THE UNIVERSE IN WHICH WE LIVE 15 



The second effect the moon produces on the earth is the pre- 

 cession of the equinoxes. The sun again plays the minor role. 

 The earth is a sphere with an added bulge about the equatorial 

 regions (an oblate spheroid see Fig. 6 A). The moon's orbit is 

 inclined to the plane of the ecliptic only 5, so the sun and moon 

 are pulling on the earth practically in the same plane." Since 

 the earth's axis is inclined 23^, this equatorial bulge is in large 



FIG. 6. 04) Diagram showing the earth's equatorial bulge and its action in 

 causing the precession of the equinoxes; (B) diagram showing the effect of the 

 moon's attraction on the motion of the earth. 



measure above or below the plane of the ecliptic. Therefore, 

 the sun and moon tend to pull the bulge back into the plane; 

 that is, the pull of the moon (and sun) acting on the bulge along 

 the line a-b is resolved into two forces, one component acting 

 along b-c. Points along the Equator such as b (Fig. 6B) are 

 therefore under the stress of two forces, one be, this pull toward 

 the plane of the ecliptic, the other, the momentum of the earth's 

 rotation indicated by bd, the resultant being a motion along, say, 

 be; that is, a point on the Equator at every turn of the earth 



