ASTRONOMY. 



tarbs their motions, and causes some ir- 

 regularities. It is this mutual attraction 

 between them and the sun, that prevents 

 them from flying off' from their orbits by 

 the centrifugal force which is generated 

 by their revolving in a curve, while the 

 centrifugal force keeps them from falling 

 into the sun by the force of gravity, as 

 they would do, if it were not for this mo- 

 tion impressed upon them. Thus these 

 two powers balance eack other, and pre- 

 serve order and regularity in the system. 

 It is well known, that if, when a body is 

 projected in a straight line, it be acted 

 upon by another force, drawing it towards 

 a centre, it will be made to describe a 

 curve, which will be either a circle or an 

 ellipsis, according to the proportion be- 

 tween the projectile and centripetal 

 force. If a planet at B (fig 3, Plate II.") 

 gravitates, or is attracted towards the sun, 

 S, so as to fall from B to y, in the time 

 that the projectile force would have car- 

 ried it from B to X, it will describe the 

 curve B Y, by the combined action of 

 these two forces, in the same time that 

 the projectile force singly would have 

 carried it from B to X, or the gravitating 

 power singly have caused it to descend 

 from B to y , and these two forces being 

 duly proportioned, the planet obeying 

 them both will move in the circle BYT 

 V. But if, whilst the projectile force 

 would carry the planet from B to b y the 

 sun's attraction should bring it down from 

 B to 1, the gravitating power would then 

 be too strong for the projectile force, and 

 would cause the planet to describe the 

 curve B C. When the planet comes to 

 C, the gravitating power (which always 

 increases as the square of the distance 

 from the sun, S, diminishes) will be yet 

 stronger for the projectile force, and by 

 conspiring in some degree therewith, 

 will accelerate the planets motion all the 

 way from C to K, causing it to describe 

 the arcs B C, C D, D E, E F, &c. all in 

 equal times. Having its motion thus ac- 

 celerated, it thereby acquires so much 

 centrifugal force, or tendency to fly off at 

 K, in the line K /?, as overcomes the sun's 

 attraction ; and the centrifugal force be- 

 ing to great too allow the planet to be 

 brought nearer to the sun, or even to 

 move round him in the circle k I m n, &c. 

 it goes off, and ascends in the curve K L 

 MN, &c. its motion decreasing as gradu- 

 ally from K to B as it increased from B to 

 K, because the sun's attraction now acts 

 against the planet's projectile motion just 

 as much as it acted with it before. When 

 the planet has got round to B, its projec- 

 tile force is as much diminished from its- 



mean state as it was augmented at K ; 

 and so th sun's attraction being more 

 than sufficient to keep the planet from 

 going off at B, it describes the same orbit 

 over again, by virtue of the same forces 

 or powers. A double projectile force 

 will always balance a quadruple power of 

 gravity. Let the planet at B have twice 

 as great an impulse from thence towards 

 X as it had before ; that is, in the same 

 length of time that it was projected from 

 B to b, as in the last example; let it novv be 

 projected from B to c, and it will require 

 four times as much gravity to retain it in its 

 orbit ; that is, it must fall as far as from B 

 to 4 in the time that the projectile force 

 would carry it from B to C, otherwise it 

 would not describe the curve B D, as is 

 evident from the figure. But in as much 

 time as the planet moves from B to C, in 

 the higher part of its orbit, it moves from 

 I to K, or from K to L, in the lower part 

 thereof; because, from the joint action of 

 these two forces, it must always describe 

 equal areas in equal times throughout its 

 annual course. These areas are repre- 

 sented by the triangles B S C, C S D, DSE, 

 E S F, &c. whose contents are equal to 

 one another, from the properties of the 

 ellipsis. We have now given a general 

 idea of the solar system ; we shall next 

 describe the bodies that compose it. 



Of the sun. The sun, as the rnost-con- 

 spicuous and most important or' ;iil the 

 heavenly bodies, would naturally claim the 

 first place in the attention of astronomers. 

 Accordingly, its motions were first stu- 

 died, and they have had considerable in- 

 fluence on all the other branches of the 

 science. That the sun has a motion of 

 its own, independent of the apparent di- 

 urnal motion common to all the heavenly 

 bodies, and in a direction contrary to that 

 motion, is easily ascertained, by observing 

 with care the changes which take place 

 in the starry hemisphere during a com- 

 plete year. If we note the time at which 

 any particular star rises, we shall find that 

 it rises somewhat sooner every success, ve 

 day, till at last we lose it altogether in 

 the west But if we note it after the in- 

 terval of a year, we shall find it rising pre- 

 cisely at the same hour as at first. Those 

 stars which are situated nearly in the 

 track of the sun, and which set soon af':oi- 

 him, in a few evenings lose themselves 

 altogether in his rays, and afterwards 

 make their appearance in the east before 

 sunrise. The sun then moves towards 

 them in a direction contrary to his diurnal 

 motio,n. It was by observations of this 

 kind that the ancients ascertained his or. 



