SECT. xi. J PRECESSI 



UNIVERSITY 



SECTION XL 



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 sphe- 

 roid 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 inclined to the plane of the ecliptic at an 

 angle of 23 27' 34"'69 ; 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 sphe- 

 roid, urge the plane of the equator from its direction, and 

 force it to move from east to west, so that the equinoctial 

 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 coin- 

 cide, but it is balanced by the tendency of the earth to re- 

 turn to stable rotation 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 pre- 

 serves the same inclination to the plane of the horizon. 

 Were the earth spherical, this effect would not be produced, 



