THE CONQUEST OF TIME AND SPACE 



instead of elliptical, and were the earth subject to no 

 gravitational influences except that of the sun and 

 moon, matters of astronomical computation would be 

 quite different from what they are. But as the case 

 stands, the axis of the earth is not only tipped at an 

 angle of about twenty-three degrees, but is subject to 

 sundry variations, due to the wobbling of the great top 

 as it whirls. 



Then the other planets, notably the massive Jupi- 

 ter, exert a perverting influence which constantly inter- 

 feres with the regular progression of the earth in its 

 annual tour about the sun. A moment's reflection 

 makes it clear that the gravitation pull of Jupiter is 

 exerted sometimes in opposition to that of the sun, 

 whereas at other times it is applied in aid of the sun, 

 and yet again at various angles. In short, on no two 

 successive days — for that matter no two successive 

 hours or minutes — is the perturbing influence of Jupi- 

 ter precisely the same. 



What applies to the earth applies also, of course, to 

 the varying action of Jupiter on the moon and to the 

 incessantly varied action of the moon itself upon the 

 earth. All in all, then, the course of our globe is by no 

 means a stable and uniform one; though it should be 

 understood that the perturbations are at most very 

 slight indeed as compared with the major motions that 

 constitute its regular action and lead to the chief phe- 

 nomena of day and night and the succession of the 

 seasons. 



Relatively slight though the perturbations may be, 

 however, they are sufficient to make noteworthy changes 



[38] 



