ASTRONOMY 



311 



een in the same relative position. The complexity 

 of these motions is increased by the confusion pre- 

 sented by the apparent motion of the other planets : 

 sometimes they seem to be hurried along with great 

 rapidity ; at other tunes they appear stationary, and, 

 at others, still retrograde. All this seeming chaos of 

 motions is reduced to order by a knowledge of the 

 fact, that, while the earth turns on its axis, it ad- 

 vances, at the same time, in absolute space from west 

 to east, and performs an entire revolution round the 

 sun in the course of a year, in a plane inclined to the 

 equator. The circle which the centre of the earth 

 Describes in this revolution, and which is the apparent 

 path of the sun, is called the ecliptic. The axis of 

 the earth remaining always parallel to itself, the 

 opposite poles will be directed towards the sun once 

 in each revolution. When a pole is directed towards 

 the sun, it receives more light and heat, and for a 

 longer period, than at any other portion of the revo- 

 lution. It is then the summer solstice in that hemis- 

 phere ; the days are longest, the nights shortest, and 

 the heat greatest. Six months, or, rather, half a 

 year from that period, every thing is reversed ; the 

 same pole is turned from the sun ; the light and 

 heat is received in small quantities, and for a short 

 period ; the days are short, the nights long ; the cold 

 intense : it is the winter solstice. At two other points 

 of tlie orbit, equidistant from each other and from the 

 solstices, the poles are equally inclined to the sun ; 

 they receive an equal supply of heat and light, and 

 during equal periods ; the days and nights are equal 

 all over the globe : it is the vernal or autumnal equi- 

 nox. The diurnal rotation of the earth on its own 

 axis produces, therefore, the alternation of day and 

 night. The annual revolution round the sun. and 

 the obliquity of the ecliptic to the equator, causes the 

 changes of the seasons. The daily rotation of the 

 earth produces, also, the phenomena of tides in the 

 ocean and the atmosphere. (See Tides.) Let us 

 now take a more general view of the celestial phe- 

 nomena. The discovery of peculiar qualities common 

 to a number of heavenly bodies, has led to the for- 

 mation of classes (see Planets, Satellites, Comets, 

 Fixed Stars) ; or convenience of description has clus- 

 tered them into groups with fanciful names (see 

 Constellations) ; or their peculiar influence on human 

 affairs has given a name to individuals (see Sun, 

 Moon, Earth, &c.). At first view, the stars in general 

 do not seem to change their relative positions ; and, 

 if they have particular motions, a long series of ob- 

 servations is necessary to render them sensible. But, 

 by continuing to compare the heavens at different 

 epochs, we perceive that some of them are distin- 

 guished by relative motions, and by the nature of the 

 light which they transmit to us. These we call 

 planets, that is, wandering stars, in distinction from 

 those, which, maintaining always the same relative 

 positions, are called fixed stars. The planets transmit 

 to us a soft, mild, steady light, never exhibiting any 

 change of colour. They are opaque Ixxiies, and their 

 light is only a reflection of that which they receive 

 from the sun, around which they revolve in regular 

 but unequal periods, turning at the same time on their 

 axes. Their number now known is eleven. We men- 

 tion them in the order of their distances from the 

 sun Mercury, Venus, Earth, Mars, Vesta, Juno, 

 Ceres, Pallas, Jupiter, Saturn, and Uranus, or Her- 

 schel. Five of these are visible to the naked eye, 

 and were known to the ancients ; five have been 

 discovered in modern times by the aid of the teles- 

 cope. Some of these bodies have smaller ones in 

 their neighbourhood, which revolve round them at 

 the same time that they accompany them in their 

 orbits of revolution round the sun, and turn on their 

 >iwn axes. The former are called primary, to distin- 



guish them from these attendants, which are called 

 the secondary planets, or satellites. The latter are 

 opaque, like the former. The earth is accompanied 

 by one, which is called the moon, Jupiter by four, 

 Saturn by seven, with his remarkable ring, and 

 Herschel by six. The interposition of one of the 

 planets between the sun and an observer stationed on 

 another planet, produces an eclipse, (q. v.) From 

 time to time, small specks appear in the heavens, of 

 a feeble lustre, moving slowly in the midst of the 

 other stars. Gradually, as they approach nearer, 

 their velocity increases ; their light is more brilliant ; 

 and, after passing into the immediate vicinity of the 

 earth and sun, they recede again, and disappear in 

 the distance. These are called comets (which signi- 

 fies hairy bodies) from the peculiar luminous train by 

 which they are attended, and which the ancients 

 called hair, and the moderns, tail. These bodies, 

 long the objects of terror to man, as the harbingers 

 of pestilence and war, are now known to be subject 

 to the common laws of nature, and to revolve round 

 the sun in regular periods. The sun, the 11 primary 

 and 18 secondary planets, and the comets, constitute 

 the solar system. Far beyond these limits, at an immea- 

 surable distance, lie the fixed stars, infinite in number, 

 of a brilliant lustre, and constantly changing colour. 

 Their distance, and the brilliancy of their light, with 

 the fact that, their magnitudes remain always the same, 

 render it probable that they are luminous bodies, like so 

 many suns. They have been formed into many groups 

 of arbitrary extent, under the names of gods, men, 

 beasts, &c. , which are called constellations. Of these 

 the ancients knew 48 ; the moderns have increased 

 the number to more than 10O. It should be under- 

 stood, that the stars, thus grouped together under one 

 name, have no connexion with each other, but are so 

 arranged for convenience of description. The first 

 astronomers, in order to estimate better the apparent 

 motion of the sun, referred it to those constellations 

 through which it appeared to pass, and which are 12 

 in number. They are, in Latin, Aries, Taurus, 

 Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagit- 

 tarius, Capricornus, Aquarius, and Pisces. The zone, 

 or band, which contains them, is called the zodiac, (q. 

 v.) and each constellation is called a sign of the zodiac. 

 In consequence of a motion of the earth's axis, the 

 constellations no longer correspond to the same points 

 of the orbit ; but as we confine the name signs to the 

 12 divisions of the circumference of the circle, which 

 measures the whole revolution of the earth, and as 

 these divisions do not change, the venial equinox al- 

 ways corresponds to the first point of the sign Aries, 

 the summer solstice to the first point of the sign Can- 

 cer, the autumnal equinox to that of Libra, and die 

 winter solstice to that of Capricorn, although the con- 

 stellations, which bear these names, have ceased to 

 be connected with these seasons. (See Precession of 

 the Equinoxes.) To penetrate yet farther into the 

 heavens, it is necessary to aid the imperfection fit 

 vision by the telescope, which discovers tousmillions 

 of stars in the infinity of space. In a clear night, 

 turn your eyes to the irregular zone of whitish light : 

 it is the milky way (q. v.) : you will find it to consist 

 of an infinite number of stars, whose inconceivable 

 distance renders their light too feeble to make a^iis- 

 tinct impression on the naked eye. Continue ^ur 

 examination, and you will observe luminous spots of 

 an undefined shape : these are nebulee, some of which 

 a further observation will show you to be assemblages 

 of stars, like the milky way, while others will appear 

 to consist of an unbroken mass of whitish light. You 

 will find, also, some stars to be variable, undergoing 

 a periodical change of brightness : some, which ap- 

 pear single to the naked eye, will be found to be 

 double, triple, &c., and to revolve round a common 



