Page:Popular Science Monthly Volume 15.djvu/854

834 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 matter bulging at the equator, which motion is retrograde, or in an opposite 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:

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

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