228 HISTORY OF PHYSICAL ASTRONOMY. 



real motions of the planets and their orbits are 

 rendered still further intricate by this, that all the 

 lines and points to which we can refer them, are 

 themselves in motion. The task of carrying order 

 and law into this mass of apparent confusion, has 

 required a long series of men of transcendent intel- 

 lectual powers ; and a perseverance and delicacy of 

 observation, such as we have not the smallest 

 example of in any other subject. It is impossible 

 here to give any detailed account of these labours ; 

 but we may mention one instance of the complex 

 considerations w r hich enter into them. The nodes 

 of Jupiter's fourth satellite do not go backwards 24 , 

 as the Newtonian theory seems to require; they 

 advance upon Jupiter's orbit. But then, it is to be 

 recollected that the theory requires the nodes to 

 retrograde upon the orbit of the perturbing body, 

 which is here the third satellite; and Lalande 

 showed that, by the necessary relations of space, 

 the latter motion may be retrograde though the 

 former is direct. 



Attempts have been made, from the time of the 

 solution of the problem of three bodies to the pre- 

 sent, to give the greatest possible accuracy to the 

 Tables of the Sun, by considering the effect of the 

 various perturbations to which the earth is subject. 

 Thus, in 1756, Euler calculated the effect of the 

 attractions of the planets on the earth (the prize- 

 question of the French Academy of Sciences) and 



24 Bailly, iii. 175. 



