PRECESSION OF THE EQUINOXES. 59 



nox, and let the apparent positions of the ecliptic and the equator, 

 or rather portions of them, be represented by the dotted lines, and 

 suppose some star S, to lie directly in the equinoctial point, or 

 node, as seen from the earth at H. Suppose the sun, commencing 

 from the point B, or S, to move around in the direction B A D C, 

 it is evident, that if the crossing point still corresponded with the 

 star S, or remained unchanged, the sun would arrive at B, or S, 

 after an interval equal to a siderial year. But this is not the case, 

 the plane of the equator E D F B, is not fixed, but while the sun 

 is performing his journey, it moves slowly backward on the ecliptic 

 contrary to the apparent yearly motion of the sun in the heavens, 

 so that, in about .the time of a year, the crossing points are at N 

 and O, and in the heavens the position of the vernal equinox will 

 appear to have shifted, contrary to the order of the signs, from S 

 to' T ; hence, as the sun arrives at T before it can come to S, the 

 equinoctial year is shorter than the siderial year. This shifting of 

 the nodes is called the Precession of the Equinoxes, because the 

 equinox seems to go forward to meet the sun, and thus precedes 

 the complete revolution of the sun in the ecliptic. Now this 



change of place, in the position of the equinox, we infer very 



