$66 OUTLINES OF NATURAL PHILOSOPHY. 



This inequality, which affects all the heavenly bodies^ 

 was discovered by Dr BRADLEY, while employed 

 in verifying his theory of the aberration. 



The period of the changes of this inequality was ob- 

 served to be eighteen years nearly, the same with 

 the period of the revolution of the Moon's nodes ; 

 and it was soon found, that the quantity of the in- 

 equality depended on the place of the node. Cer- 

 tain theoretical considerations thus led to the dis- 

 covery of the precise form which this inequality 

 assumes, when analytically expressed, and reduced 

 into a formula. It is not certain that observation 

 alone would have led to this conclusion ; but 

 there is no doubt that the places of the heavenly 

 bodies, when corrected by means of it, agree much 

 better with observation than when the correction 

 is not applied. 



218. The phenomena of the, nutation maybe 

 represented by supposing, that while a point, 

 which may be considered as defining the mean 

 place of the pole of the equator, describes a cir- 

 cle in the heavens, round the pole of the eclip- 

 tic, at a distance from it equal to the mean ob- 

 liquity of the ecliptic, and with a retrograde mo- 

 tion of 50" annually ; another point, represent- 

 ing the actual pole of the equator, moves round 

 the former at the distance of 9", so as to be al- 

 ways 90 more easterly than the Moon's ascend- 

 ing 



