Astronomy

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

THE OBSERVATORY AT NICE.

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.

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