SECT. XI. PEECESSION. 



SECTION XI. 



Precession and Nutation Their Effects on the Apparent Places of the 

 Fixed Stars. 



IT has been shown that the axis of rotation is invariable on the 

 surface of the earth ; and observation as well as theory prove 

 that, were it not for the action of the sun and moon on the matter 

 at the equator, it would remain exactly parallel to itself in every 

 point of its orbit. 



The attraction of an external body not only draws a spheroid 

 towards it, but, as the force varies inversely as the square of the 

 distance, it gives it a motion about its centre of gravity, unless 

 when the attracting body is situated in the prolongation of one 

 of the axes of the spheroid. The plane of the equator is in- 

 clined to the plane of the ecliptic at an angle of 23 27' 28" '29 ; 

 and the inclination of the lunar orbit to the same is 5 8' 47"'9. 

 Consequently, from the oblate figure of the earth, the sun and 

 moon, acting obliquely and unequally on the different parts of the 

 terrestrial spheroid, urge the plane of the equator from its direc- 

 tion, and force it to move from east to west, so that the equi- 

 noctial points have a slow retrograde motion on the plane of the 

 ecliptic of 50" - 41 annually. The direct tendency of this action 

 is to make the planes of the equator and ecliptic coincide, but it 

 is balanced by the tendency of the earth to return to stable rota- 

 tion about the polar diameter, which is one of its principal axes 

 of rotation. Therefore the inclination of the two planes remains 

 constant, as a top spinning preserves the same inclination to the 

 plane of the horizon. Were the earth spherical, this effect would 

 not be produced, and the equinoxes would always correspond 

 with the same points of the ecliptic, at least as far as this kind 

 of motion is concerned. But another and totally different cause 

 which operates on this motion has already been mentioned. The 

 action of the planets on one another and on the sun occasions a 

 very slow variation in the position of the plane of the ecliptic, 

 which affects its inclination to the plane of the equator, and gives 



