SICT. Ill ACTION OP PLANETS ON SATELLITES. 25 



are, however, not exactly commensurable ; the points in 

 which the conjunctions take place are in advance each 

 time as much as 8*37 ; so that the conjunctions do not 

 happen exactly in the same points of the orbits till after 

 a period of 850 years; and, in consequence of this small 

 advance, the planets are brought into such relative posi- 

 tions that the inequality which seemed to threaten the 

 stability of the system is completely compensated, and 

 the bodies, having returned to the same relative positions 

 with regard to one another and the sun, begin a new 

 course. The secular variations in the elements of the 

 orbit increase the period of the inequality to 918 years 

 (N. 84). As any perturbation which affects the mean 

 motion affects also the major axis, the disturbing forces 

 tend to diminish the major axis of Jupiter's orbit and 

 increase that of Saturn's during one half of the period, 

 and the contrary during the other half. This inequality 

 is strictly periodical, since it depends upon the configura- 

 tion (N. 85) of the two planets ; and theory is confirmed 

 by observation, which shows that, in the course of twenty 

 centuries, Jupiter's mean motion has been accelerated 

 by about 3 23', and Saturn's retarded by 5 13'. Sev- 

 eral instances of perturbations of this kind occur in the 

 solar system. One, in the mean motions of the Earth 

 and Venus, only amounting to a few seconds, has been 

 recently worked out with immense labor by Professor 

 Airy. It accomplishes its changes in 240 years, and 

 arises from the circumstance of thirteen times the peri- 

 odic time of Venus being nearly equal to eight times 

 that of the Earth. Small as it is, it is sensible in the 

 motions of the Earth. 



It might be imagined that the reciprocal action of such 

 planets as have satellites would be different from the 

 influence of those that have none. But the distances of 

 the satellites from their primaries are incomparably less 

 than the distances of the planets from the sun, and from 

 one another; so that the system of a planet and its 

 satellites moves nearly as if all these bodies were united 

 in their common center of gravity. The action of the 

 sun, however, in some degree disturbs the motion of the 

 satellites about their primary. 



C 



