34 



THE POPULAR SCIENCE MONTHLY. 



to the other bodies in space. Owing to the attractive force of the 

 other planets, it is slowly revolving in the same direction that the 

 earth moves in its orbit. The rate of revolution of this line of the 

 apsides or major axis is a little over 11 6" a year. 



We ordinarily speak of the earth's axis as being always parallel to 

 itself. It is, in fact, however, subject to a very slow gyratory motion, 

 caused by the attraction of the sun upon the meniscus, or ring of mat- 

 ter bulging at the equator, which motion is retrograde, or in an oppo- 

 site direction to the revolution of the earth. This retrograde motion 

 causes the phenomenon known as the precession of the equinoxes. Its 

 rate is about 50'3" a year. 



It will thus appear that a vast period of time will elapse before the 

 earth's axis will occupy the same position with respect to the line of 

 the apsides that it occupies to-day. This period is sometimes called 

 the Great Year. Its length may be ascertained by reducing three 

 hundred and sixty degrees to seconds, and dividing this quotient by 

 the annual amount of the two motions described above : 



1,296,000" -^ (50-3" + 11-6') = ^ffj?,-^' = 20,937. 



The Great Year is thus seen to be about 21,000 years in length. 



A, Perihelion ; B, Aphelion ; E, Winter Solstice ; F, Summer Solstice; D, Autumnal Equinox; 



C, Vernal Equinox. 



I have here drawn a diagram illustrating the present position of 

 the earth's orbit with respect to the line of the apsides, purposely 

 exaggerating the eccentricity of the ellipse in order to better present 

 it to the eye of the observer. In the diagram, A B is the line of the 



