. THE DISTANCES AND DIMENSIONS OF THE HEAVENLY BODIES 139
XII. THE MASSES OF CELESTIAL BODIES 151
XIII. GRAVITATIONAL EFFECTS OF SUN AND MOON UPON THE EARTH 162
XIV. INSTRUMENTAL MEASUREMENT OF ANGLES AND TIME 171
XV. TELESCOPES 176
XVI. INSTRUMENTS OF PRECISION 198
XVII. ASTROPHYSICAL INSTRUMENTS 211
SECTION III.—THE SOLAR SYSTEM.
By Agnes M. Clerke.
I. THE SOLAR SYSTEM AS A WHOLE 229
II. THE SUN 237
III. THE SUN’S SURROUNDINGS 253
IV. THE INTERIOR PLANETS 273
V. THE EARTH AND MOON 283
VI. THE PLANET MARS 297
VII. THE ASTEROIDS 310
VIII. THE PLANET JUPITER 318
IX. THE SATURNIAN SYSTEM 333
X. URANUS AND NEPTUNE 343
XI. FAMOUS COMETS 352
XII. NATURE AND ORIGIN OF COMETS 368
XIII. METEORITES AND SHOOTING STARS 385
SECTION IV.—THE SIDEREAL HEAVENS.
By J. E. Gore, F.R.A.S.
I. THE STARS AND CONSTELLATIONS 399
II. DOUBLE, MULTIPLE, AND COLOURED STARS 410
III. THE DISTANCES AND MOTIONS OF THE STARS 417
IV. BINARY STARS 431
V. VARIABLE AND TEMPORARY STARS 458
VI. CLUSTERS AND NEBULÆ 497
VII. THE CONSTRUCTION OF THE HEAVENS 538
LIST OF ILLUSTRATIONS
_Frontispiece to Volume_—PHOTOGRAPH OF THE MOON.
SECTION I.—HISTORY.
_Page_
_Frontispiece_—THE OBSERVATORY AT NICE 2
SECTION II.—GEOMETRICAL ASTRONOMY.
_Frontispiece_—THE LICK REFRACTOR OF THIRTY-SIX INCHES
APERTURE 40
_Fig._
1.— ROUGH MEASUREMENT OF EARTH’S DIAMETER 43
2.— HORIZONS AT TWO PLACES ON THE EARTH 44
3.— FOUCAULT’S PENDULUM EXPERIMENT 49
4.— SUN’S RAYS—DAY AND NIGHT 52
5.— ATMOSPHERIC REFRACTION 54
6.— APPARENT PATHS OF SUN AT EQUINOXES AND SOLSTICES 56
7.— ABERRATIONAL ORBIT OF A STAR 58
8.— THE SUN’S ALTITUDE IN SUMMER AND WINTER 62
9.— THE SUN’S ALTITUDE AT THE EQUINOXES 63
10.— THE MIDNIGHT SUN 64
11.— ALTITUDE AND AZIMUTH 66
12.— RIGHT ASCENSION, DECLINATION, ETC. 69
13.— ELLIPTIC FORM OF EARTH’S ORBIT 72
14.— THE ELLIPSE 73
15.— HOW TO DRAW AN ELLIPSE 74
16.— ILLUSTRATING KEPLER’S SECOND LAW 76
17.— EFFECT OF OBLIQUITY OF ECLIPTIC UPON THE EQUATION OF TIME 81
18.— THE MOON’S PHASES 90
19.— THE LUNAR MONTH 91
20.— THE MOON’S ROTATION 93
21.— THE MOON’S NODES 94
22.— POSITION OF ECLIPTIC AT SUNSET AT VERNAL EQUINOX 96
23.— MOVEMENT OF AN INTERIOR PLANET 99
24.— MORNING AND EVENING STARS 100
25.— MOVEMENT OF AN EXTERIOR PLANET 103
26.— APPARENT PATHS OF CERES, PALLAS, JUNO, AND VESTA, IN 1896 104
27.— OPPOSITION OF MARS 105
28.— ELEMENTS OF AN ELLIPTIC ORBIT 107
29.— THE EARTH’S SHADOW 110
30.— THE LUNAR ECLIPTIC LIMIT 112
31.— ECLIPSES OF THE SUN 114
32.— DURATION OF A SOLAR ECLIPSE 115
33.— TRACK OF ECLIPSE OF APRIL 16, 1893 117
34.— DETERMINATION OF LATITUDE 124
35.— ANCIENT MODE OF MEASURING LATITUDE 125
36.— TRIANGULATION 132
37.— MOVEMENTS OF THE EARTH’S POLE, 1890–95 138
38.— PARALLAX OF A HEAVENLY BODY 140
39.— DIAMETER OF A HEAVENLY BODY 142
40.— MEASUREMENT OF THE MOON’S DISTANCE 143
41.— RELATIVE DISTANCE OF VENUS 145
42.— THE PARALLAX OF MARS 147
43.— THE TRANSIT OF VENUS 148
44.— THE MOON’S CURVILINEAR PATH 155
45.— THE TIDES 163
46.— NUTATION 169
47.— SECTION OF READING MICROSCOPE 172
48.— THE READING MICROSCOPE 173
49.— THE ACHROMATIC OBJECT-GLASS 177
50.— THE NEWTONIAN REFLECTOR 179
51.— THE CASSEGRAIN REFLECTOR 181
52.— THE PHOTOGRAPHIC TELESCOPE 196
53.— THE MERIDIAN CIRCLE OF THE PARIS OBSERVATORY 203
54.— THE MICROMETER APPLIED TO A BINARY STAR 208
55.— THE SPECTROSCOPE ADAPTED TO THE EYE END OF THE LICK
TELESCOPE 221
SECTION III.—THE SOLAR SYSTEM.
_Frontispiece_—DONATI’S COMET 228
1.— PHOTOGRAPH OF A SUN-SPOT 243
2.— SUN-SPOTS AND MAGNETIC VARIATIONS 246
3.— CURVES SHOWING THE DEVELOPMENT OF SUN-SPOTS 257
4.— ERUPTIVE PROMINENCE 264
5.— THE SAME, 18 MINUTES LATER 265
6.— THE ECLIPSED SUN 267
7.— THE CORONA OF JANUARY 1, 1889 269
8.— MAP OF MERCURY 276
9.— VENUS, FROM A DRAWING BY MASCARI 280
10.— MAP OF THE MOON 291
11.— PHOTOGRAPH OF THE TOTALLY ECLIPSED MOON 296
12.— CHART OF MARS 300
13.— THE “EYE OF MARS” 302
14.— THE OASES OF MARS 304
15.— JUPITER, SHOWING THE RED SPOT 322
16.— PHOTOGRAPH OF JUPITER 328
17.— TRANSIT OF JUPITER’S FIRST SATELLITE 330
18.— SATURN AND HIS RINGS 335
19.— GREAT COMET OF SEPTEMBER, 1882 361
20.— DONATI’S COMET 363
21.— PHOTOGRAPH OF SWIFT’S COMET 374
22.— THE SAME, 24 HOURS LATER 375
23.— PHOTOGRAPH OF HOLMES’ COMET 378
24.— PHOTOGRAPH OF BROOKS’ COMET 381
SECTION IV.—THE SIDEREAL HEAVENS.
_Frontispiece_—NEBULA IN ANDROMEDA 31 MESSIER 398
1.— STARS VISIBLE IN THE NORTHERN HEMISPHERE 401
2.— STARS VISIBLE IN THE SOUTHERN HEMISPHERE 403
3.— DIAGRAM SHOWING “SOLAR APEX” AS ROUNDLY COMPUTED 429
4.— APPARENT ORBIT OF ZETA HERCULIS 436
5.— APPARENT ORBIT OF THE COMPANION OF SIRIUS 439
6.— APPARENT ORBIT OF 70 OPHIUCHI 443
7.— TRIPLE STARS 451
8.— THE TEMPORARY STAR OF 1572 481
9.— THE DOUBLE STAR CLUSTER IN PERSEUS 503
10.— STAR CLUSTER IN GEMINI 504
11.— 37 MESSIER 505
12.— STAR CLUSTER IN HERCULES 507
13.— THE STAR CLUSTER, OMEGA CENTAURI 512
14.— THE ORION NEBULÆ 521
15.— THE NEBULA ROUND ETA ARGUS 523
16.— THE TRIFID NEBULA, SAGITTARIUS 525
17.— SPIRAL NEBULA, 51 MESSIER 533
18.— MAGELLANIC CLOUDS 537
19.— PHOTOGRAPH OF MILKY WAY, SAGITTARIUS 555
20.— THE MILKY WAY 557
ASTRONOMY
[Illustration:
THE OBSERVATORY AT NICE.
[_See page 192_
]
SECTION I.—HISTORY.
BY AGNES M. CLERKE.
CHAPTER I.
FROM HIPPARCHUS TO LAPLACE.
In the year 134 B.C., a temporary star blazed out in the constellation
Scorpio. It was observed by a man of extraordinary genius, and furnished
the incentive to one of his most memorable works. This was the
construction, on essentially modern principles, of a catalogue of 1,080
stars. Hipparchus thus, with deliberation and singular prescience,
furnished a standard by which future changes in the heavens might be
detected. He was a native of Rhodes, but belonged to the school of
Alexandria; and at Alexandria, after three centuries, he found an able
and ambitious successor.
Claudius Ptolemæus was one of the many “inheritors of unfulfilled
renown.” He combined, completed, and preserved what his predecessors,
eminent or obscure, had done. Gathering materials from all quarters, and
adding much of his own, he reared an astronomical edifice so imposing,
coherent, and substantial, that the lapse of fourteen centuries left it
virtually unassailed, and, to a superficial judgment, unassailable.
Fitly, then, this monument of industry and ingenuity kept the title
bestowed upon it by the Arabs of “Almagest,” signifying “the Greatest.”
It bears, nevertheless, perennial witness to the possibility of
satisfying the human mind with the truth of appearances, apart from the
truth of things. For although the Almagest embodies a large amount of
real knowledge, that knowledge is throughout falsely interpreted. The
Ptolemaic system was constructed on the principle of “saving the
phenomena”—that is, of providing expedients geometrically valid, even if
physically inadmissible, by which to represent the apparent movements of
the heavenly bodies. That they might, to a great extent, be apparent
only, was obvious to the cultivated Greek mind. The rotation of the
earth on an axis was a familiar Pythagorean doctrine; it was adopted by
Plato, and Aristarchus of Samos went to the length of ranking our green
world as a planet revolving yearly round the sun. The idea, however, was
too recondite for vulgar apprehension; it was tainted with a suspicion
of impiety, and its development would, besides, have proved extremely
embarrassing to the nascent science of that age. So Hipparchus chose the
prudent alternative of treating astronomy from the purely mathematical
standpoint; he submitted to the restrictions imposed by the hypothesis
of equable circular motion; and, with wonderful skill, fitted the
Apollonian eccentrics and epicycles to expound celestial wanderings.
Ptolemy inevitably followed suit. He set some five dozen spheres in
motion, while leaving the earth at rest; and at rest it remained until,
in long meditations by the foggy shores of the Baltic, a grave-browed
ecclesiastic elaborated certain cogent arguments in favour of its
motion.
During the interval between Ptolemy and Copernicus, astronomy kept in
the Alexandrian groove. Early in the eighth century, the seat of
learning having been transferred to Baghdad, the charge of its
crystalline machinery devolved upon Arabs and Jews, men of fine
technical acquirements, but small originative power, men of the kind
described in the “Vicar of Wakefield,” who, “had they been bred
cobblers, would all their lives have only mended shoes, but never made
them.” Not but that they executed the necessary repairs with uncommon
ingenuity, modifying the cumbrous structure given into their keeping to
suit the fresh inequalities brought to light by their patient
watchfulness. But their improvements consisted in adding to already
intolerable complications—in piling orb on orb, in devising
“trepidations” and oscillations, of which nature took small heed; so
that the better they observed, the worse their system became.
The science was diligently cultivated. Al-Mamûm, son and successor of
“good Harûn-al-Raschid,” founded at Baghdad a school of astronomy, of
which Albategnius, called “the Ptolemy of the Arabs,” was the brightest
ornament. He discovered, early in the tenth century, the movement of the
“sun’s apogee”—that slow revolution of the longer axis of the earth’s
orbit, regarded by astronomical glacialists as a factor in the
production of recurring Ice Ages. The Persian grandee, Al-Sûfi (903–986)
belonged to the same group. His “Description of the Stars” was a revised
edition, not a simple reprint, of the Alexandrian list, and has the
value derived from personal consultation with the skies. Thus, Algol,
now purely white, is recorded in it as a decidedly red star. About a
century later, Aboul Wefa detected the moon’s “variation,” independently
noted, after five centuries, by Tycho Brahé. Then the Tartars had their
turn. Nasir Eddin (1201–1274) was a native of Khorassan; but his love of
learning drew him to the city of the Khalifs, where he assembled a band
of experts for the construction of new planetary tables, the old ones
having lapsed into hopeless disaccord with the heavens. Last came Ulugh
Beigh, grandson of the furious Tamerlane. He founded at Samarcand a kind
of astronomical Solomon’s House, built a grand observatory, and worked
in it assiduously. His vigorous and ennobling reign of forty years was
terminated by one of those domestic catastrophes which ordinarily fix
the chronology of Eastern dynasties. He was murdered by his son in 1447,
and the sands of the desert thereupon closed, so to speak, over his
civilising efforts. His star catalogue, edited by Francis Baily in 1843,
is the outcome of fresh observations made in the old way. A Tartar
prince, he ranks as an Arab astronomer.
Mohammedan science had already fulfilled its appointed task. A torch,
still alight, had been handed on from East to West. Its extinction would
have been a calamity. A total break in the cultivation of astronomy, for
instance, would have cost ages to repair. The Ptolemaic system, it is
true, disguised rather than revealed nature; yet it constituted a
regulated body of knowledge, only looked at from a wrong point of view.
An unbiassed spectator had merely to shift his position and open his
eyes, in order to perceive the simplicity of the real celestial
mechanism. No better illustration could be adduced of Bacon’s aphorism
that “truth emerges more easily from error than from confusion.”
It was from the Moors in Spain that Christian Europe took its first
lessons in antique science. The Alphonsine Tables were due to Oriental
industry. They were compiled at Toledo about 1270 by an assemblage of
Arab experts directed by Hassan, the Jew delegate of Alfonso X. of
Castile. But they caught Western attention, and drew Western
intelligence towards the abstruse art they exemplified. Thus a little
treatise on the Sphere composed about 1230, by John Holywood, a
Yorkshireman, known to cosmopolitan fame as Johannes de Sacrobosco,
obtained astonishing popularity; at least sixty-five Latin editions of
it appearing between 1472 and 1647, besides French, Italian, German, and
Spanish versions, and endless commentaries. With the revival of
classical learning, the Almagest, previously known in blundering Latin
translations from the Arabic, came to be read in the original Greek, and
thus re-emergent, roused fresh enthusiasm. Inspired by the afflatus,
George Purbach (1423–1461) and his brilliant pupil, Johannes Müller of
Königsberg in Franconia (Regiomontanus), successively professors of
mathematics at Vienna, applied themselves to burnishing up the ancient
epicyclical apparatus; while in Italy, the seductive opinions of the
Pythagorean school gained ground, as evidence came to light, that there
had been astronomers before Ptolemy no less than kings before Agamemnon.
The orthodox doctrine naturally continued to be taught at the
universities; but some of the professors held esoteric opinions of a
different cast, which they freely imparted to privileged disciples. The
earth’s rotation was spoken of as a matter of common knowledge by
Lionardo da Vinci; it was inculcated in rhyme, before the close of the
fifteenth century, by Girolamo Tagliavia, a Calabrese poet; it was
debated by scholars and pedants; on all sides influences wrought to
shatter the integrity of Ptolemaic convictions.
True progress, however, consists less in destruction than in
re-organisation. And this demands powers of a high order. They were
brought into play just at the right moment. Nicholas Copernicus was born
at Thorn on the Vistula, February 19, 1473. At the age of twenty-three,
having exhausted the teaching resources of the university of Cracow, he
crossed the Alps in quest of instruction in Greek and mathematics.
Towards the close of 1496, then, he was enrolled as a student at
Bologna, and shortly afterwards became the pupil, assistant, and friend
of the Ferrarese astronomer, Domenico Maria Novara. Here, beyond
reasonable doubt, Copernicus adopted Copernican opinions. The question,
_An terra moveatur?_ was incessantly mooted at Bologna; advanced
thinkers replied in the affirmative; Novara himself most likely took his
intellectual beliefs from Plato and Aristarchus, while looking to
Ptolemy for his daily bread. The transalpine scholar, at any rate,
brought back with him to Poland in 1505, an unalterable persuasion that
the heliocentric system belonged to the reality of things. He devoted
eighteen years of his abode within the cathedral precincts of
Frauenburg—from 1512 to 1530—to demonstrating its detailed conformity
with the phenomena of the heavens; but allowed only a sketch of his
results to be published. It was only at the earnest request of the
Bishop of Culm that he finally delivered up to him the manuscript of “De
Revolutionibus Orbium Coelestium,” the first printed copy of which was
laid on his deathbed, May 24, 1543.
The immediate effect was small. The new system of astronomy was admired,
but not adopted. It indeed contradicted the evidence of the senses, and
failed to compel assent from the understanding. For its author had not
completely broken with tradition. He unfortunately retained the false
supposition of equable circular motion, and thus greatly marred the
simplicity of his scheme of the heavens. Orbs still kept rolling upon
orbs, Mercury alone demanding a combination of seven to bear him over
his course. But if seven, it might have been asked, why not seven times
seven? The principle of representing appearances by transcendental means
remained the same as before. Ignorance of the laws of motion raised
other formidable objections. A whirling earth, it was thought, should
leave behind all detached objects; absolute repose was taken to be the
condition _sine quâ non_ of stability. Then the seeming immobility of
the stars implied for them a remoteness so extravagant, according to
prevalent ideas, that even Kepler admitted it to be “a big pill to
swallow.” Copernicus was fully aware that the earth’s orbital revolution
must occasion stellar perspective displacements; indeed, he staked the
truth of his theory upon future measurements of annual parallax.
Nevertheless, four centuries passed before they were successfully
executed.
Tycho Brahé was the last great mediæval observer. Like Hipparchus, he
was summoned by a star—the marvellous “new star” of 1572; and, having
obtained from Frederick II. of Denmark the grant of an islet in the
Sound, he built upon it a mansion “royal, rich and wide,” erected
magnificent instruments, and used them, not only with consummate skill,
but also with a certain princely pomp, donning robes of state before
admitting the bright “populace of heaven” to audience. His stormy
temper, however, led to disputes with the young King Christian IV.; he
forsook Uraniborg, and died at Prague in 1601. Curiously enough, the
very accuracy of his observations led him astray from speculative truth.
For it enabled him to perceive the incompatibility of many facts with
Copernican expedients for harmonising them, and intensified the
difficulty raised to Copernican views by the absence of stellar
parallax. So he devised a system of his own, in which the planets
revolved round the sun, but the sun round the earth. It scarcely
survived its contriver.
The invention of the telescope created descriptive astronomy. Without
it, the mechanism of the solar system could have been laid bare, and the
law of force regulating its action discovered; and in point of fact,
Kepler’s achievements owed nothing, and Newton’s very little, to the
optician’s art. Inquiries, on the other hand, into the nature of the
heavenly bodies were wholly inspired by it; it disclosed the amazing
multitude of the stars, and opened endless vistas of research. No one
could at first have divined the momentous character of the accident by
which Hans Lippershey, a spectacle-maker at Middleburg in Holland, hit
upon an arrangement of lenses serving virtually to abridge distance. It
happened in 1608; and Galileo Galilei (1564–1642), hearing of it shortly
afterwards at Venice, prepared on the hint a “glazed optic tube,” and
viewed with it, early in 1610, the satellites of Jupiter, the mountains
of the moon, the star streams of the Milky Way, and in 1611, the phases
of Venus, the spots on the sun, and the strange appendages of Saturn.
Thus, amid a tumult of applause, the telescopic revelation of the
heavens began. It was brilliantly illustrative, although not
demonstrative, of Copernican theory; and Galileo drove his own vivid
conviction on the subject home to general apprehension by the literary
skill with which he treated it in his famous “Dialogues” (1632). He most
substantially promoted the new views, however, by his recognition of the
laws of motion, and of force as the cause of motion. The problem of the
heavens, stript thereby of metaphysical obscurities, was laid bare to
the reason as one of pure mechanics; the planets came to be treated as
ordinary projectiles, and distinct reasoning about the nature of their
paths was rendered possible. Newton’s great task was thus prepared and
defined by Galileo.
Kepler’s (1573–1630) three generalisations formed a still more
indispensable prelude to its accomplishment. Their immediate effect was
to sweep away the Copernican remnants of Ptolemaic lumber, and to
disclose the harmonious plan upon which our system is ordered. But it
was a geometrical plan only. Kepler indeed divined the influence of a
central power, which he surmised to be of a magnetic nature; and he
aspired towards the establishment of a truly physical astronomy. Yet he
was far from perceiving the full implications of the laws he had
himself, after half a lifetime of trial and failure, at last
triumphantly discovered. These laws are:
(I.) The planets travel in ellipses of which the sun occupies one focus.
(II.) They travel at rates varying in such a manner that the “radius
vector”—or imaginary line joining each to the sun—describes equal areas
in equal times.
(III.) The cubes of their mean distances from the sun are proportional
to the squares of their periods of revolution.
Now these are precisely the conditions under which planetary circulation
should proceed if governed by a force emanating from the sun, and
decreasing as the square of the distance from him increased. Moreover,
Hooke, Halley, and Wren separately got so far as to perceive that it
could be explained on this principle. But Isaac Newton alone could
demonstrate what they divined, and even his supreme faculties were
dangerously strained by the laborious process. This was not all. He
showed that the earth exerts on the moon just the same kind of pull that
the sun exerts on the planets; a pull identical with the familiar
“attraction of gravitation,” by which the globe we inhabit holds
integrally together, retains its oceans in their beds, and bears with it
through space its “cloud of all-sustaining air.” Its domestic affairs
are thus guided by the same unchanging rule that dominates its foreign
relations.
The publication in 1687 of Newton’s “Principia” marked an unprecedented
advance in knowledge. The advance consisted in unification. A science of
celestial physics, capable of indefinite future expansion, was founded
on the sure basis of terrestrial experience. Canons of interpretation,
derived from immediate perception, were proved applicable to the
phenomena of the heavens. The line drawn in antique philosophy between
the “corruptible” things under our feet and the “incorruptible” over our
heads was forever rubbed out. Sublunary and empyreal regions were thrown
together into one vast domain.
Although Newton’s law is, in itself, of extreme simplicity, its actual
workings are highly intricate. Because dependent upon a universal and
unintermittent influence, they are self-modifying, so that each
consequence becomes a cause, and to each cause is attached an endless
train of effects. They can be dealt with only with the aid of the
infinitesimal calculus, and then, not directly, but by successive and
tedious approximations, or by arts and devices of almost superhuman
ingenuity. Hence Newton’s laurels would have remained comparatively
barren had he not found successors in a group of men of extraordinary
ability. What he had begun, Clairaut, D’Alembert, Euler, Lagrange, and
Laplace carried on by showing the adequacy of a single law to account
for every traceable deviation from undisturbed elliptical motion. In the
course of a long and arduous campaign, they carried every position that
they attacked. Over and over again, the principle of gravitation seemed
to be compromised; over and over again, it was vindicated by these
intrepid champions.
This process of gradual verification began in 1747, when Clairaut and
D’Alembert sent to the Paris Academy of Sciences, on the same day, the
first satisfactory solutions of the “Problem of three Bodies.” The
motions of the moon, nevertheless, did not at once fall in with the
general theory; they were rendered amenable only after years of anxious
toil. Barely the initial difficulties had been overcome when Euler, in
1753, published his “Theory of the Moon,” from which Tobias Mayer of
Göttingen constructed lunar tables. Now tables are the test of theories.
Every row of figures they contain is a prediction, by the fulfilment, or
non-fulfilment of which the underlying scheme must stand or fall.
Through such comparisons, mathematical astronomers find out the
shortcomings of their methods, or the insufficiency of their hypotheses,
and are incited to refine the first, and correct the second. Demands for
the application of the nicer criteria thus afforded suggest
observational improvements, which seldom fail to bring to light minor
discrepancies with theory, impelling to fresh efforts for their
abolition. Such alternations of advance along the abstract and the
practical lines result in a continual diminution in the _scale_ of
error, although not in its annihilation; absolute exactitude being, as
it were, an asymptote, continually approached, but touched only at
infinity—that is, never, under subsisting conditions. Even now the
length of the moons tether is four or five miles. To that extent, she
may go astray from her computed path, not without occasioning
disquietude to the responsible authorities.
So far as could be ascertained in the eighteenth century, her subjection
to known law was completed by the dispersal of the mystery surrounding a
slight, continuous acceleration of her orbital velocity detected by
Halley in 1693. It had been in progress since the earliest recorded
eclipse in 721 B.C., if not longer; there was no sign of its cessation
or reversal, and the grave question arose, Was the principle of
universal attraction, elsewhere unreservedly obeyed, here fatally
complicated by the action of a resisting medium involving the eventual
collapse of the earth-moon system? Laplace gave the answer, November 19,
1787, by proving the observed quickening of pace to be a necessary and
simple consequence of a secular diminution in the ellipticity of the
earths orbit. This, however, will not go on for ever in the same
direction; after many ages the tide of change will turn, and a complete
restoration to the _status quo ante_ will ensue.
Another master-stroke of Laplace’s genius was his explanation, also in
1787, of the “long inequality” of Jupiter and Saturn. He demonstrated
its strictly gravitational origin in the mutual disturbance of the two
giant planets, rendered up to a certain point cumulative by the
approximate commensurability of their periods. While Jupiter performs
five circuits Saturn accomplishes nearly two, and the perturbation set
up at their conjunction is hence both intensified and balked of
compensation for 918 years.
The epoch of trial and confirmation immediately following the
publication of the “Principia” lasted then a full century. During its
course, difficulties had arisen only to be overcome; suggested
qualifications of the single and simple law of gravity had proved
unnecessary; at its close, recalcitrance had everywhere been overcome,
and there was victory all along the line. And not only were the workings
of the planetary system exhibited as depending upon an elementary
principle, but they were further shown to be perfectly equilibrated. It
contained within itself, so far as could be ascertained, no seeds of
decay; its destruction could only come from without. This remarkable
conclusion was established in a series of splendid treatises by Lagrange
and Laplace. The special adaptation to permanence of the solar mechanism
was demonstrated in them. Ruinous disturbances were shown to be excluded
by the overwhelming disparity of mass between the central body and its
attendants, no less than by the regularity and harmony of their
movements and distribution. Thus only slight oscillatory changes can
occur. Millions of years will elapse without producing any fundamental
alteration. The machine is so beautifully adjusted as to right itself
automatically through the mutual action of its various parts. And it is
the force which perturbs that eventually restores.
The astronomical acquisitions of the century were embodied in Laplace’s
“Mécanique Céleste,” published 1799–1805. This “Almagest of the
eighteenth century,” as it has been termed, is in a rare degree
comprehensive and complete. It leaves nothing enigmatical. Every
question propounded in it receives an answer, if not definitive, at
least highly authoritative; and the range of these questions is very
wide. All the phenomena which the Greeks and Arabs had rightly observed,
but wrongly interpreted, are not merely “saved” by geometrical
artifices, but derived as a connected whole from one physical cause,
absolutely prescribing that they should be thus, and no otherwise. The
work is a record of unmixed triumphs. It seems as if the author, for
want of more worlds to conquer, had laid down the sword of the calculus
to take up the pen of the chronicler. With grave exultation, he proceeds
from point to point, recounting the events of the campaign,
commemorating the battles won by the brilliant staff of mathematical
heroes to which he himself belonged, and expatiating in the broad
subjugated plain. He scarcely looked beyond. There was indeed at that
time no “beyond” where his methods of investigation were applicable. The
“Mécanique Céleste” hints at no unsatisfied ambitions; it is a book of
the _teres atque rotundus_ sort—a world in itself well arranged and
compact, to which outlying perplexities are allowed no access. Nor
should this be counted a defect. As a monument to one of the greatest
periods in the history of science, its fitting character was that of an
ordered collection of acquired certainties.
The countrymen of Newton took no part in the striking series of
operations by which the intricate consequences of the law of gravity
were deduced and shown to correspond with reality. During the whole of
the eighteenth century, they stood aside from the race towards
verification. Their effacement was due to no lack of ability, but to a
mistaken choice of means. Newton’s synthetic method was a veritable Bow
of Ulysses. It was too tough to be bent by other hands than his own.
Thus, no sequel could be given to the “Principia.” There was no
possibility of following up the line of demonstration pursued in it.
Newton himself would have vainly attempted to carry it much further. In
order to advance, it was necessary, as Dr. Whewell remarked, to begin
afresh. This, British mathematicians were unwilling to do. The easy and
flexible analytical method brought to perfection on the continent
remained strange to them. With inadequate strength, they persisted in
wielding the cumbrous weapon of a giant—in using main force, so to
speak, where skill and agility were required. Our insularity in this
respect lasted until about 1816, when, by the joint efforts of the
younger Herschel, Charles Babbage, and George Peacock (afterwards Dean
of Ely), mathematical studies were revolutionised at the University of
Cambridge.
The neglect in England of theoretical research was, however, partly
compensated by the steady progress of practical astronomy. For a century
and a half after its foundation in 1675, the Royal Observatory at
Greenwich continued to be the main—almost the only source of information
regarding the places of the heavenly bodies. Thence were obtained the
data necessary for the correction of theory, since there alone the
visible positions of the sun, moon, and planets were systematically
determined. _Actual_, compared with _predicted_, movements gave
so-called “tabular errors”; and tabular errors indicated theoretical
shortcomings, the rectification of which led gradually, but surely,
towards a higher plane of knowledge.
John Flamsteed (1646–1719), the first astronomer-royal, was, in
Professor De Morgan’s phrase, “Tycho Brahé with a telescope.” By his
diligence and insight he set on foot modern astronomy of precision. The
“British Catalogue” of nearly 3,000 stars, was, in its day, an unique
and most valuable work. His lunar observations were indispensable to
Newton’s calculations, which, indeed, through the insufficient supply of
them, now and again came to a halt; he constructed new solar tables, and
kept watch over the careers of planets and comets. His completion, in
1689, of a seven-foot mural quadrant, constituted a marked advance in
the art of instrument-making. It was firmly fixed in the meridian, so
that the distances from the zenith of the heavenly bodies at the moment
of culmination could be read off on the limb, the time being
simultaneously noted by a clock. Their positions in the sky relative to
a set of forty otherwise known stars were thus completely determined,
and they were determined essentially after the manner still in use.
On Flamsteed’s death in 1719, Edmund Halley (1656–1742) succeeded to his
place. An expedition to St. Helena in 1677, for the purpose of observing
stars invisible in these latitudes, got him the name of the “southern
Tycho.” They were the very first so situated to be located on the sphere
(except those few that came within Ptolemy’s range), and a list of them,
to the number of 341, was appended to the “British Catalogue.” The
purpose to which Halley devoted most sustained attention was, unluckily,
that in which he was least successful. Early in life he formed the
design of observing the moon through an entire revolution of its nodes,
so as to bring lunar tables to the perfection required for solving the
prize-problem of longitudes. But the _contumax sidus_—his opprobrious
term for our satellite—proved more than a match for him. The eighteen
years’ watch was kept, notwithstanding that the watcher had reached the
age of sixty-five before he was able to set about it; but in vain;
nothing came of it. Halley’s varied performances were, nevertheless, so
considerable as to warrant Lalande in describing him as “the greatest of
English astronomers”; and he ranked next to Newton among contemporary
English men of science.
His cometary labours alone sufficed to perpetuate his name. He initiated
the computation, on Newtonian principles, of the orbits traversed by
such bodies—then a most toilsome process; and, among twenty-four, found
three so much alike as to suggest the identity of the great comets of
1531, 1607, and 1682. A renewed apparition might then be expected in
1758, and he appealed to “candid posterity to acknowledge that this was
first discovered by an Englishman.” The prediction roused widespread
interest, and as the epoch for its fulfilment drew near, Clairaut
undertook the formidable task of determining to what extent it might be
postponed by the retarding influence of Jupiter and Saturn. Many times
he despaired of its execution, even with the efficient aid of Lalande
and Madame Lepaute, the wife of a Paris clock-maker; and at last, after
months of wearisome calculation, having succeeded in forming the
differential equations representing the comet’s disturbed motion, he
threw down the paper on which they were written, with the exclamation,
“Now, integrate them who can!” Eventually this, too, was done; and the
comet, caught sight of on Christmas Day, 1758, by Palitzsch, a rustic
star-gazer in Saxony, passed the sun within the month’s “law” permitted
to it by the French geometer. This signal triumph laid the sure
foundation of cometary astronomy.
In 1679, Halley drew attention to the importance of transits of Venus
for measuring the sun’s distance; and developed later a method
extensively used in observing the eighteenth century pair of transits in
1761 and 1769. But the accuracy actually attained in determining the
instants of contact between the limbs of the sun and planet fell far
short of what he had anticipated as attainable. The “black drop”
interposed its pernicious effects, and occasioned wide discrepancies.
The margin of uncertainty regarding the value of the great unit was,
none the less, diminished, although it still remained uncomfortably
wide; while the public interest excited by such rare events, the
adventurous character of the expeditions sent to the uttermost parts of
the earth for their utilisation, and the combined efforts of various
nations towards the same end, served to popularise astronomy, and to
give it something of that cosmopolitan stamp now borne by it.
Besides the discovery of the secular acceleration of the moon’s motion,
that of the long inequality of Jupiter and Saturn was due to Halley; he
ascertained, in 1718, the proper movements of Sirius, Aldebaran, and
Arcturus, thereby virtually demonstrating the non-existence of “fixed”
stars; he associated auroræ with terrestrial magnetism; noted the
globular star clusters in Hercules and Centaur; and divined nebulæ to be
composed of “a lucid medium shining with its own proper lustre,” and
filling “spaces immensely great.” Yet, in spite of the comprehensiveness
of his genius, his administration at Greenwich was a failure. He was a
better astronomer than astronomer-royal.
James Bradley (1693–1762), who came after him, gave a narrower scope to
his abilities, yet was of unsurpassed sagacity in connecting effects
with their causes. Robert Hooke (1635–1703) had observed, in 1669,
annual displacements of γ Draconis, a star nearly crossing the zenith of
London, which he took for results of parallax; and Flamsteed, in 1694,
had similarly interpreted a similar affection of the pole-star. They had
both been misled by an “aberration,” due to the progressive transmission
of light combined with the advance of the earth in its orbit. Bradley
determined to sift the matter thoroughly, and observed Hooke’s star
continuously from 1725 until 1728, first at Kew with Molyneux, then at
Wanstead in Essex. It evidently described a small ellipse in the sky
with a period of one year; yet its place in the ellipse was not what it
should have been on the parallactic hypothesis; so he remained for some
time in the dark about it. During a water-party on the Thames, however,
in September 1728, he noticed that the slant of the pennant varied with
changes in the boat’s course, the wind remaining steady throughout. This
gave him the clue he wanted; and his discovery of the “aberration of
light” was communicated to the Royal Society in the month of January
following. That of the nutation of the earth’s axis followed in 1748.
Both, setting aside their importance in themselves, were indispensable
as preliminaries to accuracy in fixing the places of the heavenly
bodies. For they are vital elements in the process of “reduction,” by
which the ore of truth contained in observations is extricated from the
dross of casual circumstances. The raw material, collected by timing
transits and reading circles, must be so refined and purified that the
facts contained in it become mutually comparable. Before Bradley’s time
allowance was indeed roughly made for refraction in our atmosphere, and
for the precession of the equinoxes; and, in the case of the moon, for
parallax; but the effects of aberration and nutation had remained mixed
up with a mass of disguising errors. Their elimination constituted an
inestimable improvement.
In the immediate art of observation Bradley was a master. He did not
live to possess an achromatic telescope; neither astronomical circles
nor equatorial mountings were at his disposal. His leading instrument
was an eight-foot quadrant, by John Bird, certainly of admirable
workmanship; although of a type long since, and for good reasons,
superseded. He amassed with it, nevertheless, a treasure of high-class
observations. The bulk of them remained in manuscript until 1798, so
that it was reserved for this century to turn them to account; but their
value has only developed with the efflux of years. Those relating to the
moon and planets, reduced by Sir George Airy, lent efficient aid towards
perfecting the theories of those bodies. Those of 3,222 stars formed
into a catalogue by Bessel were published in 1818 with the proud, but
not unmerited title of “Fundamenta Astronomiæ.” The same original data,
again in 1886 reduced with the utmost nicety of care by Dr. Auwers of
Berlin, afforded a splendid accession to knowledge of stellar proper
motions. Acquaintance with Bradley’s stars now extends over 144 years;
and the amount and direction of their progress across the sphere during
that long interval have, for the most part, become defined with
tolerable certainty.
Nathaniel Bliss (1700–1764), the fourth astronomer-royal, filled the
post only two years. Yet the observations made under his care form a
sequel to Bradley’s well worth having. The reign of his successor, Nevil
Maskelyne (1732–1811), extended over forty-six years. His determinations
of the sun, moon, and planets, were in great demand abroad for the
correction of tables, and as criteria of theories; while, of the stars,
he paid attention only to thirty-six, catalogued as reference-points in
1790. Their proper motions served Herschel for his second investigation,
in 1805, of the sun’s translation through space. By the close of the
century, Maskelyne’s instruments had lapsed into decrepitude; and only
the stimulus supplied by Pond’s strictures roused him to order one of
Troughton’s improved circles. But he died before it was mounted, and its
employment fell to the share of his critic, John Pond (1767–1836), the
sixth astronomer-royal. Maskelyne’s most enduring title to fame is his
foundation, in 1767, of the “Nautical Almanac.”
English observers were ably seconded by English artists. Graham, Sisson,
Cary, Bird, Ramsden, had, from the beginning to the end of the
eighteenth century, no foreign competitors of note. Their quadrants and
sectors were distinguished both for stability and for refinement of
execution. The mechanical skill displayed in their construction was no
less necessary for the promotion of practical astronomy than the
subtlety of eye and hand needed to employ them to the best advantage.
Bradley’s work was conditioned by the performances of Graham and Bird.
Without Graham’s sector he could not have discovered the aberration of
light; without Bird’s quadrant the perennial worth of his Greenwich
observations would have been impaired, if not destroyed. Observatories
all over the continent were furnished in the latter half of the
eighteenth century with instruments of English make; the art of
accurately dividing circular limbs was invented in England, and nowhere
else successfully practised. The innovation of substituting entire
circles for quadrants was effectively introduced by Ramsden; and Piazzi
came from Palermo in 1788 for the purpose of securing from him a
five-foot altazimuth, at that date the finest sky-measuring machine in
the world. Edward Troughton (1753–1835) ably carried on the tradition of
his predecessors, and brought the altazimuth, transit circle, and
equatorial up to the modern standard of efficiency. But they were no
longer in exclusive demand. The foundation, in 1804, of Reichenbach’s
Institute at Munich finally abolished the British monopoly in supplying
astronomers with their exquisite and ingenious tools.
The improvement of refracting telescopes ran a somewhat similar course.
The essential step of combining flint and crown glass, so as to bring
differently-coloured rays to one focus, was taken in 1733 by Chester
More Hall, a gentleman of fortune in Essex; but he published nothing,
and the re-invention of the “achromatic” lens was left to John Dollond
(1706–1761) a Spitalfields weaver. “I obtained,” he wrote in 1758, “a
perfect theory for making object-glasses, to the apertures of which I
could scarcely conceive any limits.” The excise duty on glass, however,
which was repealed only in 1845, drew these limits very narrowly in this
country; and it was through the extraordinary perseverance of a Swiss
artisan named Guinand, in overcoming the difficulties connected with
glass-making, and the genius of Joseph Fraunhofer (1787–1826) in
moulding the material thus placed at his disposal, that refractors began
at Munich to rise towards their present power and perfection.
The history of the reflecting telescope is British throughout. It was
invented by Newton, made practically effective by John Hadley
(1682–1744), and brought very near to theoretical perfection by James
Short of Edinburgh (1710–1768); yet it is remarkable that not a single
observation of lasting interest was made with any of his instruments, a
few of which have survived, and are regarded with admiration to this
day. The career of reflectors as engines of discovery began, but did not
end, with William Herschel.
CHAPTER II.
A CENTURY OF PROGRESS.
On March 13, 1781, an event occurred without precedent in the history of
astronomy. A new member of the sun’s immediate retinue was disclosed. A
hard-worked music-teacher at Bath performed this startling—indeed,
according to antique notions—impossible feat; and the name of Herschel
became known _urbi et orbi_. It was far from being by chance that the
“new planet swam into his ken.” The Octagon Chapel organist was no
ordinary lucky amateur. He had, some time previously, made two notable
resolutions. The first was to push the improvement of telescopes to the
furthest verge of what was possible; the second, to leave no corner of
the starry heavens unexplored. And he applied himself with marvellous
energy, in despite of accumulated professional engagements, to carry
them into execution. He thus rapidly grew to be an adept in the art of
constructing specula, and a master in the art of using them.
Two lines of effort, accordingly, converged, in his case, towards
celestial discoveries. With all his diligence in “reviewing” the
heavens, he could not have distinguished at sight Uranus from a fixed
star, but for the uncommon excellence of his seven-foot reflector; nor
would the reflector, had it been used in the ordinary erratic fashion of
casual stargazers, been at all likely to have encountered the little
bluish disc of the remote orb then slowly wending its way through the
constellation of the Twins. The direct, and a momentous result of the
discovery was to secure for astronomy the undivided powers of the
extraordinary man who had made it. George III. attached him to his
Court, delivered him from the drudgery of teaching, and gave him the
means of carrying out his grand designs.
Their fulfilment involved the construction of great light-gathering
machines. Herschel ardently desired to see as far and as much as the
conditions of mortality permitted; he was the first to connect depth of
penetration into space with extent of reflective surface; and he
accordingly strained every nerve to secure the means by which to compass
the end he had mainly in view. Nor was he content with mere size. His
mirrors were as remarkable for beauty of figure as for breadth of
aperture. They bore, on proper occasions, enormously high magnifying
powers, and the precise roundness of the star-images formed by them
excited the incredulous wonder of contemporaries. The quality of some of
his largest instruments was guaranteed by the heavens themselves. Their
approval was signified to the seven-foot reflector through the detection
with it of Uranus; the “large twenty-foot,” with a speculum of eighteen
inches, revealed in January 1787, two Uranian moons, Oberon and Titania;
and the monster forty-foot, through the tube of which George III.
promenaded with the Archbishop of Canterbury, brought into view, within
three weeks of its completion, Enceladus and Mimas, the innermost and
hardest to observe of Saturn’s numerous family of satellites.
The forty-foot was “Herschel’s furthest”; he fully recognised that with
it he had touched the line which divides failure from success. If,
indeed, he had not overpassed it; for the subsequent career of the great
telescope hardly bore out the promise of its start. It was an unwieldy
engine, demanding vastly more time and labour to bring into play than
the twenty-foot; and Herschel took such account of minutes as few men do
of hours or days. His fiftieth birthday had in fact gone by before his
optical ambition was satisfied; while his appetite for exploration was
only whetted by what he had already accomplished. He estimated, however,
that a “review of the heavens” with the forty-foot would have occupied
800 years; hence it was used only on special occasions. The Orion nebula
was the last celestial object upon which, January 19, 1811, “its broad,
bright eye” rested; and it was then, with due honour, placed on the
retired list.
Two years before his death, which occurred August 25, 1822, the elder
Herschel initiated his son into the secrets of speculum-building. The
pupil was worthy of the master. John Herschel (1792–1871) aimed only at
producing generally available instruments, and his success was easy and
unqualified. His eighteen-inch mirrors seem to have been all but
faultless. They certainly afforded him better views of the nebulæ than
had been obtained by his father. Thus he first saw the “Dumb-bell” in
its true oval shape; and his remarks upon annular lines of structure in
elliptical nebulæ prove that features unmistakably imprinted upon Dr.
Roberts’ photographs had been antecedently visible to him, and probably
to him alone.
The next stride in the enlargement of reflectors was made by an Irish
nobleman, the third Earl of Rosse (1800–1867). His leviathan telescope,
six feet in aperture, and fifty-four in length, has, in point of actual
size, never been surpassed. Distinguished rather for light-grasp than
for precise definition, it found its appropriate field in the nebular
realms of the sphere; and the discovery of spiral nebulæ, with which it
made its début, was one of high and wide significance.
William Lassell (1800–1881) of Starfield, near Liverpool, set the
example, in 1840, of mounting reflectors equatorially, so as to enable
them, by the application of clock-work, to follow automatically the
diurnal movement of the heavens. His specula were of almost unrivalled
perfection in form and finish. One twenty-four inches in diameter, now
at Greenwich, left a splendid record. With it Lassell detected, October
10, 1846, the satellite of Neptune; September 18, 1848, simultaneously
with W. C. Bond of Cambridge, U.S., Hyperion, the seventh in order of
distance and last in order of discovery of Saturn’s eight moons; and
October 24, 1851, Ariel and Umbriel, the inner pair of Uranian
satellites, of which Sir William Herschel had possibly, although not
very probably, caught transient glimpses. He erected a similar
instrument of fourfold capacity at Malta in 1861, registered with its
aid 600 new nebulæ, and delineated the complex structure of many others,
previously less well seen.
The four-foot reflector built in 1870 by Thomas Grubb of Dublin for the
Melbourne Observatory disappointed expectation. An apparatus so delicate
that the abrasion of 1/20,000th of an inch makes all the difference
between good and bad definition, is ill-fitted to endure the
rough-and-tumble experiences of an ocean-voyage; and that it in some way
“suffered a sea-change” is scarcely doubtful. It was the last great
telescope of its kind, metallic specula, having, in the seventies, been
superseded by mirrors made of glass upon which a thin layer of silver
has been chemically deposited. These have many advantages over their
predecessors. They are considerably more reflective; they are more
easily constructed; their shape is less liable to injury; their
brilliancy, although more evanescent, can be readily restored. They have
the drawback, however, of being extremely sensitive to changes of
temperature. A three-foot mirror of this description by Calver, was
employed by Dr. Common at Ealing with surprising success, early in 1883,
for the purpose of photographing the Orion nebula. It was mounted at the
Lick Observatory, California, in 1896. Dr. Common has since himself
constructed a similar instrument of five feet aperture, which is the
most potent light-collector ever yet turned to the skies. It is curious
to learn that the silver spread over its surface weighs less than one of
the “fourpenny bits” some time ago withdrawn from circulation; the
reflecting film is in fact only 1/280,000 inch thick.
Reflectors are perfectly, and _naturally_, achromatic, rays of all
colours being thrown back at the same angle, and consequently meeting at
the same focus. This gives additional brilliancy to the images formed by
them, compared with those given by object-glasses, the colour-correction
of which has hitherto been so imperfect that much light has to be
“thrown away” as worse than useless. New kinds and combinations of
optical glass have, however, of late been invented, by which this grave
defect may be cured. Reflecting telescopes, on the other hand, are less
manageable, and suffer more from distortion through change of position.
Their cheapness recommends them to amateurs; but they should, on
principle, be reserved for special departments of work, such as nebular
photography and the chemical delineation of stellar and nebular spectra.
The growth of refractors, like that of reflectors, has obtained from
time to time the sanction of unexpected disclosures. Thus a superb
fifteen-inch, turned out at Munich in 1847, for Harvard College,
Cambridge, U.S., showed Hyperion to Bond, September 16, 1848, and on
November 15, 1850, surprised him with a view of Saturn’s dusky ring.
This telescope was surpassed, after fifteen years, through the energy
and genius of Alvan Clark, the famous self-taught American optician,
originally a portrait-painter at Cambridgeport, Massachusetts. Before it
had left the workshop, an eighteen-inch achromatic, now the leading
instrument at the Dearborn Observatory, Evanston, Illinois, won maiden
honours by disclosing to Alvan G. Clark, one of the maker’s sons,
January 31, 1862, the dim companion of Sirius, which, before being seen,
had made itself _felt_ by gravitational disturbances of its radiant
primary. The Washington twenty-six-inch, by the same firm, was rendered
illustrious by Professor Hall’s discovery, in 1877, of a pair of Martian
moons; the Lick thirty-six-inch, by bringing within the range of
Professor Barnard’s keen eyesight, September 9, 1892, Jupiter’s tiny
“fifth satellite.” The diploma performance of the Yerkes forty-inch,
mounted in 1896 at the Chicago University Observatory, is yet to come.
Meanwhile, several very perfect refractors, up to thirty-two inches of
aperture, have been built on this side of the Atlantic by Sir Howard
Grubb of Dublin, and the MM. Henry of Paris; and a twenty-five-inch,
finished so long ago as 1868, and at the cost of his life through the
labours which it entailed, by Thomas Cooke of York, after having lain
for upwards of a score of years choked by the fog and smoke of
Gateshead, has recently begun a promising career at Cambridge, under the
care of Mr. Frank Newall, son of the original owner.
And now we cannot but ask ourselves, has the _ne plus ultra_ in
telescopic magnitude been attained? There is no reason to suppose that
it has, provided that due allowance be made for inexorable conditions.
Climate is one of these. The largest instruments are those most readily
crippled by atmospheric hindrances. The greater their powers, the fewer
are the nights on which they are likely to be available. If they are to
“shine in use,” and not “rust unburnished,” they must then be erected in
exceptionally favourable localities, such as the summit of Mount
Hamilton (the site of the Lick Observatory), or the Harvard College
southern station at Arequipa in Peru. In South Africa, too, but “up
country”—not in the Cape peninsula—splendid facilities for astronomical
observation are to be found.
From Professor Keeler’s report it can readily be gathered, and he indeed
explicitly states, that the Yerkes forty-inch marks the limit of useful
size in equatorials. For the character of the star-images formed by it
slightly change their character when it is directed to different parts
of the sky; and this implies that its lenses become, as it moves,
infinitesimally deformed through the effects of their own weight. No
larger instrument, accordingly, can safely be permitted to swing in
mid-air. The huge light-concentrating machines of the future will lie in
wait for the objects to be observed, instead of pursuing them. They will
either be supported horizontally, or mounted in the “Coudé” fashion
invented by M. Loewy. In either case, the necessary movement will be
performed vicariously by a plane mirror.
Thus, the optical and mechanical outlook is decidedly better than the
atmospheric. The question, How to build giant telescopes? is more easily
answered than the question, Where to place them when built? The ultimate
barrier to seeing indefinitely far into space is the rigid circumstance
that we live on an air-girt globe. The prospects of astronomy are deeply
involved in the forecast of its hampering effects. The dependence of
those prospects upon telescopic improvements became obvious when
Herschel took the whole contents of the sphere “for his province.” These
are indefinitely numerous, indefinitely far-off, indefinitely faint. The
task of their correlation undertaken by Herschel, and inherited from him
by modern astronomers, can at no time be more than approximately
fulfilled; but for each successive approximation more light is needed.
Those who would investigate the universe can never get enough of that
too scarce commodity.
Until Herschel conceived the novel idea of a comprehensive science of
the stars, they had been chiefly regarded as convenient sky-marks, by
which to track the wanderings of our nearer neighbours in space. When it
was perceived that the sky-marks were not fixed, it became necessary to
determine their movements; and this was very roughly done for
fifty-seven stars by Tobias Mayer of Göttingen, in 1757; and more
accurately for thirty-six by Maskelyne, a third of a century later. But
if the stars were travelling, the sun could not be supposed to stand
still; and the possibility of laying down his line of march through
space, by extricating a common element from the confused network of
mutually-crossing stellar paths, occurred to Mayer, and was actually
realised by Herschel in 1783. His inquiry, with the scanty materials
then at command, was a wonderful stroke of audacity, which very nearly
hit the mark; yet few believed in his result until it was confirmed by
Argelander in 1837.
The various attempts made, prior to 1782, to measure the parallaxes of
some of the brighter stars were instigated by the wish to find a
demonstrative argument in favour of the Copernican theory of our system.
They had no reference to sidereal structure. Herschel, however, took up
the subject simply for the purpose of fixing the scale of that vast
edifice. Before sounding the skies, he sought to ascertain the length of
his fathom-line. He never ascertained it. To the end of his life, he
could only make plausible assumptions as to the distances of the stars.
Their real parallaxes were insensible with his instrumental means. But
he fortunately chose for his experiments Galileo’s “double-star method.”
This consisted in determining the relative positions of two close stars,
one of which, taken to be indefinitely remote, was designed to serve as
a standard of reference for the perspective shiftings of the other. It
was thus that Herschel’s attention was directed to double stars. He
found them to be astonishingly numerous—far more numerous than could
have been anticipated by the doctrine of probabilities. In January,
1782, he presented to the Royal Society a catalogue of 269 star-pairs,
and he had collected 434 more by December, 1784. From their abundance
alone, the Rev. John Michell inferred their character of binary systems;
and Herschel, after twenty years of observation, was able, in 1802, to
announce the fact of their mutual revolutions. Thus was taken the second
great step towards the unification of the Cosmos. Newton proved that
terrestrial gravity dominates the solar system; Herschel showed that a
law of attraction, presumably (and assuredly) identical in its mode of
operation, extends through sidereal space.
One cannot reflect without amazement that the special life-task set
himself by this struggling musician—originally a penniless deserter from
the Hanoverian Guard—was nothing less than to search out the
“construction of the heavens.” He did not accomplish it, for that was
impossible; but he never relinquished, and, in grappling with it, laid
deep and sure the foundations of sidereal science. No one before him had
thought of approaching the subject otherwise than by way of speculation;
he alone had the boldness to attack it experimentally. Having invented
for the purpose an ingenious method of “star-gauging,” based upon the
hypothesis that the stars are, on an average, scattered evenly through
space, he concluded in 1784, from its application, that the Milky Way is
the visual projection of a disc-shaped stellar aggregation, within which
our sun is somewhat excentrically placed. The progress, however, of his
telescopic studies convinced him that the continued action of a
“clustering power” had long ago drawn the stars into many separate
allotments, and annulled the original uniformity of their distribution.
So the disc theory was given up, and the Milky Way came to be regarded
as a collection of genuine clusters, arranged into an irregular ring
encircling the solar system. This view, implicitly held by the elder
Herschel from 1802, was explicitly stated by his son in 1847. The
results that Herschel expected from star-gauging may, in the future, be
derived from the more elaborate process of star-gauging by magnitudes,
photographically executed; and the sky-charting work, rapidly
progressing in all parts of the world, will at least supply ample
materials for sounding the star-depths.
These are stored besides with the curious objects called “nebulæ.” They
were little noticed until Herschel, on March 4, 1774, made
“That marvellous round of milky light
Below Orion,”
the subject of his earliest recorded observation. Except, indeed, as
impediments to comet-hunting. Thus, Messier, one of the keenest
sportsmen in that line who have ever scanned the sphere, tried to
eliminate by enumerating them, and drew up in 1771 a list of 45 such
misleading objects, enlarged in 1781 to 103. And Lacaille, during an
expedition to the Cape in 1752–1755, picked up 42 more. So far this
department of knowledge had been cultivated when Herschel began to
“sweep the heavens.” To _sweep_ them, be it remembered. Not merely to
gaze at hap-hazard, or to look out for show specimens, but to gather in
the celestial harvest methodically, zone by zone, so as to “leave no
spot of the heavens unvisited.” The fruits were proportioned to his
diligence. The nebulæ discovered by him amounted, in 1802, to 2,500. And
he did not merely discover; he investigated them as well. He separated
them into classes, noted the mode of their distribution, and searched
out their relationships. To begin with, he believed them to be of a
purely stellar nature—to be, in fact, independent galaxies. Miss Burney
was informed by him in 1786 that he had “discovered fifteen hundred
universes.” A few years later, however, he reasoned out for himself the
gaseous nature of a great many nebulæ, such as that in Orion, and those
of the “planetary” sort; and published in 1811 a complete theory,
strikingly illustrated with examples taken from his telescopic
experiences, of stellar development out of nebulous stuff. The
supposition that they included the revelation of “exterior universes”
was thus rendered, to say the least, superfluous; yet it was not
perhaps, even by him, wholly abandoned. It was, moreover, revived in
consequence of the performances of the great Rosse reflector, from 1845
onwards, in resolving apparent nebulæ into “bee-like swarms” of stars.
Meanwhile Sir John Herschel’s examination of those wonders of the
southern heavens, the Magellanic Clouds, had virtually decided nebular
standing. For they contain within a limited compass, as Dr. Whewell
argued in 1853, “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.” That stars
and nebulæ co-exist in every part of the heavens, has since been fully
established; while the laws respectively governing their distribution
over the sphere are related in such a manner as to leave no doubt that
these two classes of sidereal objects unite to form the grand galactic
whole. Hence, to all reasonable apprehension, “island universes” have
vanished into the inane.
Sir John Herschel accomplished the unparalleled feat of sweeping the
heavens from pole to pole. Having, within eight years from 1825, revised
his father’s work at Slough, he conceived the noble idea of rounding it
off in the southern hemisphere; and, in 1833–4, transported his
instruments from Slough to Feldhausen near Cape Town. During the four
years of his residence there, he not only executed his proposed survey,
registering 1,790 nebulæ—300 of them for the first time—and discovering
and measuring 2,100 double stars, but carried out a number of special
researches. He catalogued the miscellaneous contents of the Magellanic
Clouds—systems _sui generis_, as he justly termed them—made a detailed
and laborious study of the Argo nebula, applied pretty extensively the
paternal method of star-gauging, observed Halley’s comet at its second
predicted return, measured the sun’s heat-emissions, carefully watched
the spot-maximum of 1837, and finally, struck with a sudden rise in
magnitude of η Argûs, brought to general knowledge that star’s
extraordinary character. These varied results were embodied in a
monumental volume, published in 1847.
One of the greatest triumphs of modern science has been the
establishment of an “Astronomy of the Invisible.” It was primarily due
to Bessel’s inquiries into the disturbed proper motions of the
“Dog-stars,” Sirius and Procyon. They convinced him that each of these
brilliant orbs is attended by a massive satellite, round which it
revolves as it advances, its path in the sky being thus not straight but
wavy. Telescopic verification of his forecast was, nevertheless, delayed
until 1862 in the case of Sirius, until 1896 as regards Procyon. The
earliest, and still the most memorable result in this line is the
discovery of Neptune. Bessel knew that the thing was to be done, and in
1840 planned the doing of it. But his powers began, soon afterwards, to
be crippled by deadly illness, to which he succumbed, March 17, 1846.
_Uno avulso, non deficit alter._ Adams and Leverrier separately
undertook the enterprise he had relinquished, and each with perfect
success. It was a formidable one. The _direct_ problem of perturbations
taxes the highest mathematical resources; the _inverse_ problem is not
only more arduous, but was then untried. Laplace and Lagrange had shown
how to determine the perturbations produced by a known disturbing body;
it was left for Adams and Leverrier to find an unknown body through its
disturbing effects. Irregularities in the movements of Uranus betrayed
the presence of Neptune, and by the powerful analysis brought to bear
upon them, were made to serve as an index to his actual place in the
heavens at a given epoch. This was done by Adams in September, 1845; but
his calculations, deposited at the Royal Observatory in the hope that
they would incite to a telescopic search for the new planet, remained
there buried in a drawer. Sir George Airy had no faith in them, and he
unaccountably received no reply to a test-question addressed to their
author. In the following June, however, he was roused by the
intelligence of Leverrier’s advance towards the goal already attained by
Adams, to arrange an exploratory campaign with the Cambridge
“Northumberland equatorial.” But here again, disbelief—reinforced by the
absence of a detailed star-map—stepped in to retard proceedings
conducted by Professor Challis in so leisurely a fashion that the object
“wanted” was found before he had sifted his observations, September 23,
1846, by Galle of Berlin, acting under Leverrier’s precise directions.
It proved on inquiry to have been twice observed at Cambridge during the
previous couple of months.
Gravitational astronomy won its crowning distinction by the discovery of
Neptune. It afforded the first instance of a body made known as an
unseen power previously to being visually detected. Many stellar
systems, however, have since then been ascertained to include members
which can only be _felt_, owing to their partial, if not total
obscurity. Again, the spectroscope tells of the existence of others
entirely beyond the range of direct vision with the most powerful
optical appliances; not because they do not shine (although this is
sometimes also the case), but because they revolve so close to their
primaries as to form with them single and indissoluble telescopic
objects.
The spectroscope and the photographic camera have been mentioned as aids
to astronomy. Their adoption has profoundly modified the science,
widening its borders, inviting it to undertake novel tasks, endowing it
with previously undreamt-of powers. Realms of knowledge deemed
inaccessible to human faculties have, as if at the touch of a magician’s
wand, been thrown open; and of the many paths leading into the interior,
only a few have yet been pursued, and that for a short distance. The
prospects of exploration are hence unlimited, and of bewildering
variety.
Spectrum analysis is essentially a chemical method. It depends upon the
principle firmly established in 1859 by Kirchhoff and Bunsen, two
professors at the university of Heidelberg, that different kinds of
glowing vapour give out distinctive rays of variously coloured light,
commonly called “lines,” simply because, for the purpose of getting rid
of overlapping images, and for convenience of measurement, they are
transmitted through a narrow slit. Thus, the presence of a familiar, and
almost ubiquitous deep-yellow line, named by Fraunhofer “D,” and shown
by a moderately powerful apparatus to be double, _infallibly_ testifies
to the presence of sodium; iron, rendered gaseous by heat, gives out
several thousand lines ranging from end to end of the spectrum, not one
of which is common to any other substance; hydrogen shows a radiant
sequence exclusively its own; and so of all the remaining elements. To
apply this mode of detection, the light from the source to be studied
must be analysed, or dispersed into its various component colours
through the unequal action upon them of a prism, or train of prisms.
Dispersion can also be effected by “diffraction”; and since the spectrum
thus produced is “normal,” or dependent wholly upon wave-length, it is
always employed where a high degree of exactitude is aimed at. The
coloured fringes of shadows originate in this way, through the
interference of ethereal undulations; while the rainbow is a prismatic
phenomenon, drops of water performing the refractive office of actual
prisms.
The rainbow exemplifies too—although less perfectly than the electric
light—what is called a “continuous spectrum.” Its tints merge one into
the other insensibly, without any sensible dark interruption. Now,
incandescent liquids and solids of every kind and quality give
rainbow-like spectra; they emit light which _rolls out_ into an unbroken
band of colour. Hence there is nothing characteristic about them. They
are to the chemical enquirer absolutely uncommunicative. Vapours and
gases alone can be induced to show the _badge_ of their particular
nature.
Celestial spectrum analysis began with the sun. The solar spectrum is
furrowed transversely by a multitude of fine dark lines, known as
“Fraunhofer lines,” because Fraunhofer brought them within scientific
cognisance by carefully mapping and measuring them. Their significance
remained a standing puzzle until Kirchhoff, in 1859, furnished the key
to it, by demonstrating the correlation of radiation and absorption. In
other words, vapours and gases have the faculty of arresting those
precise rays of light which they are in a condition to emit. Hence, the
ignited, although relatively cool vaporous envelope of a white-hot body
like the sun, or the carbons of the electric arc, acts predominantly as
an intercepting medium, stopping more than it sends out of its peculiar
rays. There results a continuous spectrum crossed by dark lines of the
same chemical significance as if they were bright. They would, in fact,
show as bright if the brilliant background, upon which they are seen
projected, could be withdrawn. The interpretation, upon this principle,
of the Fraunhofer lines, proved the sun to be surrounded by hydrogen in
vast quantities, by incandescent sodium, magnesium, iron, calcium, and a
number of other metals. Spectrum analysis in this way assumed a double
aspect. The hieroglyphics of coloured light were rendered legible,
whether positively or negatively written. And the spectra of the
heavenly bodies are actually found to be inscribed, some in one way,
some in the other; not unfrequently, in both combined.
The new and marvellous power of investigation thus acquired was in 1864
applied to the stars by Dr. Huggins and his coadjutor, Professor W. A.
Miller. They ascertained the presence in the atmospheres of Aldebaran
and Betelgeuse, of nine or ten terrestrial elements, thereby setting on
foot the science of stellar chemistry. Moreover, on August 29, in the
same year, Dr. Huggins made the signal discovery of gaseous nebulæ.
Admitting the dim rays of a “planetary” in Draco through the slit of his
spectroscope, he perceived it to be composed of three bright green
lines, one of them Fraunhofer’s “F”—an emanation of hydrogen. This one
observation verified after seventy-three years Herschel’s inference of
the existence in the heavens of a “fiery haze,” destined, according to
his long forecast of creative processes, eventually to “subside into
stars.”
By the discovery of celestial spectrum analysis, a third stadium of
progress towards the unification of the sciences was reached. The first
step was taken with the demonstration that the force retaining the
planets in their orbits is no other than that which causes rivers to
flow, and apples to fall upon the earth. The extension of the same law
to the stellar universe through the discovery of binary stars, showing
that matter, wherever existing, possesses at least one unchanging
quality, constituted the second. It was now learned that the sun and
stars were composed of the identical _species_ of matter scattered in
the dust of the earth, dug up from its bowels, condensed to make its
oceans, entering into the very framework of our own bodies. An universal
chemistry was established, based upon the relations of light to material
molecules, and of material molecules to the ether filling space; and, as
an inevitable consequence, the new branch of knowledge, termed
“astrophysics,” made its ardently welcomed advent. By it astronomy has
entered into close alliance with the rest of the sciences. No laboratory
experiment is any longer indifferent to her; and laboratory experiments,
on the other hand, derive from the connexion vastly augmented
importance. The youth of learning seems renewed. Secrets of nature,
formerly believed to lie beyond the scope of investigation, have been
penetrated; _nil desperandum_ is the motto which astro-physicists have
earned the title to adopt as their own.
The old art of direct observation has, during the latter half of the
present century, developed in sundry novel directions. By the use of
auxiliary appliances, the telescope has gained a wonderful increase of
subtlety and power. Modern astronomical work may be divided into four
classes:—telescopic, spectroscopic, photographic, and spectrographic or
spectrophotographic. Daguerre’s invention was almost immediately tried
with the sun and moon; J. W. Draper and the two Bonds in America,
Foucault and Fizeau in France, and Warren de la Rue in this country,
being among the pioneers of celestial photography. But it was not until
after the introduction of the collodion process that really useful
results were obtained. With the regular employment at Kew, from 1858
onwards, of De la Rue’s “photoheliograph,” began the daily
selfregistration of sun-spots, suggested by Sir John Herschel in 1847;
and pictures of the eclipsed sun, obtained with the same instrument at
Rivabellosa in Spain, July 18, 1860, terminated a prolonged dispute as
to the nature of the red prominences by exhibiting them as undeniably
solar appendages. Lunar photography was meanwhile successfully
prosecuted, and Henry Draper’s picture, of September 3, 1863, remained
unsurpassed for a quarter of a century. Star-prints were first secured
at Harvard College, under the direction of W. C. Bond in 1850; and his
son, G. P. Bond, made, in 1857, a most promising start with double-star
measurements on sensitive plates, his subject being the well-known pair
in the Tail of the Great Bear. The competence of the new method to meet
the stringent requirements of exact astronomy was still more decisively
shown in 1866 by Dr. Gould’s determination from his plates of nearly
fifty stars in the Pleiades. Their comparison with Bessel’s places for
the same objects proved that the lapse of a score of years had made no
sensible difference in the configuration of that immemorial cluster; and
Professor Jacoby’s recent measures of Rutherfurd’s photographs, taken in
1872 and 1874, enforced the same conclusion. To the “collodion period”
also belongs the earliest spectrograph, taken by Dr. Huggins in 1863;
but the analysed light of Sirius left an uncharacteristic, although a
strong impression. No lines were visible in it; a “virgin page” was
presented. Before prosecuting the subject, fresh developments had to be
awaited.
The invention of gelatine dry plates was the decisive event in the
history of celestial photography. Dr. Huggins turned it to account with
marked success for depicting the spectrum of Vega, December 21, 1876,
and was able, three years later, to exhibit to the Royal Society
photographs of the spectra of six white, or Sirian stars, stamped with
the ultra-violet series of hydrogen lines, then for the first time
recognised, whether on the earth, or in the sky. The uses of the camera
have since then multiplied at a prodigious rate. Its versatility appears
unbounded. There are very few departments of astronomy left in which the
eye has the advantage over it. A volume might be written on its
successes; its comparative failures would scarcely fill a page. Its
extraordinary power of penetrating space would have amazed and delighted
William Herschel. This is due to the indefinitely prolonged exposures
rendered practicable by the employment of dry plates; and these
exposures can be interrupted and resumed at pleasure. Three-night
photographs are now quite commonly taken, following the example given by
Dr. Roberts in 1889. Now every additional minute of exposure brings
intelligence from further and further sky-depths, owing to the happy
faculty of sensitive plates for accumulating impressions. The eye sees
at once, or not at all; the chemical retina sees by degrees, storing up
insensible effects until they become sensible, and this without
definable limit. This is its most essential prerogative. For the
portrayal of nebulæ and comets, it is inestimable; and by its means the
boundaries of the sidereal system may be laid down before the twentieth
century is far on its way. A picture of the great comet of 1882,
standing out from a richly spangled background, taken at the Cape
Observatory under Dr. Gill’s direction, was the object-lesson by which
the advantages of photographic star-charting were effectually learnt.
They have been practically illustrated in the _Cape Durchmusterung_, a
southern continuation, by photographic means, of Argelander’s
corresponding telescopic work at Bonn; and are being turned to account
on a magnified scale, in the International Survey of the heavens, now in
progress at seventeen observatories scattered over the face of the
globe. Special problems have, meanwhile, been investigated with striking
success, by the chemical method, and its fresh applications are
innumerable. Hitherto, performance has usually outrun promise; but
promise has now so quickened its pace as to make the issue of the race
dubious. We can only be sure that the future will be full of surprises.
ASTRONOMY
[Illustration:
THE LICK REFRACTOR OF THIRTY-SIX INCHES APERTURE.
]
SECTION II.—GEOMETRICAL ASTRONOMY AND ASTRONOMICAL INSTRUMENTS.
BY A. FOWLER, A.R.C.S., F.R.A.S.
CHAPTER I.
THE EARTH AND ITS ROTATION.
It is a common remark that we are creatures of circumstances, and in no
sense is this truer than in its application to the conditions under
which we view the heavenly bodies. At the commencement of a study of
astronomy it is accordingly important to first ascertain as far as
possible the nature of the earth on which we are situated, and to
determine in what way our observations are affected by our local
conditions.
THE HORIZON.—When we look at the sky we see a vast hemispherical vault
of which we seem to occupy the centre. If we are at sea, the water and
sky appear to meet at a certain distance, in whatever direction we look.
Where these meet we have what is called the visible horizon. On land,
the horizon is usually broken up by terrestrial objects, such as hills,
buildings, or vegetation, but otherwise the appearances are the same as
at sea.
SHAPE OF THE EARTH.—When we observe the horizon, whether from land or
sea, our eyes are at a certain elevation above the level of the ground
or water, as the case may be, and the higher we are situated, the
greater is the distance of the visible horizon, although the circular
outline is retained. No matter where we may be, the same appearances are
noted, and we are thus led to infer that the earth is a globe, as no
other shape could appear circular from all points of view.
There are other considerations which lead to the same conclusion with
regard to the shape of the earth. One of the most familiar proofs that
the earth cannot be flat is found in the aspects of a ship putting out
to sea or coming into port, when observed from a somewhat elevated
position on shore. A ship does not become visible in its entirety, as it
would if diminishing distance were the only cause affecting its
visibility; the masts are seen first, and then the lower parts of the
vessel gradually make their appearance. This finds a simple explanation
in the curvature of the surface of the sea, and as similar appearances
can be seen in all parts of the world, a globular form is indicated.
The fact that one may continue to travel westward and yet return to the
point of starting, is quite in harmony with the supposition that the
earth is globular, but it does not furnish a proof. This facility would
evidently be equally afforded by a cylindrical earth, or even by a flat
earth of which the Pole occupied the centre.
Still another indication of the rotundity of the earth is given by the
phenomena of an eclipse of the moon. On these occasions, as will appear
later, the moon passes through the shadow of the earth, and as this
shadow is always circular, nothing but a spherical, or nearly spherical,
body can be in question.
SIZE OF THE EARTH ROUGHLY MEASURED.—Granting then that the earth is
spherical, a measurement of its curvature will enable us to determine
its size. To do this it is necessary to measure the distance of the
visible horizon from the eye at a known elevation. Then it can be shown
that if the height of the eye is only a small fraction of the diameter
of the earth, the diameter is as many times larger than the distance of
the horizon as that distance is greater than the height of the eye.
Thus, to an observer whose eye is 5 feet above sea level, the horizon is
2¾ miles distant, while from the top of a lighthouse 66 feet high the
sky would appear to meet the sea at a distance of 10 miles. One way in
which an approximate measurement may be made is illustrated in Fig. 1.
Three posts are placed in line, with their tops at the same height above
the surface of some calm stretch of water such as is afforded by a
canal. A telescope fixed to the first post, so that its centre is at the
top, is directed to the upper end of the third post, and it is seen to
sight the middle one at some distance from the top. When the posts are a
mile apart, the line joining the two extremes turns out to be 8 inches
below the top of the middle one.
[Illustration:
FIG. 1.—_Rough Measurement of Earth’s Diameter._
]
In our diagram this 8 inches is represented by the distance _b d_, and
if we imagine an arc of a circle _d e_ concentric with the surface of
the water, the part which it intersects on the end post, namely _a e_,
will also be 8 inches. This means that to an eye at _a_, 8 inches above
the surface represented by _d e_, the visible horizon at _d_ would be a
mile distant. Applying the proportion named above, it results that the
earth is 7,920 miles in diameter.
Owing to various causes, this method only furnishes a rough indication
of the dimensions of our globe; but, if we had no other evidence, the
result would suffice to explain that the irregularities of the earth’s
surface, though seeming so large to us who dwell upon it, are not
inconsistent with the idea that the surface forms part of a sphere. The
highest mountains with which we are acquainted do not exceed 5½ miles in
height, and this is only ¹⁄₁₄₀₀th part of the earth’s diameter. On a
globe 14 inches in diameter, representing the earth, the highest
mountains would be less than a hundredth of an inch on the same scale;
so that, taking the earth generally, it is practically a smooth globe.
DIFFERENT HORIZONS AT DIFFERENT PLACES.—So far then we have learned that
the earth is a globe about 8,000 miles in diameter. This enables us to
understand that persons in different parts of the earth will see the sky
in different ways. At any given place we can see only what is above our
horizon, and it results from the spherical form of the earth that no two
observers have precisely the same horizon. If we consider a section of
the earth, such as is shown in Fig. 2, an observer at the point _a_ will
have a horizon represented in section by the line _b c_, while the
horizon of an observer at _d_ will be represented be _e f_. It is clear
then that an external distant object, such as the sun or a star, which
may appear on the horizon in the direction _a b_, as seen from the point
_a_, will be at a considerable angle above the horizon when seen from
the point _d_.
[Illustration:
FIG. 2.—_Horizons at Two Places on the Earth._
]
SENSIBLE AND RATIONAL HORIZON.—Having this conception of the horizon as
a thing terrestrial, we may consider its astronomical relationships a
little further. If we imagine the plane of the horizon prolonged until
it cuts the distant sphere on which the stars and other celestial bodies
seem to lie, it will meet that sphere in what is called the _sensible
horizon_. A parallel plane passing through the centre of the earth is
called the _rational horizon_, but as the starry sphere is at an almost
infinite distance, the rational and sensible horizons coalesce into one
celestial horizon.
Closely associated with the horizon is the point vertically overhead
which is called the _zenith_, and the point vertically below which is
called the _nadir_. As the plane of the horizon is tangential to the
earth’s surface at the point of observation, the zenith is simply the
prolongation into space of the line joining the centre of the earth with
the place of observation; at the point _a_ in Fig. 2, for example, the
zenith is in the direction _o a z_.
The zenith as thus defined, however, is not the astronomical zenith, but
what is called the geocentric zenith. As will appear later, the earth is
not truly spherical, so that the direction of gravity does not pass
exactly through the earth’s centre, and the astronomical zenith is
overhead in the direction of gravity.
DIURNAL MOTION OF THE HEAVENS.—In the day-time, when the sky is clear,
we see the sun; at night, we sometimes see the moon, always some stars,
and occasionally a comet. If we continue our observations, even for a
few hours, we begin to recognise that the heavenly bodies have an
apparent movement towards the west, very similar to the daily motion of
the sun, with which everyone must have been familiar from childhood.
Continuing such observations, it is found that the great majority of the
stars do not appear to change their positions relatively to each other,
although their apparent places in the sky are different at different
times. These have consequently been called the “fixed stars,” but in the
light of our present knowledge, the name is not to be taken too
strictly. On account of this seeming fixity, the stars have been divided
from very remote times into _constellations_, or groups, which enable us
to name and identify individual members of the starry host. Other bright
objects having the appearance of stars, when they are viewed merely by
the naked eye, may be seen to change their positions with regard to the
stars in that part of the sky in which they appear. These are the
_planets_—the “wandering stars” of the ancients, to whom five were
known, namely, Mercury, Venus, Mars, Jupiter, and Saturn.
Comets also are seen to share in the general westward movement of the
heavenly bodies, but, in addition, they have another movement relative
to the stars situated in the same part of the sky.
If we closely observe the stars in Europe, we shall find some of them
rising due east, and setting due west; others, again, will be found to
rise in the north-east, and to travel nearly overhead; still others will
be seen to rise south of east, attain only a small elevation above the
horizon, and pass from our view as far south of west as they rise south
of east. One point in the heavens appears stationary, and all the stars
seem to traverse their daily courses round this as a centre. This
stationary point is the north _celestial pole_. It is marked by no star,
but a fairly conspicuous star is at present only about a degree and a
half removed from it The name given to this star is the Pole Star, or
Polaris. As seen from London, stars within 51½° from the celestial pole
never set, and such stars are said to be _circumpolar_.
When our place of observation is changed from one of middle latitude to
one very near the Equator, these appearances are modified. We still see
the stars rising and setting daily, but there will be _two_ points which
do not seem to move, one on the northern and the other on the southern
horizon. One of these stationary points is identical with that seen from
higher latitudes, and the other, which is called the _south celestial
pole_, is diametrically opposite to it What is more, stars which were
not visible at all at our first place of observation will be seen in the
south. All the stars will rise and set, and will alike be above the
horizon for twelve hours.
If we could see the stars from the North Pole, the Pole Star, which is
on the horizon of places at the Equator, would be found overhead, and
all the stars visible to us would be ever above the horizon. Not only
this, not one of the glittering stars which adorns the southern heavens
would ever be seen at all.
In place of the rising and setting of stars, which lends such a great
interest to their observation in other parts of the world, as seen from
the poles the stars will simply travel round and round in circles
parallel to the horizon.
To produce the apparent daily revolution of the heavens, and the changes
in the appearances observed at different places, one of two causes must
be at work; either the celestial bodies themselves must be performing a
daily majestic movement from east to west round a motionless earth, or
the earth itself must be whirling round from west to east, and so
changing the situation of the observer’s horizon with regard to external
bodies. In the early days of astronomical observations this observed
revolution of the heavens was thought to be real, but, with our present
knowledge, we are no longer justified in regarding the earth as
occupying a place of any such importance as that of the centre of the
universe. By the earth’s rotation, an observer, unless situated exactly
at the North or South Pole, is carried round in a circle, and his
horizon is gradually swept round so that on one side stars are setting
and on the other side rising. The appearances at different places find a
simple and sufficient explanation in the varying inclination of the
observer’s horizon to the earth’s axis of rotation as the place of
observation is changed.
A very simple experiment will assist one to comprehend the varying
position of the horizon in different latitudes, and its effect upon the
apparent diurnal movement of the heavens. Through the middle of an
orange pass a knitting-needle, so that the two together may be taken to
represent the earth and its axis. A circular piece of thin card pushed
on to the needle at one end will represent the polar horizon, and, if
the orange be rotated, it will be at once realised that such movement
produces no change in the plane of this horizon, although different
points on the visible horizon will be successively brought in line with
different groups of stars or other external bodies.
Another piece of card should next be fixed on the orange by means of a
pin at a point corresponding to the Equator. Again spinning the model
earth on its axis, this horizon will be seen to constantly change its
plane with regard to outside objects, and in a manner which perfectly
accounts for the apparent movement of the heavens as observed from a
point on the Equator.
A third piece of card touching the surface of the orange at an
intermediate place will have an oblique movement, and as referred to
this plane, the stars appear to traverse their daily rounds in oblique
circles.
EXPERIMENTAL PROOFS OF ROTATION.—Not only does a supposed rotation of
the earth accord perfectly with all that we can glean from observations
of the heavens, but actual demonstrations of the reality of this
movement are forthcoming. Sir Isaac Newton suggested one experimental
method of setting the matter at rest. The further a thing is removed
from the centre of the earth, the greater is the circle which it
describes in a day, and the greater, consequently, the speed with which
it must travel. Thus the top of a high tower moves more quickly than its
base, and the surface of a mine than the bottom of the shaft. A stone
let fall from the top of a tower thus starts with a greater forward
velocity than that of objects at the base, and when it reaches the
earth’s surface, it will be a little east of the point where a
plumb-line let down from its starting-point reaches the surface. This
experiment has been tried, but there are so many disturbing causes
affecting the movement of the falling stone that the results are not
very satisfactory, although generally confirming the earths rotation
from west to east. Evidently this method would fail at the Pole, and
would be most effective at the Equator.
A much more beautiful and perfect proof is furnished by the celebrated
Foucault’s pendulum experiment. Again fancying ourselves at the North
Pole, let us imagine a long and heavy pendulum, suspended in such a
manner that the plane in which it swings is not affected by the earth’s
rotation. The trace of such a pendulum on a bed of sand placed beneath
it would remain in a constant position if the earth were at rest. As the
earth rotates, the bed of sand is twisted round, and the path of the
pendulum apparently changes. The experiment was first actually carried
out by Foucault in 1851, at the Pantheon in Paris, and it created a
widespread interest. Since then, pendulums have been erected in various
parts of the world, and all agree in essential results. The experiment
can be seen in actual operation in the science section of the South
Kensington Museum. The pendulum bob is a very heavy one, and before
commencing the experiment, it is held out of the vertical by a loose
band, which is fixed to the wall by a piece of string. On burning the
string, the band falls off, and the pendulum starts its swing with
little or no movement out of a plane. The pendulum bob is suspended by a
long piano wire which is attached to a bracket carrying a conical pivot.
The pivot rests on an agate plate at the end of a beam, and the weight
of the bracket is compensated by an adjustable weight (Fig. 3). When
swinging, the pendulum has a constant tendency to remain in one plane,
and the turning of the beam beneath the pivot has no effect on the
absolute direction of the plane of swing. Beneath the pendulum is a
table divided into degrees, and the hourly apparent movement of the
plane of swing at Kensington is observed to be nearly 12°.
[Illustration:
FIG. 3.—_Foucault’s Pendulum Experiment._
]
If the experiment could be performed at the North Pole, the pendulum
plane would apparently rotate from east to west, making a complete
rotation once a day. At the South Pole the direction of movement would
be reversed, but the rate would be the same as at the North Pole. The
experiment, however, fails altogether at the Equator, while at places
between the Poles and Equator the rate of movement varies with the
latitude.
A more compact piece of apparatus for demonstrating the earth’s rotation
is the gyroscope, which we also owe to Foucault’s ingenuity. The
principle is exactly the same as in the case of the pendulum. A heavy
disc is set in very rapid rotation, and is suspended in such a way that
its points of support may be turned round without disturbing its plane
of rotation. The results obtained with this instrument substantiate
those derived from pendulums.
These experimental proofs of the rotation of the earth further teach us
the same fact that we learn from observations of the stars, namely, that
the earth makes a complete turn on its axis once a day.
LATITUDE AND LONGITUDE.—Having thus arrived at the conclusion that the
earth is a globe turning on an axis once in twenty-four hours, the
_North and South Poles_ may be defined as the points where the axis of
rotation meets the surface, while the _Equator_ is the circle passing
through places midway between the Poles. Imaginary circles passing round
the earth through the Poles are called _meridians_, while circles
parallel to the Equator are called _parallels_. These conceptions enable
us to define very precisely the situation of any particular place upon
the terrestrial sphere. We measure its angular distance from the
Equator, as seen from the centre of the earth, and call this its
_latitude_; London, for instance, is 51½° north of the Equator, and this
is abbreviated to lat. 51½°N. All places on the same parallel have the
same latitude, so that another measurement is required to designate the
exact location of any one place. For this purpose the meridian passing
through some place is agreed upon as a start-point, and we can then say
that the place in question is so many degrees east or west; such a
measurement represents the _longitude_ of the place. At present there is
no universal agreement as to the initial meridian, but in all British
maps the meridian passing through the centre of the transit instrument
at the Royal Observatory, Greenwich, is taken as the start-point.
Longitudes are reckoned up to 180° E. and 180° W. New York, for example,
is in long. 73° 58′ W., and Berlin in long. 13° 24′ E.
THE CARDINAL POINTS.—For general convenience in expressing the situation
of an object, it is usual to say that it is towards the north, south,
south-west, etc., as the case may be. A north or south line at any
place, or a _meridian line_, as it is called, is in the direction of the
terrestrial meridian passing through the place. The north point of the
horizon is thus the point in which the meridian line meets the horizon
towards the North Pole. The opposite point is south; while the east and
west points lie in the directions at right angles. There are various
ways in which a meridian line may be drawn. One of the simplest is to
erect a vertical rod and to observe when its shadow thrown by the sun is
shortest; at that moment the shadow marks the direction of north and
south. This method is not very exact, as it is so difficult to tell when
the shadow is shortest. A more accurate result may be obtained by
drawing a circle round the stick as centre, and noting the points on
this circle reached by the end of the shadow before and after noon; the
point midway between these, marks the position of the shadow when
shortest. By taking the average result of observations made with more
than one circle, a good approximation can be obtained.
For a somewhat rough determination of the direction of the cardinal
points, a watch showing the correct time may be utilised. Directing the
hour hand to the sun, the south point will lie midway between that and
XII. In the case of a watch having a dial marked up to XXIV., and
reading XII. at mid-day, the latter figure would always point to the
south when the hand indicating the hour was directed towards the sun.
This will be easily understood if it be remembered that the sun is in
the south at intervals of (approximately) twenty-four hours.
[Illustration:
FIG. 4.—_Day and Night._
]
DAY AND NIGHT.—The succession of days and nights by which our daily
arrangements are regulated is at once explained by the fact that the
earth is round, and turns on its axis once a day. At any particular
instant of time the sun can only shine on that half of the earth which
is turned towards it. At all places included in the illuminated part the
sun will be above the horizon, and it will be day. One half of the earth
will be turned away from the sun, and to all places in that part it will
be night. Under the conditions represented in Fig. 4, to a person
situated at the point P it will be midnight; he will, however, be
carried by the earth’s rotation along the circle P Q R; when he arrives
at a point on _a b_, the sun will be rising to him, and his day will
commence. On reaching the point R the sun will be on the spectator’s
meridian, and it will be noon. After another interval he will arrive at
the boundary of light and shade, and his night will commence.
ATMOSPHERIC REFRACTION.—In common with other substances through which
light can pass, the atmosphere by which the earth is surrounded has the
effect of bending rays of light out of their courses, and on account of
this we do not see the heavenly bodies in their true positions. If the
air were of uniform density the effect of this refraction would be as
illustrated to the left in Fig. 5. The light from a star S will reach
the observer at O after striking the atmospheric shell at _a_ and being
refracted along the line _a_ O; consequently the observer will see it in
the direction O S′, and not in the direction O S, which it would have if
the air were absent. As a matter of fact, the atmosphere becomes less
dense in passing upwards, so that the rays of light are subjected to a
succession of small deviations; two such refractions are illustrated at
the right of Fig. 5. When a star is overhead there is no refraction, and
the greatest displacements of a star’s positions are produced on the
horizon, where the light has to pass through a great thickness of
atmosphere.
Refraction always makes the heavenly bodies appear higher in the sky
than they otherwise would be, and some very curious effects can be
traced to it. Thus the sun becomes visible on account of refraction some
time before it has actually risen, and remains visible for a little
while after it has really descended below the horizon. The amount of
refraction varies with the temperature and pressure of the air, but the
average amounts for different elevations above the horizon are as
follows:
TABLE OF MEAN
REFRACTIONS.
┌───────────┬───────────┐
│ Altitude. │Refraction.│
├───────────┼───────────┤
│ 0°│ 34′ 54″│
│ 2°│ 18′ 9″│
│ 4°│ 11′ 39″│
│ 6°│ 8′ 23″│
│ 8°│ 6′ 29″│
│ 10°│ 5′ 15″│
│ 12°│ 4′ 23″│
│ 14°│ 3′ 45″│
│ 16°│ 3′ 17″│
│ 18°│ 2′ 54″│
│ 20°│ 2′ 35″│
│ 25°│ 2′ 2″│
│ 30°│ 1′ 38″│
│ 40°│ 1′ 8″│
│ 50°│ 0′ 48″│
│ 60°│ 0′ 33″│
│ 70°│ 0′ 21″│
│ 90°│ 0′ 0″│
└───────────┴───────────┘
Refraction is responsible, among other things, for the curiously
distorted appearances of the sun and moon, when they are very near the
horizon.
TWILIGHT.—The atmosphere, or rather the solid and liquid particles which
it always contains, has the property of reflecting light, and hence it
does not suddenly become dark when the sun has set. Even until the sun
has descended 18° below the horizon, the upper parts of the air continue
to reflect his beams, and this is the origin of _twilight_. In the
tropics the sun sets almost vertically, so that it gets below the
twilight limit comparatively quickly, and this explains the short
twilight which is remarked by all who have visited a tropical country.
In our own country the sun has an apparent oblique motion, and a
relatively long period elapses before twilight ends. The increase in the
duration of twilight is, indeed, very noticeable in merely travelling
from London to the north of Scotland in summer-time.
[Illustration:
FIG. 5.—_Atmospheric Refraction._
]
Within the Arctic Circle, at places where the sun itself is never
visible for months together, its reflected beams in the form of twilight
may be seen for months.