ASTRONOMY. 



air. This motion of the earth round its 

 axis, which, from the instances already 

 given, has been sufficiently proved, is 

 called its diurnal or daily motion, and is 

 that which occasions the regular return 

 of day and night, and all the celestial ap- 

 pearances before mentioned. But there 

 is also another motion of the earth, called 

 its annual or yearly motion, which occa- 

 sions the various vicissitudes of the sea- 

 sons, summer, winter, spring, and autumn. 

 And the proofs of this second motion may 

 be easily gathered from celestial appear- 

 ances, in nearly the same manner as the 

 former. For as that luminary seems to 

 move round the earth from east to west, 

 in the space of a day, which is really 

 owing to the diurnal revolution of the 

 earth upon its axis, in a contrary direc- 

 tion ; so likewise he seems to have an 

 annual motion in the heavens, and to rise 

 and set continually in different parts of 

 them ; which is certainly occasioned by 

 the daily motion of the earth in its orbit, 

 or path round the sun, which it completes 

 in the space of a year. 



OF THE SOLAR SYSTEM. 



It is fully proved that the planets, with 

 the earth which we inhabit, and also the 

 moon, revolve round the sun, which is 

 fixed in the centre of the system. There 

 are two kinds of planets, primary and 

 secondary. The first move round the 

 sun, and respect him only as the centre of 

 their motions. The secondary planets, 

 called also satellites or moons, are small- 

 er planets, revolving round the primary, 

 while they, with the primary planets, 

 about which they move, are carried round 

 the sun. The planets move round the 

 sun at various distances, some being much 

 nearer to him than our earth, and others 

 being much farther off. There are 11 

 primary planets, which are situated, with 

 respect to their distances from the sun, as 

 follows : Mercury $ ; Venus 9 ? tne 

 Earth ; Mars ; Ceres, Pallas, Juno, 

 Vesta, Jupiter ^r ; Saturn ^ ; and the 

 Ilerschel planet, or the Georgium Sidus 

 ft . (See plate I. Astronomy.) Of these 

 our earth is accompanied by one moon, 

 Jupiter has four moons, Saturn has seven 

 moons, and the Ilerschel planet has six 

 moons. None of these moons, except our 

 own, can be seen without a good tele- 

 scope. The other five planets do not ap- 

 pear to have any satellites, or moons. All 

 the planets move round the sun from west 

 to east, and in the same direction do the 

 moons revolve round their primaries, ex- 

 cepting those of the Herschel planet, 



VOL. II, 





which seem to move in a contrary direc- 

 tion. The paths in which they move 

 round the sun are called their orbits. 

 These orbits are elliptical ; but the ec- 

 centricity of the ellipses is so small, that 

 they approach very nearly to circles. 

 They perform their revolutions also in 

 very different periods of time. The time 

 of performing their revolutions is called 

 their year. The planets are evidently 

 opaque bodies, and they shine only by 

 reflecting the light which they receive 

 from the sun ; for Mercury and Venus, 

 when viewed by a telescope, often ap- 

 pear to be only partly illuminated, and 

 have the appearance of our moon when 

 she is cusped or horned, having the illu- 

 mined part alwaysturned towards the sun. 

 From the appearance of the boundary of 

 light and shadow upon their surfaces, we 

 conclude that they are spherical; which 

 is confirmed by some of them having been 

 found to turn periodically on their axes. 

 Venus and Mercury, being nearer to the 

 sun than our earth, are called inferior pla- 

 nets, and all the rest, which are without 

 the earth's orbit, are called superior pla- 

 nets. That the first go round the sun is 

 certain, because they are seen sometimes 

 passing between us and the sun,and some- 

 times they go behind it. That their or- 

 bits are within that of the earth is evident, 

 because they are never seen in opposi- 

 tion to the sun, that is, appearing to rise 

 from the horizon in the east when the sun 

 is setting in the west, which is another 

 proof that the earth is not the centre of 

 celestial motions. On the contrary, the 

 orbits of all the other planets surround 

 that of the earth ; for they sometimes are 

 seen in opposition to the sun, and they 

 never appear to be horned, but always 

 nearly or quite full, though sometimes 

 Mars appears a little gibbous, or some- 

 what deficient from full. 



Since all the planets move round the 

 sun in elliptical orbits, the sun itself is 

 situated in one of the foci of each ellipse. 

 That focus is called the lower focus. If 

 we suppose the plain of the earth's orbit, 

 which passes through the centre of the 

 sun, to be extended in every direction as 

 far as the fixed stars, it will mark out 

 among them a great circle, which is the 

 ecliptic ; and with this the situations of 

 the orbits of all the'other planets are com. 

 pared. The planes of the orbits of all 

 the other planets must necessarily pass 

 through the centre of the sun ; but if ex- 

 tended as far as the fixed stars, they form 

 circles different from one another, as also 

 from the ecliptic ; one part of each orbit 

 being on the north, and the other on the 



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