Chapter XXXVI: Appendix: Astronomical Quantities

THE SUN.

The sun's mean distance from the earth is 92,900,000 miles; his diameter is 866,000 miles; his mean density, as compared with water, is 1.4; his ellipticity is insensible; he rotates on his axis in a period between 25 and 26 days.

THE MOON.

The moon's mean distance from the earth is 239,000 miles. The diameter of the moon is 2,160 miles; and her mean density, as compared with water, is 3.5. The time of a revolution around the earth is 27.322 days.

THE PLANETS.

___________________________________________________________________________ | |Distance from the Sun in | | Mean | |Density | | | Millions of Miles. | Periodic |Diameter| Axial |compared| | |-------------------------| Time | in | Rotation.| with | | | Mean. | Least.|Greatest.| in Days. | Miles. | | Water. | |-------+-------+-------+---------+----------+--------+----------+--------| |Mercury| 36.0| 28.6| 43.3 | 87.969| 3,030 | (?) | 6.85(?)| |Venus | 67.2| 66.6| 67.5 | 224.70 | 7,700 | (?) | 4.85 | |Earth | 92.9| 91.1| 94.6 | 365.26 | 7,918 |23 56 4.09| 5.58 | |Mars | 141 | 128 | 155 | 686.98 | 4,230 |24 37 22.7| 4.01 | |Jupiter| 483 | 459 | 505 | 4,332.6 | 86,500 | 9 55 -- | 1.38 | |Saturn | 886 | 834 | 936 |10,759 | 71,000 |10 14 -- | 0.72 | |Uranus |1,782 |1,700 | 1,860 |30,687 | 31,900 | Unknown | 1.22 | |Neptune|2,792 |2,760 | 2,810 |60,127 | 34,800 | Unknown | 1.11 | ---------------------------------------------------------------------------

THE SATELLITES OF MARS.

Mean Distance from Periodic Time.
Name. Centre of Mars. hrs. mins. secs.

Phobos 5,800 miles 7 39 14
Deimos 14,500 miles 30 17 54

THE SATELLITES OF JUPITER.

Mean Distance from Periodic Time.
Name. Centre of Jupiter. days. hrs. mins. secs.

New Inner
Satellite
Barnard 112,500 miles 0 11 57 22
I. 261,000 miles 1 18 27 34
II. 415,000 miles 3 13 13 42
III. 664,000 miles 7 3 42 33
IV. 1,167,000 miles 16 16 32 11

THE SATELLITES OF SATURN.

Mean Distance from Periodic Time.
Name. Centre of Saturn. days. hrs. mins. secs.

Mimas 115,000 miles 0 22 37 6
Enceladus 148,000 miles 1 8 53 7
Tethys 183,000 miles 1 21 18 26
Dione 235,000 miles 2 17 41 9
Rhea 329,000 miles 4 12 25 12
Titan 760,000 miles 15 22 41 27
Hyperion 921,000 miles 21 6 38 31
Iapetus 2,215,000 miles 79 7 56 40

THE SATELLITES OF URANUS.

Mean Distance from Periodic Time.
Name. Centre of Uranus. days. hrs. mins. secs.

Ariel 119,000 miles 2 12 29 21
Umbriel 166,000 miles 4 3 27 37
Titania 272,000 miles 8 16 56 30
Oberon 364,000 miles 13 11 7 6

THE SATELLITE OF NEPTUNE.

Mean Distance from Periodic Time.
Name. Centre of Neptune. days. hrs. mins. secs.

Satellite 220,000 miles 5 21 2 44

INDEX.

A

Aberration of light, 503-512;
and the apparent movements of stars, 504, 507;
Bradley's discoveries, 503;
causes, 507-511;
circles of stars, 505-507;
dependent upon the velocity of light, 511;
effect on Draco, 505;
telescopic investigation, 510

Achromatic combination of glasses, 11

Adams, Professor J.C., and the discovery of Neptune, 324-327, 330-332;
and the Ellipse of the Leonids, 386

Aerolite, the Chaco, 398;
the Orgueil, 399

Airy, Sir George, 325

Alban Mount Meteorites, the, 393

Alcor, 438

Aldebaran, 209, 418, 419;
spectrum of, 480;
value of velocity of, 484

Algol, 485, 487

Almagest, the, 7

Alphonsus, 92

Alps, the great valley of the (lunar), 88

Altair, 424

Aluminium in the Sun, 50

Ancients, astronomy of the, 2-7

Andrews, Professor, and basaltic formation at Giant's Causeway, 407

Andromeda, 414;
nebula in, 469, 489

Andromedes, The, shooting star shower, and Biela's comet, 390

Antares, 423

Apennines (lunar), 83

Aphelion, 163

Aquarius, 215, 413

Aquila, or the Eagle, 424

Arago, 326

Archimedes, 88

Arcturus, 358, 480;
value of velocity of, 484

Argelander's Catalogue of Stars, 431, 476

Argus, 481

Ariel, 309, 559

Aristarchus, 90

Aristillus, 88

Aristotle, lunar crater named after him, 88;
credulity respecting his writings, 267;
the Moon and the tides and, 535

Asteroids, 229-244

Astrea, 328

Astronomers of Nineveh, 156

Astronomical quantities, 558

Astronomy, ancient, 2-7;
Galileo's achievements in, 10;
the first phenomenon of, 2

_Athenaeum_, the, and Sir John Herschel's letter on Adams's share
in the discovery of Neptune, 330

Atmosphere, height of the Earth's, 100

Attraction, between the Moon and the Earth, 75;
between the planets, 148;
between the Sun and the planets, 144, 148;
of Jupiter, 248, 249;
producing precession, 498

Auriga, 414, 489

Aurora borealis, 42

Autolycus, 88

Auwers and star distances, 449;
and the irregularity in movement of Sirius, 427

Axis, Polar, 196, 497;
precession and nutation of the Earth's, 492-502

B

Backlund, and Encke's comet, 349, 351

Barnard, Professor E.E., and Saturn, 271, 278, 282;
and Titan, 294;
and the comet of 1892, 355;
and the Milky Way, 475

Beehive, the, 422

Belopolsky, M., and Binaries, 487, 488

Benares meteorite, the, 392

Bessel, and Bradley, 501;
and the distance of 61 Cygni, 446, 448, 449;
and the distances of stars, 442;
and the irregular movements of Sirius, 426;
receives gold medal of Royal Astronomical Society, 442

Betelgeuze, 209, 418, 419, 482;
value of velocity of, 484

Biela's comet, and Sir John
Herschel, 357;
and the Andromedes, 390

Binaries, spectroscopic, 487

Binocular glass, 27

Biot and the L'Aigle meteorites, 392

Bode's law, 230;
list of double stars, 435

Bond, Professor, and Saturn's satellites, 296;
and the nebula in Orion, 469;
and the third ring of Saturn, 280

Booetes, 422

Bradley, and nutation, 501;
and the aberration of light, 503;
his observations of Uranus, 312

Bredichin, Professor, and the tails of comets, 365, 366, 367

Breitenbach iron, the, 397

Bristol Channel, tides in the, 538

Bruennow, Dr., observations on the parallax of 61 Cygni, 449

_Burial of Sir John Moore_, 72

Burnham, Mr., and the orbit of Sirius, 427;
his additions to the known number of double stars, 439

Butler, Bishop, and probability, 460

Butsura meteorite, 397

C

Cadmium in the Sun, 50

Calais, tides at, 536

Calcium in the Sun, 50

Campbell, Mr., and Argus, 481;
and Mars, 223

Canals on Mars, 220

Cancri 20, 154

Cancri, z, 154

Cancri, th, 154

Canis major, 419

Canopus, 422

Cape Observatory, 27

Capella, 414, 480, 487

Carboniferous period, 518

Cardiff, tides at, 538

Cassini, J.D., and double stars, 434;
and Saturn's satellites, 294;
and the rings of Saturn, 278

Cassiopeia, 412

Castor, 420, 487;
a binary star, 437;
revolution of, 437

Catalogues of stars, 310, 311;
Messier's, 529

Catharina, 92

Centauri, a, 422;
Dr. Gill's observations of, 451;
Henderson's measurement of distance of, 442, 451

Ceres, 231, 232, 238;
and meteorites, 404, 405

Chaco meteorite, the, 398

Chacornac, and the lunar crater Schickard, 90

_Challenger_, the cruise of the, and magnetic particles in
the Atlantic, 408

Challis, Professor, 326;
his search for Neptune, 327, 328, 331, 332

Chandler, Mr., and Algol, 485

Charles's Wain, 28

Chepstow, tides at, 538

Cheseaux, discoverer of comet of 1744, 367

Chicago, telescope at Yerkes Observatory, 16

Chladni and the meteorite of Siberia, 392

Chromium in the Sun, 50

Chromosphere, the, 54

Chronometers tested by the Moon, 80

Clairaut and the attraction of planets on comets, 342, 343

Clavius, 91;
and Jupiter's satellites, 267

Clock, astronomical, 23

Clusters, star, 461-464

Cobalt in the Sun, 50

Coggia's comet, 1874, 337

Colour of light and indication of its source, 46

Colours, the seven primary, 45

Columbiad, the, 401

Columbus, 7

Comets, 112, 149, 250, 336;
and the spectroscope, 355;
attraction from planets, 342, 360;
Biela's, 357;
Biela's and the Andromedes, 390;
Clairaut's investigations, 342, 343;
Coggia's, 337;
Common's (1882), 354;
connection of, with shooting star showers, 388;
constitution of, 336;
containing sodium and iron, 356;
Donati's (1858), 353, 358, 366;
eccentricity of, 360;
Encke's, 344-352;
existence of carbon in, 356, 367;
gravitation and, 343, 348;
Halley's investigations about, 341-344;
head or nucleus of, 337;
Lexell's, 370;
mass of, 359;
movements of, 336;
Newton's explanations of, 338;
non-periodic, 353-356;
of 1531, 341;
of 1607, 341;
of 1681, 338, 339;
of 1682, 341;
of 1744 (Cheseaux's), 367;
of 1818, 345;
of 1843, 352;
of 1866, 388;
of 1874, 337;
of 1892, 355;
origin of, 369;
parabolic orbits of, 338-340, 360;
periodic return of, 338-341;
shape of, 336;
size of, 337;
tailless, 370;
tails of, 337, 361;
Bredichin's researches, 365;
Cheseaux's, 367;
composition of, 365, 369;
condensation of, 369;
electricity and, 368;
gradual growth of, 363;
law of direction of, 362;
repelled by the Sun, 364;
repulsive force of, 364, 368;
various types of, 365;
Tebbutt's (1881), 353;
tenuity of, 357

Common, Dr., constructor of reflectors, 21;
and the comet of 1882, 354;
and the nebula in Orion, 469

Cook, Captain, and the transit of Venus, 184

Copeland, Dr., and Schmidt's star, 489;
and the lunar crater, Tycho, 92;
and the spectra of nebula, 473;
and the transit of Venus, 189

Copernicus and Mercury, 156;
confirmation of his theory by the discovery of Jupiter's satellites, 267;
his theory of astronomy, 7;
lunar crater called after him, 89

Copper in the Sun, 50

Cor scorpionis, 423

Corona Borealis, 423, 488

Corona of Sun, during an eclipse, 62-64, 151

Coronium, 64

Cotopaxi and meteorites, 401

Crab, the, 422

Crabtree, and the transit of Venus, 180

Crape ring of Saturn, 281

Craters in the Moon, 83-85, 87-98

Critical velocity, 103, 104, 237

Crown, the, 423

Cryptograph of Huyghens, the, 277

Cygni, b, 439

Cygni 61, annual parallax of, 450;
Bessel's measurement of distance of, 442, 446, 447;
Bruennow's observations of, 449;
distance from the Sun of, 452;
disturbing influence of, 452;
double, 446;
Professor A. Hall's measurement of, 449;
Professor Pritchard's photographic researches concerning, 449;
proper motion of, 446;
Struve's observations of, 448, 449;
velocity of, 452

Cygnus, 424

Cyrillus, 92

Cysat, and the Belt of Orion, 467

D

D line in solar spectrum, 48

Darwin, Professor G.H., and tidal evolution, 531

Dawes, Professor, and Saturn's third ring, 281

Day, length of, and the Moon, 542;
and the tides, 541

Deimos, 226, 558

Denebola, 423

Diffraction, 56

Dione, 559

Dispersion of colours, 47

Distances, astronomical, 558, 559

Doerfel, and comets, 339

Dog star (_see_ Sirius)

Dog, the Little, 420

Donati's comet, 353, 358;
tails, 366

Double stars, 434-440

D Q, 236

Draco, nebula in, 470

Dragon, the, 415

Draper, Professor, and the nebula in Orion, 469

Dunsink Observatory, 12, 184, 447, 449

Dynamical stability, 547;
theory of Newton, 214

Dynamics and the Earth-Moon system, 546

Dynamics, Galileo the founder of, 10

E

Eagle, the, 424

Earth, The, ancient ideas respecting, 3;
annual movement of, and the apparent movement of the stars, 507, 512;
attraction of Jupiter, 319;
attraction of on Encke's comet, 350;
attraction of, on the Leonids, 386;
attraction of Saturn, 319;
attraction of the Moon, 75, 497;
attraction of the Sun, 496;
axial rotation of, 558;
carboniferous period on, 518;
change of climate on, 518;
composition of, 496;
contact of atmosphere of, with meteors, 377-379;
density of, 558;
diameter of, 558;
distance of, from Mars, 213;
distance of, from the Moon, 73, 558;
distance of, from the Sun, 31, 114, 184, 240, 265, 351, 512, 558;
energy from rotation of, 540;
formerly a molten globe, 200, 201;
geological records and, 517;
glacial period on, 518;
gravitation and, 204, 206, 207, 497;
heat in the interior of, 94, 197, 198, 251, 514;
how it is measured, 193-196;
its mass increasing owing to the fall of meteoric matter, 408;
its oceans once vapour, 251;
once in immediate proximity to the Moon, 542;
orbit of, 114;
orbit of, its elliptic form, 139;
path of deranged by Venus and Mars, 319;
periodic time of, 558;
plane of orbit of, 309;
polar axis of, 196, 492-502;
position of, relatively to the Sun and the Moon, 76, 77;
precession and nutation of axis of, 492-502;
radius of, 193, 512;
rotation of, 75, 196, 200, 494, 496;
shape of, 192, 195, 197, 201, 207;
size of, compared with Jupiter, 119,
and with other planets, 119;
size and weight of, compared with those of the Sun, 30,
and Moon, 74, 75;
velocity of, 115, 139, 146, 512,
and periodic time, 143;
volcanic outbreaks on, 197,
and the origin of meteorites, 405;
weight of, 202, 248,
as compared with Saturn, 271, 272

Earthquakes, astronomical instruments disturbed by, 24

Eccentricity of planetary ellipses, 136, 211

Eclipse of Jupiter's satellites, 261, 262, 265-267

Ellipse of the Moon, 77-80;
of the Sun, 53

Eclipses, ancient explanations of, 6;
calculations of the recurrence of, 79, 80

Ecliptic, the, 5, 233;
Pole of the, 493, 500, 505

Electric Light, the, 44

Ellipse, the, 136;
eccentricity of, 137;
focus of, 137;
Kepler's discoveries respecting, 136, 138, 142-144, 505;
the form which the orbit of a planet takes, 136;
the parallactic, 444;
variety of form of, 139

Enceladus, 559

Encke, and the distance of the Sun from the Earth, 147, 184;
his comet, 344-352

Encke's comet, 344-352;
approach to Jupiter of, 349;
and Mercury, 349;
and the Sun, 346;
diminution in periodic time of, 351;
distance from Mercury of, 347;
disturbed by the Earth, 350,
and by Mercury, 348;
irregularities of, 347, 351;
orbit of, 346;
periodical return of, 351;
Von Asten's calculations concerning, 349-350

Energy supplying the tides, 539

Ensisheim meteorite, the, 393

Equatorial diameter, 196, 497;
telescope, 14

Eratosthenes, 89

Eros, 236

Eruptions, 197

Evening star, 109, 169

Eye, structure of the, 10

F

Faculae of the Sun, 37

Fire ball of 1869, 375

Fire balls, 374

"Fixed" stars, 503

Flamsteed, first Astronomer-Royal, 311;
his _Historia Coelestis_, 311

Focus of planetary ellipse, 137-139

Fomalhaut, 413

Fraunhofer, 478

Fraunhofer lines, 48

Fundy, Bay of, tides in, 538

G

Galileo, achievements of, 10;
and Jupiter's satellites, 267;
and Saturn's rings, 273, 274;
and the Pleiades, 418

Galle, Dr., and Neptune, 328-330

Gassendi, and the transit of Mercury, 164;
and the transit of Venus, 178;
lunar crater named after him, 90

Gauss, and the minor planet Ceres, 232

Gemini, constellation of, 303, 420

Geminids, the, 400

Geologists and the lapse of time, 453

Geometers, Oriental, 5

Geometry, cultivation by the ancients of, 6

George III. and Sir W. Herschel, 299, 306

Giant's Causeway, 407

Gill, Dr. D., 27;
and Juno, 243;
and the minor planets, 242;
and the parallax of a Centauri, 451;
and the parallax of Mars, 214

Glacial period, 518

Gravitation, law of, 122-149;
and binary stars, 437;
and precession, 497;
and the Earth's axis, 495, 497, 499;
and the parabolic path of comets, 340;
and the periodical return of comets, 343;
and the weight of the Earth, 203, 204;
illustrated by experiments, 123, 124, 127, 129-132;
its discovery aided by lunar observations, 108, 125;
its influence on the satellites, 149;
its influence on stars, 149;
its influence on tides, 149;
Le Verrier's triumphant proof of, 330;
Newton's discoveries, 125, 126, 147;
on the Moon, 96;
universality amongst the heavenly bodies, 128, 373

Great Bear, 27, 28, 241;
configuration, 410;
double star in the, 438;
positions of, 409, 411

Green, Mr., and Mars, 220

Greenwich Observatory, 26, 311

Griffiths, Mr., and Jupiter, 252

Grimaldi, 90

Grubb, Sir Howard, 14

"Guards," the, 412

_Gulliver's Travels_ and the satellites of Mars, 228

H

Hadley's observations of Saturn, 282

Hall, Professor Asaph, and the satellites of Mars, 225

Halley, and the periodicity of comets, 341-343;
and the transit of Venus, 180

Heat, bearings on astronomy, 513;
in the interior of the Earth, 197-199, 514;
of the Sun, 515-526

Heliometer, the, 243

Helium, 55

Henderson, and the distance of a Centauri, 442, 451

Hercules, star cluster in, 269, 462

Herodotus (lunar crater), 90

Herschel, Caroline, 299, 465

Herschel, Sir John, address to British Association, 328;
address on the presentation of gold medal to Bessel, 443;
and Biela's comet, 357;
and nebulae, 464;
letter to _Athenoeum_ on Adams's share in the discovery of
Neptune, 330

Herschel, Sir W., and double stars, 435, 436;
and Saturn, 279;
and Saturn's satellites, 295;
and the Empress Catherine, 301;
and the movement of solar system towards Lyra, 457;
discovery of satellite of Uranus by, 308, 309;
discovery of Uranus by, 305, 308;
early life of, 299;
friendship with Sir W. Watson of 302;
he makes his own telescopes, 301;
"King's Astronomer," 307;
method of making his telescopes, 302;
musical talent of, 299;
organist of Octagon Chapel, Bath, 300;
pardon for desertion from George III., 299;
passion for astronomy of, 300, 301;
relinquishes musical profession, 307;
sidereal aggregation theory of, 529;
study of the nebulae by, 464-465, 529

Herschelian telescope, 19

_Historia Coelestis_, 311

Hoedi, the, 414

Holmes's, Mr., comet (1892), 355

Horrocks, and the transit of Venus, 179

Howard, Mr., and the Benares meteorite, 392

Huggins, Sir W., 479, 483;
and nebulae, 472

Huyghens, and Saturn's rings, 275-278;
discovers first satellite of Saturn, 293

Hyades, the, 419

Hydrogen in Sirius and Vega, 479;
in the Sun, 50

Hyginus, 93

Hyperion, 559

I

Iapetus, 559

Iberians, the, 3

Inquisition, the, and Galileo, 10

Iris, 242

Iron, dust in the Arctic regions, 408;
in the Sun, 50;
of meteorites, the, 396;
spectrum of, 50

J

Janssen, M., 34, 53;
and the transit of Venus, 177

Juno, 233, 238

Jupiter, ancient study of, 6;
and the Leonids, 386;
attraction of, 248;
axial rotation of, 558;
belts of, 252;
brilliancy of, 257;
composition of, 250;
covered with an atmosphere of clouds, 253, 254;
density of, 558;
diameter of, 247, 558;
distance from the Earth of, 110, 111;
distance from the Sun of, 246, 558;
habitability of, 257;
heat received from the Sun by, 256;
internal heat of, 252, 256, 515;
lack of permanent features of, 253;
lack of solidity of, 248, 253, 254;
moment of momentum of, 554, 555;
occultation of, 255;
orbit of, 114, 115, 246;
path of, perturbed by the attraction of Saturn, 316;
periodic time of, 558;
a planet, or "wanderer," 111;
red spot in 1878, 253;
revolution of, 246;
rotation of, 201, 202;
satellites of, 247, 249, 257-261, 265, 559;
satellites of, and gravitation, 266;
satellites of, and the Copernican theory, 267;
shadow from satellites of, 257;
shape of, 201, 202, 247, 252;
size of, compared with the Earth, 19, 246, 248,
and other planets, 114;
and the Sun, 114;
storms on, 256;
tides on, 555;
weight of, 248, 250,
and Encke's comet, 350

K

Keeler, Professor, and Saturn's ring, 288

Kempf, Dr., and the Sun's velocity, 484

Kepler, and comets, 360;
and laws of planetary motion, 10;
and meteors, 386;
and the orbit of Mars, 209;
explanation of his laws, 147, 148, 533;
his discovery of the shape of the planetary orbits, 136, 138;
his first planetary law, 138;
lunar crater called after him, 90;
prediction of the transit of Venus and Mercury, 163, 178;
second law, 141;
third law, 142

Kids, the, 414

Kirchhoff, and spectrum analysis, 478

Kirkwood, Professor, and the movements of Saturn's satellites, 296

Klinkerfues, Professor, 390

L

Lagrange, and the theory of planetary perturbation, 320-322;
his assumption of planetary rigidity, 531

L'Aigle meteorites, the, 392

Lalande, and Neptune, 332, 333

Landscapes, lunar, 98

Lane, Mr. J. Homer, 522

Laplace, and the nebular theory, 526;
and the satellites of Jupiter, 266;
and the theory of planetary perturbation, 320

Lassell, Mr., and Saturn's eighth satellite, 296;
discovers Neptune's satellite, 334

Law of gravitation (_see_ Gravitation)

Laws of Planetary Motion (_see_ Planetary Motion)

Lead in the Sun, 50

Ledger, Mr., and Mercury, 163

Leibnitz, lunar mountains named after him, 93

Lemonnier, and Uranus, 312

Leo, and shooting stars, 380, 420

Leonids, attractions of planets on, 386;
breadth of stream of, 387;
change of shape of, 383;
decrease of, 385;
enormous number of, 382;
historical records, 383;
length of stream of, 387;
Le Verrier, and the cause of their introduction into the solar
system, 388;
meteor shoal of, 382;
periodic return of, 382;
their connection with comets and Professor, Schiaparelli, 388

Leonis g, value of velocity of, 484

Leverage by equatorial protuberance, 498

Le Verrier, and Mars, 214;
and the discovery of Neptune, 324-332;
and the introduction of the Leonids into the solar system, 388;
and the weight of Mercury, 349

Lexell's comet, 370

Libration, 84

Lick Observatory, 16

Light, aberration of, 503-512;
velocity of, 261, 262, 265, 505, 512

Linne, 87, 94

Lion, the, 420, 421

Little Bear, the, 412

Little Dog, the, 420

Livy, and meteorites, 393

Lloyd, Provost, 407

Lockyer, Sir Norman, and Betelgeuze, 482;
and solar light, 52

London, tides at, 538

Louvain, F. Terby, and Titan, 295

Lowell, Mr., and Mercury, 165

Lunar tides, 548, 549

Lyra, motion of solar system towards, 459

Lyre, the, 424;
Nebula in, 469

Lyrids, the, 400

M

Maedler, and the lunar craters, 88, 90, 91

Magellanic clouds, 463

Magnesium, colour of flame from, 46;
in the Sun, 50

Magnetism, connection with Sun spots, 42

Manganese in the Sun, 50

Maraldi, and the rings of Saturn, 279

Mare crisium, 83;
foecunditatis, 83;
humorum, 83;
imbrium, 83, 98;
nectaris 83;
nubium, 83;
serenitatis, 83;
tranquillitatis, 83;
vaporum, 83

Mars, ancient study of, 6;
appearance of, through the telescope, 218;
atmosphere of, 222;
axial rotation of, 558;
canals on, 220;
density of, 558;
diameter of, 558;
distance, from the Earth of, 213;
distance from the Sun of, 213, 558;
gravitation on, 225;
Le Verrier's discovery of, 214;
life improbable on, 224;
marking on, 218;
movements of, 211-213;
opposition of, 209-211;
orbit of, 116, 209, 210, 213;
orbit of, and the laws of Kepler, 209;
parallax (1877), and Dr. D. Gill, 214;
periodic time of, 558;
a planet or "wanderer," 111;
"Polar Caps" on, 218, 219;
proximity to the Earth of, 110;
rising and setting of, 209;
rotation of, 218;
satellites of, 225-228, 558;
size of compared with other planets, 116, 216;
tides on, 551;
water and ice on, 219, 224

Maximilian, Emperor, 393

Mayer, Tobias, and Uranus, 312

Measurement of the Earth, 193-196

Mediterranean, tides in the, 537

Mercury, ancient study of, 6;
antiquity of its discovery, 155-157;
atmosphere of, 166;
attraction on comets of, 347;
climate of, 163;
comparative proximity to the Earth of, 111;
composition of, 160;
crescent-shaped, 160;
density of, 558;
diameter of, 558;
distance from the Sun of, 151, 558;
habitability of, 163;
movement of, 160, 161;
its elliptic form, 139, 161;
orbit of, 114;
period of revolution of, 161;
periodic appearances of, 158;
periodic time of, 558;
perturbations of, 350;
a planet or "wanderer," 111;
revolution of, 165;
rotation of, and Professor Schiaparelli, 165;
size of, compared with other planets, 116;
surface of, 162;
transit of, 152;
transit of, and Gassendi's observations, 164;
transit of, predicted by Kepler, 163;
velocity of, 162;
weight of, 166, 349

Meridian circle, 22, 24

Messier's Catalogue of Stars, 529

Meteors (_see_ Stars, shooting)

Meteorites, 391;
Alban Mount, 393;
ancient accounts, 392, 393;
Benares, 392;
Butsura, 397;
Chaco, 398;
characteristics of, 397;
Chladni's account of discovery in Siberia, 392;
composition of, 397-399;
Ensisheim (1492), 393;
Hindoo account of, 391;
L'Aigle, 392;
not connected with comets, 400;
not connected with star showers, 400;
Orgueil, 399;
origin, 400-408;
Ovifak, 407;
Rowton, 395-396;
Wold Cottage, 392

Micrometer, 86

Milky Way, 462-3, 474-6

Mimas, 559

Minor planets, 229-244

Mira Ceti, 430, 482

Mizar, 438, 486

Moment of momentum, the, 552-554

Month of one day, 547

Moon, The, absence of air on, 85, 99;
absence of heat on, 95;
agent in causing the tides, 70, 535-537;
ancient discoveries respecting, 5;
apparent size of, 73;
attraction to the Earth of, 75;
brightness of, as compared with that of the Sun, 71;
changes during the month of, 71, 74;
chart of surface of, 81;
craters on, 83, 84, 87-98, 514;
density of, 558;
diameter of, 558;
distance from the Earth of, 73, 75, 568;
eclipses of, 6, 77-80;
illustration of the law of gravitation, 96, 131, 133;
landscapes on, 98;
life impossible on, 99;
measuring heights of mountains, etc., of, 85, 86;
micrometer, 86; motion of, 75;
mountains on, 83, 85, 88, 89, 91, 93;
phases of, 71, 76;
plane of orbit of, 310, 500, 501;
poets and artists and, 72;
pole, 500;
possibility of ejecting meteorites, 402;
possibly fractured off from the earth, 543;
prehistoric tides on, 548, 549;
produces precession, 497-499;
proximity to the Earth of, 73, 75;
receding from the Earth, 545;
relative position of with regard to the Earth and the Sun, 76, 77;
revolution of, round the Earth, 75, 76, 558;
"seas" on, 82, 83;
shadows of, 85;
size of, compared with that of the Earth, 74;
test for chronometers, a, 80;
thraldom of terrestrial tides, 549;
waterless, 100;
weather not a affected by the phases of, 82;
weight of, 74

Motion, laws of planetary, 138, 141, 142, 147, 148

Mountains of the Moon, 83, 85, 93

N

Nasmyth, Mr., and the formation of lunar craters, 95

Natural History Museum, meteorites, 394

_Nautical Almanack_, 189

Neap Tides, 538

Nebula, in Andromeda, 469;
annular, in Lyra, 469;
in Orion, 269, 461, 466-469;
colour of, 468;
magnitude of, 468;
nature of, 467;
planetary, in Draco, 470;
simplest type of a, 528;
various grades of, 528

Nebulae, 464-472;
condensation, 528;
distances of, 464;
double, 470;
Herschel's labours respecting, 464-465, 528, 529;
number of, 466;
planetary, 470;
self-luminous, 464;
smallest greater than the Sun, 464;
spiral, 470

Nebular theory, the, 526

Neptune, 112;
Adams's researches, 324-326, 332;
Challis's observations of, 326-328;
density of, 558;
diameter of, 333, 558;
disc of, 332;
discovery (1846) of, 315;
distance from the Sun of, 334, 558;
Lalande's observations of, 332, 333;
Le Verrier's calculations, 324-332;
moment of momentum of, 554;
orbit of, 117;
periodic time of, 558;
revolution of, 334;
rotation of uncertain, 333;
satellite of, discovered by Mr. Lassell, 559;
size of, compared with other planets, 119;
vaporous atmosphere of, 333;
weight of, 333

Newall, Mr. H.F., and Capella, 487;
and the values of velocity of stars, 483

Newcomb, Professor, 9, 264, 267, 522

Newton, Professor, and meteoric showers, 377, 384

Newton, Sir Isaac, discovery of gravitation verified Kepler's laws, 144;
dynamical theory, 214;
illustrations of his teaching, 144-147;
law of gravitation and, 125, 126, 537;
parabolic path of comets and, 338-340;
reflecting telescope, 19;
weight of the Earth and, 203

Nickel in the Sun, 50

Nineveh, astronomers of, 156

Nordenskjoeld, and the Ovifak meteorite, 407

Nova Cygni, 431;
brilliancy of, 454;
decline of, 455;
distance of, 456;
parallax of, 455

November meteors, 376, 377, 379

Nutation, and Bradley, 501

O

Oberon, 309, 559

Object-glasses, 11, 12, 14, 16, 19

Observatories, 9-28

Observatory, Cape of Good Hope, 27;
Dunsink, 12, 184;
Greenwich, 26, 314;
Lick, 16;
Paris, 22;
Uraniborg, 10;
Vienna, 14;
Washington, 226;
Yerkes, 16

Occultation, 102, 215

Oceanus Procellarum, 83

Opera-glass, 27, 28

Opposition of Mars, 209

Orbital moment of momentum, 552

Orbits of planets, 114, 115, 117;
dimensions, 139-143;
elliptical form, 138-140;
minor planets, 232, 234, 239;
not exactly circles, 135;
of satellites of Uranus, 310;
Sun the common focus, 139

Orgueil meteorite, the, 399

Orion, 4, 418

Orion, belt of, 418, 467;
brilliancy of, 418;
nebula in, 269, 461, 466-469

Orionis, a, 418, 482

Orionis, th, a multiple star, 318, 467

Ovifak meteorite, the, 407

P

Palisa and the minor planets, 234

Pallas, 233, 238

Parabolic path of comets, 338-340

Parallactic ellipse, 444

Parallax, 181, 182, 214, 443;
of stars, 507

Paris telescope, 22, 23

Pegasus, great square of, 413, 414

Peg-top, the, and the rotation of the Earth, 494

Pendulum for determining the force of the Earth's attraction, 205

Penumbra of Sun-spot, 51

Perihelion, 163

Periodic times of planets, 139-143, 558

Periodicity of Sun-spots, 41

Perseids, 400

Perseus, 415, 416, 429;
sword-handle, 462

Perturbation, planetary, 317-324, 346

Perturbations, theory of, 296

Petavius, 93

Peters, Professor, and charts of minor planets, 234;
and the derangement of Sirius, 427

Phases of the Moon, 71, 76

Phobos, 226, 551, 558

Photography, and practical astronomy, 25;
and the distance of 61 Cygni, 449;
Dr. Roberts and the nebula in Andromeda, 469;
Mr. Common and the nebula in Orion, 469;
Sir W. Huggins and the spectra of nebulae, 473

Photosphere, the, 37, 54

Physical nature of the stars, 477

Piazzi, discoverer of the first known minor planet, 203

Pickering, Professor, 218, 220, 255, 265;
and Betelgeuze, 482;
and planetary nebulae, 474;
and Saturn's satellites, 296;
and spectroscopic binaries, 486, 487

Pico, 89

Planetary motion, Kepler's laws of, 138, 141, 142, 147, 148

Planetary nebulae, 470

Planetary perturbation, 317-324

Planets, ancient ideas respecting, 2, 6;
approximate number of, 112;
attract each other, 148, 317;
attracted by comets,360;
Bode's law, 230;
comparative sizes of, 118, 119;
distance of, from the Earth, 109-111;
distance of, from the Sun, 558;
how distinguished from stars, 111;
irregularity of motions of, 317-324;
Lagrange's theory of rigidity of, 531;
light of, derived from the Sun, 113;
minor, 229-244;
orbits of the four giant, 117;
orbits of the four interior, 114;
orbits have their focus in the centre of the Sun, 139;
orbits not exactly circles, 135;
orbits take the form of an ellipse, 136-138;
origin of, as suggested by the nebular theory, 526;
periodic times of, 139-143, 558;
relative distances of, 229;
uniformity of direction in their revolution, 120, 322;
velocity of, 139-142, 144, 146, 237

Plato (lunar crater), 89

Pleiades, 241, 416;
invisible in the summer, 416

Pliny, the tides and the Moon, 535

Plough, the, 28

Pogson, Mr., 390

Pointers in the Great Bear, 28, 411

Polar axis, 196

Polar caps on Mars, 218, 219

Pole, the, distance of from Pole Star lessening, 494;
elevation of, 195;
movement of, 492;
near a Draconis, 494;
near Vega or a Lyra, 494

Pole Star, 194;
belongs to the Little Bear, 412;
distance of, from the pole of the heavens, 412, 492, 494;
position of, 411;
slow motion of, 412

Pollux, 420, 480;
value of velocity of, 484

Pons, and the comet of 1818, 345

Posidonius, 87

Potassium in the Sun, 50

Praesepe, 422

Precession and nutation of the Earth's axis, 492-502

Proctor, and the stars in Argelander's atlas, 476

Prism, the, 45;
its analysing power, 46

Pritchard, Professor, stellar photographic researches of, 449

Procyon, 420;
value of velocity of, 484

Prominences on the Sun, 53-59

Ptolemy, his theory of astronomy, 6;
lunar crater named after him, 92

Q

Quarantids, the, 400

R

Radius of the Earth, 193, 512

Rainbow, the, 45

Ram, the, 420

Reflectors, 19, 21, 25

Refraction by the prism, 45

Refractors, 11, 14, 16

Regulus, 421, 479

Reservoir formed from tidal water, 538

Retina, the, and the telescope, 10, 11

Rhea, 559

Rigel, 418, 420, 480

Rigidity of the planets, 532, 533

Roberts, Dr. Isaac, and the nebula in Andromeda, 469;
and the nebula in Orion, 469

Roemer, and the velocity of light, 261

Romance, planet of, 151-154

Rosse telescope, the, 19, 20, 468, 470

Rotational moment of momentum, 553

Rowland, Professor, and spectral lines, 491

Rowton Siderite, 395

Royal Astronomical Society and Bessel, 442

S

Sappho, 242

Satellites of Jupiter, 249, 250, 257-261, 266, 559;
confirmation of the Copernican theory, 267

Satellites of Mars, 209, 225-228, 551, 558

Satellites of Neptune, 334, 559

Satellites of Saturn, 559;
Bond's discoveries, 296;
Cassini's discoveries, 294;
distances, 559;
Herschel's discoveries, 295;
Huyghens' discovery, 293;
Kirkwood's deduction, 296;
Lassell's deduction, 296;
movements, 296;
origin as suggested by the nebular theory, 526

Satellites of Uranus, 308, 309, 310, 559

Saturn, ancient study of, 6;
attraction on Uranus, 322;
axial rotation of, 558;
beauty of, 209;
comparative proximity to the Earth of, 110;
density of, 558;
diameter of, 271, 558;
distance of, from the Sun, 268, 271, 558;
elliptic path of, 271;
gravitation paramount, 283;
internal heat of, 272, 515;
Leonids and, 386;
low density of, 272;
moment of momentum of, 554;
motion of, 271;
orbit of, 117, 118;
path of, perturbed by the attraction of Jupiter, 316;
periodic time of, 558;
period of revolution of, 269;
picturesqueness of, 291;
position of, in the solar system, 269;
rings of, 269;
rings, Bonds discovery, 280;
rings, Cassini's discovery, 278;
rings, consistency, 286;
rings, Dawes's discovery, 281;
rings, Galileo's discovery, 273, 274;
rings, Hadley's observations, 282;
rings, Herschel's researches, 279;
rings, Huyghens' discovery, 275-278;
rings, Keeler's measurement of the rotation, 288;
rings, Maraldi's researches, 279;
rings, rotation of, 285, 288;
rings, spectrum of, 291;
rings, Trouvelot's drawing, 278;
satellites of, 293, 294, 295, 296, 559;
size of, compared with other planets, 119, 269, 272;
spectrum of, 291;
unequal in appearance to Mars and Venus, 269;
velocity of, 271;
weight of, compared with the Earth, 272

Savary and binary stars, 436

Schaeberle, Mr., and Mars, 224

Scheiner, and the values of velocity of stars, 483;
observations on Sun-spots, 36

Schiaparelli, Professor, and Mars, 220;
and the connection between shooting-star showers and comets, 388;
and the rotation of Mercury, 165

Schickard, 90

Schmidt, and Nova Cygni, 454, 489;
and the crater Linne, 87;
and the Leibnitz Mountains, 93

Schroeter, and the crater Posidonius, 87

Schwabe, and Sun-spots, 40

Seas in the Moon, 82

Secchi, and stellar spectra, 479

Shoal of shooting stars, 377;
dimensions, 377

Shooting stars (_see_ Stars, shooting)

Sickle, the, 421

Sidereal aggregation theory of Sir W. Herschel, 529

Siderite, Rowton, 395

Sinus Iridum, 83

Sirius, change in position of, 425;
companion of, 427, 428;
exceptional lustre of, 110;
irregularities of movement of, 426;
larger than the Sun, 110;
most brilliant star, 419;
periodical appearances of, 157;
proper motion of, 425;
spectrum of, 479;
velocity of, 426;
weight of, 427

Smyth, Professor C.P., 493

Sodium, colour of flame from, 49;
in the Sun, 50

Solar corona, prominences etc. (_see under_ Sun)

Solar system, 107-121;
Copernican exposition of the, 7;
influence of gravitation on, 149;
information respecting, obtained by observing the transit of Venus, 174;
island in the universe, 121;
minor planets, 229-244;
moment of momentum, 554;
movement of, towards Lyra, 457;
origin of, as suggested by the nebular theory, 526;
position of Saturn and Uranus in, 297, 305

South, Sir James, 12

Spectra of stars, 479

Spectro-heliograph, 58

Spectroscope, 43-56;
detection of iron in the Sun by the, 50

Spectroscopic binaries, 487

Spectrum analysis, 47;
dark lines, 49, 50;
gaseous nebulae, 474;
line D, 48, 49

Speculum, the Rosse, 20

Spica, 423, 487

Spider-threads for adjusting the micrometer, 86;
for sighting telescopes, 22

Spots on the Sun, 36-43;
connection with magnetism, 42;
cycles, 41;
duration, 41;
epochs of maximum, 42;
motion, 36;
period of revolution, 40;
Scheiner's observations, 36;
zones in which they occur, 39

Star clusters, 461-464;
in Hercules, 462;
in Perseus, 462

Stars, apparent movements due to precession, nutation, and aberration, 504;
approximate number of, 28;
attraction inappreciable, 316;
catalogues of, 310, 311, 409, 431;
charts of, 325, 328;
circular movement of, 505-507

Stars, distances of, 441;
Bessel's labours, 442-449;
Henderson's labours, 442;
method of measuring, 443-445;
Struve's work, 442, 448, 449;
parallactic ellipse, 444-449

Stars, double, 434;
Bode's list, 435;
Burnham's additions, 439;
Cassini, 434;
Herschel, 435, 436;
measurement, 435, 436;
revolution, 436;
Savary, 436;
shape of orbit, 436;
variation in colour, 438

Stars, elliptic movement of, 506;
gravitation and, 149;
how distinguished from planets, 111;
physical nature of, 477;
probability of their possessing a planetary system, 121;
real and apparent movements of, 504;
really suns, 32, 121

Stars, shooting, attractions of the planets, 386;
connection with comets, 388-390;
countless in number, 372;
dimensions of shoal, 377;
features of, 373;
length of orbit, 387;
orbit, 378;
orbit, gradual change, 386;
period of revolution, 384;
periodic return, 378, 379;
shower of November, 1866, 377, 379-380;
shower of November, 1866, and Professor Adams, 384, 386;
shower of November, 1866, radiation of tracks from Leo, 380;
shower of November, 1872, 389;
showers, 376;
showers and Professor Newton, 377;
track, 377;
transformed into vapour by friction with the Earth's
atmosphere, 374, 376;
velocity, 373, 386

Stars, spectra of, 479;
teaching of ancients respecting, 3;
temperature of, 515;
temporary, 430, 488;
values of velocity of, 484;
variable, 429

Stoney, Dr. G.J., 387

Strontium, flame from, 46;
in the Sun, 50

Struve, Otto, and the distance of Vega, 442, 447;
and the distance of 61 Cygni, 448, 449

Sun, The, and the velocity of light, 265;
apparent size of, as seen from the planets, 117, 118;
as a star, 32;
axial rotation of, 558;
compared with the Earth, 29;
connection of, with the seasons, 4;
corona of, during eclipse, 62-64;
density of, 65, 558;
diameter of, 558;
distance of, from Mars, 213;
distance of, from Saturn, 271;
distance of, from the Earth, 31, 114, 184, 240, 558;
eclipse of, 6, 53;
ellipticity of, 558;
faculae on surface of, 37;
focus of planets' orbits, 138;
gradually parting with its heat, 95;
granules on surface of, 34;
heat of, and its sources, 515-526;
heat of, thrown on Jupiter, 256;
minor planets and, 240;
movement of, towards Lyra, 457;
nebular theory of its heat, 526;
photographed, 34;
precession of the Earth's axis, 497;
prominences of, 53-59;
relation of, to the Moon, 71;
rising and setting of, 2;
rotation of, 40, 201;
size of, 29;
spectrum of, 48;
spots on, 36-43;
spots, connection with magnetism, 42;
storms and convulsions on, 42, 43;
surface of, gaseous matter, 34;
surface of, mottled, 34;
teaching of early astronomers concerning, 3-7;
temperature of, 30, 31, 516;
texture of, 34;
tides on, 530;
velocity of, 484;
weight of, compared with Jupiter, 250, 350;
zodiacal light and, 67;
zones on the surface of, 39

Sunbeam, revelations of a, 44

Swan, the, 424, 439, 445

Sword-handle of Perseus, 462

Syrtis major, 222

T

Taurus, constellation of, 231, 419

Tebbutt's comet, 353

Telescope, construction of the first, 10;
equatorial (Dunsink), 12-14, 185;
Greenwich, 26;
Herschelian, 19;
Lick, 16, 19;
Paris, 22, 23;
reflecting, 19, 21;
refracting, 11, 14;
Rosse, 19, 20, 468, 470;
sighting of a, 23;
structure of the eye illustrates the principle of the, 10;
Vienna, 14-16;
Washington, 226;
Yerkes, 16

Temporary stars, 430, 488

Tethys, 559

Theophilus, 92

Tides, The, actual energy derived from the Earth, 539;
affected by the law of gravitation, 149, 535;
affected by the Moon, 70, 535-537;
at Bay of Fundy, 538;
at Cardiff, 538;
at Chepstow, 538;
at London, 538;
at St. Helena, 538;
excited by the Sun, 537;
formation of currents, 538;
in Bristol Channel, 538;
in Mediterranean, 537;
in mid-ocean, 538;
Jupiter and, 552;
length of the day and, 541;
lunar, 548, 549;
moment of momentum and, 552;
neap, 537;
rotation of the Earth, and revolution of the Moon, 549;
satellites of Mars, 551;
solar, 550;
spring, 537;
variations in, 538;
waste of water power, 538;
work effected, 539

Tin in the Sun, 50

Titan, 294, 295, 559

Titania, 309, 559

Transit of Mercury, 152, 163, 164

Transit of Venus, 152;
Captain Cook, 184;
Copeland's observations of, 189;
Crabtree's observations of, 180;
Gassendi's observations of, 178;
Halley's method, 180, 181;
Horrocks' observations of, 179, 180;
importance of, 173;
Kepler's prediction of, 163;
observations of, at Dunsink, 184-188

Transit of Vulcan, 152-153

Triesnecker, 84, 93

Trouvelot, Mr. L., and Saturn's rings, 278

Tschermak, and the origin of meteorites, 400, 401

Tycho (lunar crater), 91

Tycho Brahe, and the Observatory of Uraniborg, 9, 10, 430

U

Umbra of Sun-spot, 51

Umbriel, 309, 559

Unstable dynamical equilibrium, 543

Uraniborg, Observatory of, 10

Uranus, 112;
attraction of Saturn, 322;
Bradley's observations of, 312;
composition of, 308;
density of, 558;
diameter of, 308, 558;
diameter of orbit of, 305;
disc of, 308;
discovery of, by Herschel, 305, 308;
distance from Sun of, 558;
ellipse of, 313;
first taken for a comet, 304;
Flamsteed's observations of, 311, 312;
formerly regarded as a star, 311, 312;
investigations to discover a planet outside the orbit, 323-324;
irregular motion of, 314, 323;
Lemonnier's observations of, 312;
Leonids and, 386;
Mayer's observations of, 312;
moment of momentum of, 554;
orbit of, 117, 310;
periodic time of, 558;
period of revolution of, 312;
rotation of, 308;
satellites of, 559;
satellites, discovery by Herschel, 308;
satellites, movement nearly circular, 309;
satellites, periodical movements, 309;
satellites, plane of orbits, 309, 310;
size of compared with the Earth, 308;
and with other planets, 119;
subject to another attraction besides the Sun, 314

Ursa major (_see_ Great Bear)

V

Variable Stars, 429

Vega, 414, 423, 424, 479;
Struve's measurement of, 442

Velocity, of light, 261, 262, 265;
of light, laws dependent upon, 511;
of planets, 140-143, 146, 237;
of stars, values of, 483-4

Venus, ancient study of, 6;
aspects of, 171;
atmosphere of, 189;
brilliancy of, 168;
density of, 558;
diameter of, 191, 558;
distance of, from the Sun, 191, 558;
habitability of, 173;
movement of, 168;
neighbour to the Earth, 109;
orbit of, 114, 135;
orbit form of, 139, 191;
periodic time of, 558;
a planet or "wanderer," 111;
rotation of, 191;
shape of, 169;
size of, compared with other planets, 116, 169;
surface of, 171;
transit of, 152, 176-190;
transit, importance of, 173;
transit predicted by Kepler, 163;
velocity and periodic time of, 142, 143, 191;
view of the ancients about, 157

Vesta, 233, 238

Victoria, 242

Vienna telescope, 14-16

Virgo, 423

Vogel and Algol, 485;
and Spica, 486, 487;
and the spectra of the stars, 479, 483

Volcanic origin of meteorites, 400;
outbreaks on the Earth, 197

Von Asten and Encke's comet, 349, 350;
and the distance of the Sun, 351;
and the weight of Mercury, 166

Vortex rings, 469

Vulcan, 152, 153;
and the Sun, 3

W

Wargentin, 90

Watson, Professor, and Mercury, 154

Watson, Sir William, friendship with Herschel, 302

Wave-lengths, 60

Weather, not affected by the Moon, 82

Wilson, Mr. W.E., and the nebula in Orion, 469

Witt, Herr G., and Eros, 236

Wold Cottage meteorite, the, 392

Wright, Thomas, and the Milky Way, 474

Y

"Year of Stars," the, 377

Yerkes Observatory, Chicago, 16

Young, Professor, account of a marvellous Sun-prominence, 42;
and Sun-spots, 38;
observations on magnetic storms, 39

Z

Zeeman, Dr., and spectral lines, 491

Zinc in the Sun, 50

Zodiac, the, 5

Zodiacal light, 67

Zone of minor planets, 234

PRINTED BY CASSELL & COMPANY, LIMITED, LA BELLE SAUVAGE, LONDON, E.C.

FOOTNOTES:

[1] It may, however, be remarked that a star is never _seen_ to set, as, owing to our atmosphere, it ceases to be visible before it reaches the horizon.

[2] "Popular Astronomy," p. 66.

[3] _Limb_ is the word used by astronomers to denote the _edge_ or circumference of the apparent disc of a heavenly body.

[4] "The Sun," p. 119.

[5] It has been frequently stated that the outburst in 1859, witnessed by Carrington and Hodgson, was immediately followed by an unusually intense magnetic storm, but the records at Kew and Greenwich show that the magnetic disturbances on that day were of a very trivial character.

[6] Some ungainly critic has observed that the poet himself seems to have felt a doubt on the matter, because he has supplemented the dubious moonbeams by the "lantern dimly burning." The more generous, if somewhat a sanguine remark has been also made, that "the time will come when the evidence of this poem will prevail over any astronomical calculations."

[7] This sketch has been copied by permission from the very beautiful view in Messrs. Nasmyth and Carpenter's book, of which it forms Plate XI. So have also the other illustrations of lunar scenery in Plates VIII., IX. The photographs were obtained by Mr. Nasmyth from models carefully constructed from his drawings to illustrate the features on the moon. During the last twenty years photography has completely superseded drawing by eye in the delineation of lunar objects. Long series of magnificent photographs of lunar scenery have been published by the Paris and Lick Observatories.

[8] At the British Association's meeting at Cardiff in 1892, Prof. Copeland exhibited a model of the moon, on which the appearance of the streaks near full moon was perfectly shown by means of small spheres of transparent glass attached to the surface.

[9] The duration of an occultation, or, in other words, the length of time during which the moon hides the star, would be slightly shorter than the computed time, if the moon had an atmosphere capable of sensibly refracting the light from the star. But, so far, our observations do not indicate this with certainty.

[10] I owe my knowledge of this subject to Dr. G. Johnstone Stoney, F.R.S. There has been some controversy as to who originated the ingenious and instructive doctrine here sketched.

[11] The space described by a falling body is proportional to the product of the force and the square of the time. The force varies inversely as the square of the distance from the earth, so that the space will vary as the square of the time, and inversely as the square of the distance. If, therefore, the distance be increased sixty-fold, the time must also be increased sixty-fold, if the space fallen through is to remain the same.

[12] See Newcomb's "Popular Astronomy," p. 78.

[13] Recent investigation by Newcomb on the motion of Mercury have led to the result that the hypothesis of a planet or a ring of very small planets between the orbit of Mercury and the sun cannot account for the difference between theory and observation in the movements of Mercury. Harzer has come to the same result, and has shown that the disturbing element may possibly be the material of the Solar Corona.

[14] "The Sun: its Planets, and their Satellites." London: 1882 (page 147).

[15] James Gregory, in a book on optics written in 1667, had already suggested the use of the transit of Venus for this purpose.

[16] _See_ "Astronomy and Astrophysics," No. 128.

[17] _See_ "Astronomy and Astrophysics," No. 128.

[18] This is the curved marking which on Plate XVIII. appears in longitude 290 deg. and north of (that is, below) the equator. Here, as in all astronomical drawings, north is at the foot and south at the top. _See_ above, p. 82 (Chapter III.).

[19] Now Director of the Lick Observatory.

[20] The heliometer is a telescope with its object-glass cut in half along a diameter. One or both of these halves is movable transversely by a screw. Each half gives a complete image of the object. The measures are effected by observing how many turns of the screw convey the image of the star formed by one half of the object-glass to coincide with the image of the planet formed by the other.

[21] See "Astronomy and Astrophysics," No. 109.

[22] It is only right to add that some observers believe that, in exceptional circumstances, points of Jupiter have shown some slight degree of intrinsic light.

[23] Professor Pickering, of Cambridge, Mass., has, however, effected the important improvement of measuring the decline of light of the satellite undergoing eclipse by the photometer. Much additional precision may be anticipated in the results of such observations.

[24] "Newcomb's Popular Astronomy," p. 336.

[25] _See_ Grant, "History of Physical Astronomy," page 255.

[26] Now Director of the Lick Observatory.

[27] We are here neglecting the orbital motion of Saturn, by which the whole system is moved towards or from the earth, but as this motion is common to the ball and the ring, it will not disturb the relative positions of the three spectra.

[28] According to Prof. Barnard's recent measures, the diameter of Titan is 2,700 miles. This is the satellite discovered by Huyghens; it is the sixth in order from the planet.

[29] Extract from "Three Cities of Russia," by C. Piazzi Smyth, vol. ii., p. 164: "In the year 1796. It then chanced that George III., of Great Britain, was pleased to send as a present to the Empress Catharine of Russia a ten-foot reflecting telescope constructed by Sir William Herschel. Her Majesty immediately desired to try its powers, and Roumovsky was sent for from the Academy to repair to Tsarskoe-Selo, where the Court was at the time residing. The telescope was accordingly unpacked, and for eight long consecutive evenings the Empress employed herself ardently in observing the moon, planets, and stars; and more than this, in inquiring into the state of astronomy in her dominions. Then it was that Roumovsky set before the Imperial view the Academy's idea of removing their observatory, detailing the necessity for, and the advantages of, such a proceeding. Graciously did the 'Semiramis of the North,' the 'Polar Star,' enter into all these particulars, and warmly approve of the project; but death closed her career within a few weeks after, and prevented her execution of the design."

[30] _See_ Professor Holden's "Sir William Herschel, his Life and Works."

[31] Arago says that "Lemonnier's records were the image of chaos." Bouvard showed to Arago one of the observations of Uranus which was written on a paper bag that in its time had contained hair-powder.

[32] The first comet of 1884 also suddenly increased in brightness, while a distinct disc, which hitherto had formed the nucleus, became transformed into a fine point of light.

[33] The three numbers 12, 1, and 1/4 are nearly inversely proportional to the atomic weights of hydrogen, hydrocarbon gas, and iron vapour, and it is for this reason that Bredichin suggested the above-mentioned composition of the various types of tail. Spectroscopic evidence of the presence of hydrogen is yet wanting.

[34] This illustration, as well as the figure of the path of the meteors, has been derived from Dr. G.J. Stoney's interesting lecture on "The Story of the November Meteors," at the Royal Institution, in 1879.

[35] On the 27th November, 1885, a piece of meteoric iron fell at Mazapil, in Mexico, during the shower of Andromedes, but whether it formed part of the swarm is not known. It is, however, to be noticed that meteorites are said to have fallen on several other occasions at the end of November.

[36] Hooke had noticed, in 1664, that the star Gamma Arietis was double.

[37] Perhaps if we could view the stars without the intervention of the atmosphere, blue stars would be more common. The absorption of the atmosphere specially affects the greenish and bluish colours. Professor Langley gives us good reason for believing that the sun itself would be blue if it were not for the effect of the air.

[38] The declination of a star is the arc drawn from the star to the equator at right angles to the latter.

[39] The distance of 61 Cygni has, however, again been investigated by Professor Asaph Hall, of Washington, who has obtained a result considerably less than had been previously supposed; on the other hand, Professor Pritchard's photographic researches are in confirmation of Struve's and those obtained at Dunsink.

[40] I am indebted for this drawing to the kindness of Messrs. De la Rue.

[41] _See_ Chapter XIX., on the mass of Sirius and his satellite.

[42] As the earth carries on the telescope at the rate of 18 miles a second, and as light moves with the velocity of 180,000 miles a second very nearly, it follows that the velocity of the telescope is about one ten-thousandth part of that of light. While the light moves down the tube 20 feet long, the telescope will therefore have moved the ten-thousandth part of 20 feet--_i.e._, the fortieth of an inch.

[43] _See_ Newcomb's "Popular Astronomy," p. 508, where the discovery of this law is attributed to Mr. J. Homer Lane, of Washington. The contraction theory is due to Helmholtz.

[44] The theory of Tidal Evolution sketched in this chapter is mainly due to the researches of Professor G.H. Darwin, F.R.S.

[45] The hour varies with the locality: it would be 11.49 at Calais; at Liverpool, 11.23; at Swansea Bay, 5.56, etc.

[46] Having decided upon the units of mass, of angle, and of distance which we intend to use for measuring these quantities, then any mass, or angle, or distance is expressed by a certain definite number. Thus if we take the mass of the earth as the unit of mass, the angle through which it moves in a second as the unit of angle, and its distance from the sun as the unit of distance, we shall find that the similar quantities for Jupiter are expressed by the numbers 316, 0.0843, and 5.2 respectively. Hence its orbital moment of momentum is 316 x 0.0843 x (5.2) squared.

End of Project Gutenberg's The Story of the Heavens, by Robert Stawell Ball