26 INEQUALITY OF THE EARTH AND VENUS. SECT. III. 



vance, the planets are brought into such relative positions, that 

 the inequality, which seemed to threaten the stability of the 

 system, is completely compensated, and the bodies, having re- 

 turned 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 configuration (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'. 

 Several 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 labour by Professor Airy. It accomplishes its 

 changes in 240 years, and arises from the circumstance of thirteen 

 times the periodic 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 centre of gravity. The action of the 

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

 about their primary. 



