SEQUEL TO THE EPOCH OF NEWTON. 235 



the Obliquity of the Ecliptic, which has been going 

 on from the earliest times to the present. This 

 change has been explained by theory, and shown to 

 have, like all the other changes of the system, a 

 limit, after which the diminution will be converted 

 into an increase. 



We may mention here some subjects of a kind 

 somewhat different from those just spoken of. The 

 true theoretical quantity of the Precession of the 

 Equinoxes, which had been erroneously calculated 

 by Newton, was shown by D'Alembert to agree with 

 observation. The constant coincidence of the Nodes 

 of the Moon's Equator with those of her Orbit, was 

 proved to result from mechanical principles by La- 

 grange. The curious circumstance that the Time of 

 the Moon's rotation on her axis is equal to the 

 Time of her revolution about the earth, was shown 

 to be consistent with the results of the laws of 

 motion by Laplace. Laplace also, as we have seen, 

 explained certain remarkable relations which con- 

 stantly connect the longitudes of the three first 

 satellites of Jupiter ; Bailly and Lagrange analyzed 

 and explained the curious librations of the nodes 

 and inclinations of their orbits ; and Laplace traced 

 the effect of Jupiter's oblate figure on their motions, 

 which masks the other causes of inequality, by 

 determining the direction of the motions of the 

 perijove and node of each satellite. 



