 |
The tides which the moon raises in the earth act as a brake on the rotation of the earth. They now constantly tend to bring the period of rotation of the earth to coincide with the period of revolution of the moon. As the moon revolves once in twenty-seven days, the earth is at present going too fast, and consequently the tidal control at the present moment endeavours to retard the rotation of the earth. The rotation of the moon long since succumbed to tidal control, but that was because the moon was comparatively small and the tidal power of the earth was enormous. But this is the opposite case. The earth is large and more massive than the moon, the tides raised by the moon are but small and weak, and the earth has not yet completely succumbed to the tidal action. But the tides are constant, they never for an instant relax the effort to control, and they are gradually tending to render the day and the month coincident, though the progress is a very slow one.
The theory of the tides leads us to look forward to a remote state of things, in which the moon revolves around the earth in a period equal to the day, so that the two bodies shall constantly bend the same face to each other, provided the tidal control be still able to guide the moon's rotation. So far as the mutual action of the earth and the moon is concerned, such an arrangement possesses all the attributes of permanence. If, however, we venture to project our view to a still more remote future, we can discern an external cause which must prevent this mutual accommodation between the earth and the moon from being eternal. The tides raised by the moon on the earth are so much greater than those raised by the sun, that we have, in the course of our previous reasoning, held little account of the sun-raised tides. This is obviously only an approximate method of dealing with the question. The influence of the solar tide is appreciable, and its importance relatively to the lunar tide will gradually increase as the earth and moon approach the final critical stage. The solar tides will have the effect of constantly applying a further brake to the rotation of the earth. It will therefore follow that, after the day and the month have become equal, a still further retardation awaits the length of the day. We thus see that in the remote future we shall find the moon revolving around the earth in a shorter time than that in which the earth rotates on its axis.
A most instructive corroboration of these views is afforded by the discovery of the satellites of Mars. The planet Mars is one of the smaller members of our system. It has a mass which is only the eighth part of the mass of the earth. A small planet like Mars has much less energy of rotation to be destroyed than a larger one like the earth. It may therefore be expected that the small planet will proceed much more rapidly in its evolution than the large one; we might, therefore, anticipate that Mars and his satellites have attained a more advanced stage of their history than is the case with the earth and her satellite.
When the discovery of the satellites of Mars startled the world, in 1877, there was no feature which created so much amazement as the periodic time of the interior satellite. We have already pointed out in Chapter X. how Phobos revolves around Mars in a period of 7 hours 39 minutes. The period of rotation of Mars himself is 24 hours 37 minutes, and hence we have the fact, unparalleled in the solar system, that the satellite is actually revolving three times as rapidly as the planet is rotating. There can hardly be a doubt that the solar tides on Mars have abated its velocity of rotation in the manner just suggested.
It has always seemed to me that the matter just referred to is one of the most interesting and instructive in the whole history of astronomy. We have, first, a very beautiful telescopic discovery of the minute satellites of Mars, and we have a determination of the anomalous movement of one of them. We have then found a satisfactory physical explanation of the cause of this phenomenon, and we have shown it to be a striking instance of tidal evolution. Finally, we have seen that the system of Mars and his satellite is really a forecast of the destiny which, after the lapse of ages, awaits the earth-moon system.
It seems natural to enquire how far the influence of tides can have contributed towards moulding the planetary orbits. The circumstances are here very different from those we have encountered in the earth-moon system. Let us first enunciate the problem in a definite shape. The solar system consists of the sun in the centre, and of the planets revolving around the sun. These planets rotate on their axes; and circulating round some of the planets we have their systems of satellites. For simplicity, we may suppose all the planets and their satellites to revolve in the same plane, and the planets to rotate about axes which are perpendicular to that plane. In the study of the theory of tidal evolution we must be mainly guided by a profound dynamical principle known as the conservation of the "moment of momentum." The proof of this great principle is not here attempted; suffice it to say that it can be strictly deduced from the laws of motion, and is thus only second in certainty to the fundamental truths of ordinary geometry or of algebra. Take, for instance, the giant planet, Jupiter. In one second he moves around the sun through a certain angle. If we multiply the mass of Jupiter by that angle, and if we then multiply the product by the square of the distance from Jupiter to the sun, we obtain a certain definite amount. A mathematician calls this quantity the "orbital" moment of momentum of Jupiter.[46] In the same way, if we multiply the mass of Saturn by the angle through which the planet moves in one second, and this product by the square of the distance between the planet and the sun, then we have the orbital moment of momentum of Saturn. In a similar manner we ascertain the moment of momentum for each of the other planets due to revolution around the sun. We have also to define the moment of momentum of the planets around their axes. In one second Jupiter rotates through a certain angle; we multiply that angle by the mass of Jupiter, and by the square of a certain line which depends on his internal constitution: the product forms the "rotational" moment of momentum. In a similar manner we find the rotational moment of momentum for each of the other planets. Each satellite revolves through a certain angle around its primary in one second; we obtain the moment of momentum of each satellite by multiplying its mass into the angle described in one second, and then multiplying the product into the square of the distance of the satellite from its primary. Finally, we compute the moment of momentum of the sun due to its rotation. This we obtain by multiplying the angle through which the sun turns in one second by the whole mass of the sun, and then multiplying the product by the square of a certain line of prodigious length, which depends upon the details of the sun's internal structure.
If we have succeeded in explaining what is meant by the moment of momentum, then the statement of the great law is comparatively simple. We are, in the first place, to observe that the moment of momentum of any planet may alter. It would alter if the distance of the planet from the sun changed, or if the velocity with which the planet rotates upon its axis changed; so, too, the moment of momentum of the sun may change, and so may those of the satellites. In the beginning a certain total quantity of moment of momentum was communicated to our system, and not one particle of that total can the solar system, as a whole, squander or alienate. No matter what be the mutual actions of the various bodies of the system, no matter what perturbations they may undergo—what tides may be produced, or even what mutual collisions may occur—the great law of the conservation of moment of momentum must be obeyed. If some bodies in the solar system be losing moment of momentum, then other bodies in the system must be gaining, so that the total quantity shall remain unaltered. This consideration is one of supreme importance in connection with the tides. The distribution of moment of momentum in the system is being continually altered by the tides; but, however the tides may ebb or flow, the total moment of momentum can never alter so long as influences external to the system are absent.
We must here point out the contrast between the endowment of our system with energy and with moment of momentum. The mutual actions of our system, in so far as they produce heat, tend to squander the energy, a considerable part of which can be thus dissipated and lost; but the mutual actions have no power of dissipating the moment of momentum.
The total moment of momentum of the solar system being taken to be 100, this is at present distributed as follows:—
Orbital moment of momentum of Jupiter 60 Orbital moment of momentum of Saturn 24 Orbital moment of momentum of Uranus 6 Orbital moment of momentum of Neptune 8 Rotational moment of momentum of Sun 2 — 100
The contributions of the other items are excessively minute. The orbital moments of momentum of the few interior planets contain but little more than one thousandth part of the total amount. The rotational contributions of all the planets and of their satellites is very much less, being not more than one sixty-thousandth part of the whole. When, therefore, we are studying the general effects of tides on the planetary orbits these trifling matters may be overlooked. We shall, however, find it desirable to narrow the question still more, and concentrate our attention on one splendid illustration. Let us take the sun and the planet Jupiter, and, supposing all other bodies of our system to be absent, let us discuss the influence of tides produced in Jupiter by the sun, and of tides in the sun by Jupiter.
It might be hastily thought that, just as the moon was born of the earth, so the planets were born of the sun, and have gradually receded by tides into their present condition. We have the means of enquiry into this question by the figures just given, and we shall show that it is impossible that Jupiter, or any of the other planets, can ever have been very much closer to the sun than they are at present. In the case of Jupiter and the sun we have the moment of momentum made up of three items. By far the largest of these items is due to the orbital revolution of Jupiter, the next is due to the sun, the third is due to the rotation of Jupiter on its axis. We may put them in round numbers as follows:—
Orbital moment of momentum of Jupiter 600,000 Rotational moment of momentum of Sun 20,000 Rotational moment of momentum of Jupiter 12
The sun produces tides in Jupiter, those tides retard the rotation of Jupiter. They make Jupiter rotate more and more slowly, therefore the moment of momentum of Jupiter is decreasing, therefore its present value of 12 must be decreasing. Even the mighty sun himself may be distracted by tides. Jupiter raises tides in the sun, those tides retard the motion of the sun, and therefore the moment of momentum of the sun is decreasing, and it follows from both causes that the item of 600,000 must be increasing; in other words, the orbital motion of Jupiter must be increasing, or Jupiter must be receding from the sun. To this extent, therefore, the sun-Jupiter system is analogous to the earth-moon system. As the tides on the earth are driving away the moon, so the tides in Jupiter and the sun are gradually driving the two bodies apart. But there is a profound difference between the two cases. It can be proved that the tides produced in Jupiter by the sun are more effective than those produced in the sun by Jupiter. The contribution of the sun may, therefore, be at present omitted; so that, practically, the augmentations of the orbital moment of momentum of Jupiter are now achieved at the expense of that stored up by Jupiter's rotation. But what is 12 compared with 600,000. Even when the whole of Jupiter's rotational moment of momentum and that of his satellites has become absorbed into the orbital motion, there will hardly be an appreciable difference in the latter. In ancient days we may indeed suppose that Jupiter being hotter was larger than at present, and that he had considerably more rotational moment of momentum. But it is hardly credible that Jupiter can ever have had one hundred times the moment of momentum that he has at present. Yet even if 1,200 units of rotational momentum had been transferred to the orbital motion it would only correspond with the most trivial difference in the distance of Jupiter from the sun. We are hence assured that the tides have not appreciably altered the dimensions of the orbit of Jupiter, or of the other great planets.
The time will, however, come when the rotation of Jupiter on his axis will be gradually abated by the influence of the tides. It will then be found that the moment of momentum of the sun's rotation will be gradually expended in increasing the orbits of the planets, but as this reserve only holds about two per cent. of the whole amount in our system it cannot produce any considerable effect.
The theory of tidal evolution, which in the hands of Professor Darwin has taught us so much with regard to the past history of the systems of satellites in the solar system, will doubtless also, as pointed out by Dr. See, be found to account for the highly eccentric orbits of double star systems. In the earth-moon system we have two bodies exceedingly different in bulk, the mass of the earth being about eighty times as great as that of the moon. But in the case of most double stars we have to do with two bodies not very different as regards mass. It can be demonstrated that the orbit must have been originally of slight eccentricity, but that tidal friction is capable not only of extending, but also of elongating it. The accelerating force is vastly greater at periastron (when the two bodies are nearest each other) than at apastron (when their distance is greatest). At periastron the disturbing force will, therefore, increase the apastron distance by an enormous amount, while at apastron it increases the periastron distance by a very small amount. Thus, while the ellipse is being gradually expanded, the orbit grows more and more eccentric, until the axial rotations have been sufficiently reduced by the transfer of axial to orbital moment of momentum.
And now we must draw this chapter to a close, though there are many other subjects that might be included. The theory of tidal evolution is, indeed, one of quite exceptional interest. The earlier mathematicians expended their labour on the determination of the dynamics of a system which consisted of rigid bodies. We are indebted to contemporary mathematicians for opening up celestial mechanics upon the more real supposition that the bodies are not rigid; in other words, that they are subject to tides. The mathematical difficulties are enormously enhanced, but the problem is more true to nature, and has already led to some of the most remarkable astronomical discoveries made in modern times.
* * * * *
Our Story of the Heavens has now been told. We commenced this work with some account of the mechanical and optical aids to astronomy; we have ended it with a brief description of an intellectual method of research which reveals some of the celestial phenomena that occurred ages before the human race existed. We have spoken of those objects which are comparatively near to us, and then, step by step, we have advanced to the distant nebulae and clusters which seem to lie on the confines of the visible universe. Yet how little can we see with even our greatest telescopes, when compared with the whole extent of infinite space! No matter how vast may be the depth which our instruments have sounded, there is yet a beyond of infinite extent. Imagine a mighty globe described in space, a globe of such stupendous dimensions that it shall include the sun and his system, all the stars and nebulae, and even all the objects which our finite capacities can imagine. Yet, what ratio must the volume of this great globe bear to the whole extent of infinite space? The ratio is infinitely less than that which the water in a single drop of dew bears to the water in the whole Atlantic Ocean.
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.
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