NEWTON S PRINCIPIA 343 



in the second quarter-month than the first ; that is, the 

 effect will be greater while the moon is approaching the 

 equator, than while it is receding from it, that is, by what 

 precedes, the whole effect of the disturbing force is to lessen 

 the diminution of i. Similarly, when i is increasing, this 

 increase is lessened by the action of the disturbing force. 

 This is exactly what would take place if we neglected this 

 disturbing cause and supposed the obliquity of the ecliptic 

 to be less than it really was. Hence we arrive at this 

 conclusion, which we shall state in the words of Laplace : 

 &quot; The non-sphericity of the earth produces in the lati 

 tude of the moon but one sensible inequality. We can 

 represent its effect by supposing the orbit of the moon 

 instead of moving in the plane of the ecliptic with a con 

 stant inclination, to move with the same condition on a 

 plane passing through the equinoxes between the ecliptic 

 and equator. This inequality is well adapted for deter 

 mining the ellipticity of the earth.&quot; 



Y. 4 



