Chapter IV: The Structure of the Stellar Universe

391. _Sir William Herschel's View._--Sir William Herschel assumed that the stars are distributed with tolerable uniformity throughout the space occupied by our stellar system. He accounted for the increase in the number of stars in the field of view as he approached the plane of the Milky-Way, not by the supposition that the stars are really closer together in and about this plane, but by the supposition that our stellar system is in the form of a flat disk cloven at one side, and with our sun near its centre. A section of this disk is shown in Fig. 457.

An observer near _S_, with his telescope pointed in the direction of _S b_, would see comparatively few stars within the field of view, because looking through a comparatively thin stratum of stars. With his telescope pointed in the direction _S a_, he would see many more stars within his field of view, even though the stars were really no nearer together, because he would be looking through a thicker stratum of stars. As he directed his telescope more and more nearly in the direction _S f_, he would be looking through a thicker and thicker stratum of stars, and hence he would see a greater and greater number of them in the field of view, though they were everywhere in the disk distributed at uniform distances. He assumed, also, that the stars are all tolerably uniform in size, and that certain stars appear smaller than others, only because they are farther off. He supposed the faint stars of the Milky-Way to be merely the most distant stars of the stellar disk; that they are really as large as the other stars, but appear small owing to their great distance. The disk was assumed to be cloven on one side, to account for the division of the Milky-Way through nearly half of its course. This theory of the structure of the stellar universe is often referred to as the _cloven disk_ theory.

392. _The Cloven Ring Theory._--According to Mädler, the stars of the Milky-Way are entirely separated from the other stars of our system, belonging to an outlying ring, or system of rings. To account for the division of the Milky-Way, the ring is supposed to be cloven on one side: hence this theory is often referred to as the _cloven ring_ theory. According to this hypothesis, the stellar system viewed from without would present an appearance somewhat like that in Fig. 458. The outlying ring cloven on one side would represent the stars of the Milky-Way; and the luminous mass at the centre, the remaining stars of the system.

393. _Proctor's View._--According to Proctor, the Milky-Way is composed of an irregular spiral stream of minute stars lying in and among the larger stars of our system, as represented in Fig. 459. The spiral stream is shown in the inner circle as it really exists among the stars, and in the outer circle as it is seen projected upon the sky. According to this view, the stars of the Milky-Way appear faint, not because they are distant, but because they are really small.

394. _Newcomb's View._--According to Newcomb, the stars of our system are all situated in a comparatively thin zone lying in the plane of the Milky-Way, while there is a zone of nebulæ lying on each side of the stellar zone. He believes that so much is certain with reference to the structure of our stellar universe; but he considers that we are as yet comparatively ignorant of the internal structure of either the stellar or the nebular zones. The structure of the stellar universe, according to this view, is shown in Fig. 460.

INDEX

A.

Aberration of light, 38.

Aerolites, 304.

Aldebaran, star in Taurus, 340, 342.

Algol, a variable star, 343, 358.

Almanac, perpetual, 82.

Alps, lunar mountains, 126.

Altair, star in Aquila, 336.

Alt-azimuth instrument, 13.

Altitude, 12.

Andromeda (constellation), 343, 346.
nebula in, 376.

Angström's map of spectrum, 164.

Antares, star in Scorpio, 347.

Apennines, lunar mountains, 122, 124.

Aphelion, 47.

Apogee, 44.

Aquarius, or the Water-Bearer, 350.
cluster in, 354.

Aquila, or the Eagle, 336.

Arcturus, star in Boötes, 335, 365, 370.

Argo, or the Ship, 360.
nebula in, 383.
variable star in, 360.

Aries, or the Ram, 350.

Asteroids, 223, 241.

Astræa, an asteroid, 241.

Auriga, or the Wagoner, 342.

Azimuth, 13.

B.

Betelgeuse, star in Orion, 340, 370.

Berenice's Hair (constellation), 334.

Bode's law, 241.
disproved, 273.

Boötes (constellation), 334, 335.

C.

Calendar, the, 80.

Callisto, moon of Jupiter, 250.

Cancer, or the Crab, 350.
tropic of, 61.

Canes Venatici, or the Hunting-Dogs, 334.

Canes Venatici, nebula in, 384.

Canis Major, or the Great Dog, 342.

Canis Minor, or the Little Dog, 340.

Capella, star in Auriga, 340, 343.

Capricorn, tropic of, 61.

Capricornus, or the Goat, 350.

Cassiopeia (constellation), 332.
new star in, 362.

Castor, star in Gemini, 340, 370.

Caucasus, a lunar range, 124.

Centaurus, star-cluster in, 355.

Cepheus (constellation), 334.
nebula in, 387.

Ceres, the planet, 241.

Cetus, or the Whale, 346.
variable star in, 359.

Charles's Wain, 330.

Circles, great, 4.
diurnal, 8.
hour, 16.
small, 4.
vertical, 12.

Clock, astronomical, 18.
time, 78.

Coma Berenices, or Berenice's Hair, 334.

Comet, Biela's, 293.
and earth, collision of, 316.
Coggia's, 297.
Donati's, 296.
Encke's, 293.
Halley's, 291.
of 1680, 290.
of 1811, 290.
of 1843, 295.
of 1861, 297.
of June, 1881, 300.

Comets, appearance of, 274.
and meteors, 313.
bright, 274.
chemical constitution of, 318.
development of, 277.
number of, 288.
orbits of, 282.
origin of, 287.
periodic, 286.
physical constitution of, 314.
tails of, 279.
telescopic, 275, 281.
visibility of, 281.

Conic sections, 48.

Conjunction, 91.
inferior, 130.
superior, 130, 136.

Constellations, 325.
zodiacal, 32.

Copernican system, the, 44, 53.

Copernicus, a lunar crater, 120, 129.

Corona Borealis, or the Northern Crown, 336.

Corona Borealis, new star in, 363.

Corvus, or the Crow, 339.

Crystalline spheres, 41.

Cycles and epicycles, 42.

Cygnus, or the Swan, 338.

D.

Day and night, 57.
civil, 77.
lunar, 108.
sidereal, 74.
solar, 74.

Declination, 16.

Deimos, satellite of Mars, 239.

Delphinus, or the Dolphin, 338.

Deneb, star in Cygnus, 338, 370.

Dione, satellite of Saturn, 259.

Dipper, the Great, 330, 366, 369, 370.
the Little, 331.
the Milk, 347.

Dissociation, 163.

Dominical Letter, the, 81.

Draco, or the Dragon, 331.

E.

Earth, density of, 85.
flattened at poles, 55.
form of, 53.
in space, 56.
seen from moon, 109.
size of, 55.
weight of, 83.

Eccentric, the 43.

Eccentricity, 46.

Eclipses, 210.
annular, 219.
lunar, 210, 214.
solar, 216.

Ecliptic, the, 27.
obliquity of, 28.

Ellipse, the, 45, 49.

Elongation, of planet, 130.

Enceladus, moon of Saturn, 259.

Epicycles, 42.

Epicycloid, 107.

Epsilon Lyræ, a double star, 356.

Equator, the celestial, 7.

Equinoctial, the, 7.
colure, 16.
elevation of, 9.

Equinox, autumnal, 29.
vernal, 16, 29.

Equinoxes, precession of, 31, 85.

Eta Argus, a variable star, 360, 383.

Europa, moon of Jupiter, 250.

F.

Faculæ, solar, 177.

Fomalhaut, star in Southern Fish, 350.

Fraunhofer's lines, 164, 371.

G.

Galaxy, the, 326.

Ganymede, moon of Jupiter, 250.

Gemini, or the Twins, 340.

Georgium Sidus, 271.

H.

Hercules (constellation), 336.
cluster in, 353.
orbit of double star in, 357.
solar system moving towards, 367.

Herschel, the planet (see Uranus).

Herschel's hypothesis, 392, 396.

Horizon, the, 5.

Hyades, the, 342, 350.

Hydra, or the Water-Snake, 340.

Hyperbola, the, 49.

Hyperion, moon of Saturn, 259.

I.

Io, moon of Jupiter, 250.

Irradiation, 90, 113.

J.

Japetus, moon of Saturn, 259.

Job's Coffin (asterism), 338.

Juno, the planet, 241.

Jupiter, apparent size of, 245.
distance of, 245.
great red spot of, 249.
orbit of, 244.
periods of, 246.
physical constitution of, 246.
rotation of, 248.
satellites of, 250.
eclipses of, 252.
transits of, 254.
volume of, 245.
without satellites, 255.

K.

Kant's hypothesis, 391.

Kepler, a lunar crater, 129.

Kepler's system, 44.
laws, 46.
star, 362.

Kirchhoff's map of spectrum, 164.

L.

Laplace's hypothesis, 392.

Latitude, celestial, 30.

Leap year, 81.

Leo, or the Lion, 334.

Leonids (meteors), 312.

Libra, or the Balances, 347.

Libration, 102.

Longitude, celestial, 30.

Lyra, or the Lyre, 338.
double star in, 356.

M.

Magellanic clouds, the, 389.

Magnetic storms, 190.

Magnetism and sun-spots, 190.

Mars, apparent size of, 236.
brilliancy of, 237.
distance of, 235.
orbit of, 235.
periods of, 237.
rotation of, 239.
satellites of, 239.
volume of, 236.

Mercury, apparent size of, 226.
atmosphere of, 228.
distance of, 225.
elongation of, 227.
orbit of, 225.
periods of, 227.
volume of, 226.

Meridian, the, 12.

Meridian circle, 17.

Meridians, celestial, 31.

Meteoric iron, 305, 307.
showers, 310.
stones, 305.

Meteors, 300.
August, 311.
light of, 309.
November, 312.
sporadic, 310.

Meteoroids, 308.

Micrometers, 20, 153.

Milky-Way, the, 326.

Mimas, moon of Saturn, 259.

Mira, a variable star, 359.

Moon, apparent size of, 87, 89.
aspects of, 91.
atmosphere of, 109.
chasms in, 123.
craters in, 119.
day of, 108.
distance of, 86.
eclipses of, 210.
form of orbit, 97.
harvest, 101.
hunter's, 102.
inclination of orbit, 97.
kept in her path by gravity, 51.
librations of, 102.
mass of, 90.
meridian altitude of, 98.
mountains of, 116.
orbital motion of, 91.
phases of, 93.
real size of, 88.
rising of, 99.
rotation of, 102.
sidereal period of, 92.
surface of, 115.
synodical period of, 92.
terminator of, 115.
wet and dry, 98.

N.

Nadir, the, 6.

Neap-tides, 72.

Nebula, in Andromeda, 376.
crab, 376.
dumb-bell, 383.
in Argus, 383.
in Canes Venatici, 384.
in Cepheus, 387.
in Orion, 378.
in the Triangle, 387.
in Ursa Major, 386.

Nebulæ, 281, 330, 373.
annular, 373.
circular, 373.
condensation of, 385.
double, 375.
elliptical, 373.
irregular, 376.
multiple, 375.
spiral, 373, 384.

Nebular hypothesis, the, 391.

Neptune, discovery of, 271.
orbit of, 271.
satellite of, 274.

New style, 80.

Newcomb's theory of the stellar universe, 398.

Newton's system, 48.

Nodes, 97.

Nubecula, Major, 389.
Minor, 389.

Nutation, 34.

O.

Olbers's hypothesis, 241.

Old style, 80.

Ophiuchus (constellation), 347.
new star in, 362.

Opposition, 91, 136.

Orion, 341.
nebula in, 378.
the trapezium of, 356.

P.

Pallas, the planet, 241.

Parabola, the, 49.

Parallax, 37.

Pegasus (constellation), 343, 346.
triple star in, 356.

Perigee, 44.

Perihelion, 47.

Perseids (meteors), 311.

Perseus (constellation), 346.
cluster in, 353.

Phobos, satellite of Mars, 239.

Pico, a lunar mountain, 127.

Pisces, or the Fishes, 350.

Piscis Australis, or the Southern Fish, 350.

Planets, 39.
inferior, 130.
periods of, 132.
phases of, 132.
inner group of, 221.
intra-Mercurial, 230.
minor, 223.
outer group of, 222, 244.
superior, 134.
motion of, 134.
periods of, 137.
phases of, 137.
three groups of, 221.

Pleiades, the, 328, 342, 351.

Pointers, the, 330.

Polar distance, 16.

Pole Star, the, 7, 330, 365.

Poles, celestial, 7, 9.

Pollux, star in Gemini, 340, 370.

Præsepe, or the Beehive, 350.

Precession of equinoxes, 31, 85.

Prime vertical, the, 12.

Proctor's theory of the stellar universe, 398.

Procyon, star in Canis Minor, 340.

Ptolemaic system, the, 41.

Q.

Quadrature, 91, 137.

R.

Radiant point (meteors), 310.

Radius vector, 47.

Refraction, 35.

Regulus, star in Leo, 334, 370.

Rhea, moon of Saturn, 259.

Rigel, star in Orion, 340, 370.

Right ascension, 16.

S.

Sagittarius, or the Archer, 347.

Saturn, apparent size of, 256.
distance of, 256.
orbit of, 255.
periods of, 256.
physical constitution of, 257.
ring of, 261.
changes in, 268.
constitution of, 269.
phases of, 263.
rotation of, 258.
satellites of, 259.
volume of, 256.

Scorpio, or the Scorpion, 347.
cluster in, 355.

Seasons, the, 64.

Sirius, the Dog-Star, 340, 342, 365, 370, 371.

Solar system, the, 41.

Solstices, 29, 59, 60.

Sound, effect of motion on, 168.

Spectra, bright-lined, 158.
comparison of, 154.
continuous, 158.
displacement of lines in, 171.
of comets, 318.
reversed, 161.
sun-spot, 193.
types of stellar, 371.

Spectroscope, the, 152.
diffraction, 157.
direct-vision, 155.
dispersion, 152.

Spectrum analysis, 159.
solar, 164.

Sphere, defined, 3.
the celestial, 5.
rotation of, 7.

Spring-tides, 72.

Stars, circumpolar, 7.
clusters of, 328, 350.
color of, 357.
constellations of, 325.
constitution of, 371.
distance of, 364.
double, 355.
drift of, 368.
four sets of, 10.
magnitude of, 322.
motion of, in line of sight, 369.
multiple, 356.
names of, 325.
nebulous, 373.
new, 361.
number of, 323.
parallax of, 364.
proper motion of, 365.
secular displacement of, 366.
temporary, 361.
variable, 358.

Sun, atmosphere of, 149.
brightness of, 151.
chemical constitution of, 164.
chromosphere of, 149, 196.
corona of, 149, 196, 204.
distance of, 142.
faculæ of, 177.
heat radiated by, 150.
inclination of axis of, 187.
mass of, 140.
motion of, among the stars, 26.
at surface of, 168.
in atmosphere of, 172.
secular, 366.
photosphere of, 149, 175.
prominences of, 149, 197.
rotation of, 186.
spectrum of, 164, 171.
temperature of, 149.
volume of, 140.
winds on, 174.

Sun-spots, 179.
and magnetism, 190.
birth and decay of, 185.
cause of, 194.
cyclonic motion in, 182.
distribution of, 188.
duration of, 181.
groups of, 181.
periodicity of, 189.
proper motion of, 187.
size of, 181.
spectrum of, 193.

T.

Taurus, or the Bull, 342.
quadruple star in, 356.

Telescope, Cassegrainian, 23.
equatorial, 19.
front-view, 22.
Gregorian, 23.
Herschelian, 22.
Lord Rosse's, 25.
Melbourne, 25.
Newall, 20.
Newtonian, 22.
Paris, 26.
reflecting, 21.
Washington, 20.
Vienna, 20.

Telespectroscope, the, 155.

Telluric lines of spectrum, 165.

Tethys, moon of Saturn, 259.

Tides, 67.

Time, clock, 78.
sun, 78.

Titan, moon of Saturn, 259, 261.

Toucan, star cluster in, 354, 389.

Transit instrument, 17.

Transits of Venus, 145.

Triesneker, lunar formation, 123.

Tropics, 61.

Twilight, 62.

Tycho Brahe's star, 361.
system, 44.

Tycho, a lunar crater, 129.

U.

Universe, structure of the stellar, 396.

Uranus, discovery of, 271.
name of, 270.
orbit of, 269.
satellites of, 271.

Ursa Major, or the Great Bear, 330.
nebula in, 386.

Ursa Minor, or the Little Bear, 330.

V.

Vega, star in Lyra, 336, 365, 370.

Venus, apparent size of, 231.
atmosphere of, 234.
brilliancy of, 232.
distance of, 231.
elongation of, 231.
orbit of, 230.
periods of, 232.
volume of, 231.
transits of, 145, 234.

Vernier, the, 15.

Virgo, or the Virgin, 338.

Vesta, the planet, 241.

Vulcan, the planet, 230.

Y.

Year, the, 78.
anomalistic, 79.
Julian, 80.
sidereal, 79.
tropical, 79.

Z.

Zenith, the, 6.
distance, 12.

Zodiac, the, 32.

Zodiacal constellations, 32.
light, 318.

Zones, 61.

* * * * * *

Transcriber's note:

Missing or obscured punctuation was corrected.

Typographical errors were silently corrected.