xxxiv INTRODUCTION TO ASTRONOMY. 



the quantity of matter, but to the degree of proximity of the attracting 

 body. This power being weakened by diffusion, diminishes as the squares 

 of the distances increase. The square is the product of a number multi- 

 plied by itself; so that a planet situated at twice the distance at which 

 we are from the sun would gravitate four times less than we do, the pro- 

 duct of two multiplied by itself being four. The more distant planets, 

 therefore, move slower in their orbits, for their projectile force must be 

 proportioned to that of attraction. This diminution of attraction by the 

 increase of distance also accounts for the motion of the secondary round 

 the primary planets, in preference to the sun ; for the vicinity of the 

 primary planets renders their attraction stronger than that of the sun. 

 But since tjje attraction between bodies is mutual, the primary planets are 

 also attracted by their satellites. The moon attracts the earth, as well as 

 the earth the moon ; but as the latter is the smaller body, her attraction 

 is proportionally less. The result is, that neither the earth revolves 

 round the moon, nor the moon round the earth ; but they both revolve 

 round a point, which is their common centre of gravity, and which is 

 as much nearer the earth than the moon, as the weight of the former 

 exceeds that of the latter. It has been already stated (p. xix.) that if 

 two bodies were fastened together by a wire or bar, their common centre 

 of gravity would be in the middle of the bar, provided the bodies were 

 of equal weight ; and if they differed in weight, it would be nearer the 

 larger body. Attraction is the tie which unites the earth and moon ; 

 and if these bodies had no projectile force which prevented their mutual 

 attraction from bringing them together, they would meet at their common 

 centre of gravity. 



The earth then has three different motions : it revolves round the sun, 

 upon its axis, and round the point towards which the moon attracts it; 

 and this is the case with every planet which is attended by satellites. The 

 complicated effect of this variety of motions produces certain irregularities, 

 which, however, it is not necessary to notice at present. The planets act 

 on the sun in the same manner as they are themselves acted on by their 

 satellites ; but the gravity of the planets (even when taken collectively) is 

 so trifling compared with that of the sun, that they do not cause the latter 

 to move so much as one-half of its diameter. The planets do not, there- 

 fore, revolve round the centre of the sun, but round a point at a small 

 distance from its centre, about which the sun also revolves. The sun 

 also revolves on his axis. This motion is ascertained by observing certain 

 spots which disappear and re-appear regularly at stated times. 



The great distance of the planets renders their motion apparently so 

 slow, that the eye is not sensible of their progress in their orbit, unless 

 we watch them for some considerable length of time : in different seasons 

 they appear in different parts of the heavens. The most accurate idea 

 which can be 'given of the situation and motion of the planets will be 

 by the examination of the diagram (fig. 5), representing the solar system, 

 in which the principal planets, with their orbits, are delineated. The sun 

 is in the common centre of the whole, but, to avoid confusion in the figure, 

 he is not represented. 



The orbits of the planets are so nearly circular, and the common centre 

 of gravity of the solar system so near the centre of the sun, that these 

 deviations are not noticed in the diagram. The dimensions of the pla- 

 nets, in their true proportions, will be found delineated in Jig. 6 : the 

 signs annexed to them are those used to represent the planets, which are 

 also used in fig. 1. 



