The New Student's Reference Work/Precession
Preces′sion, is the name given to that slow motion of the earth which causes the equinoxes to recede slowly along the ecliptic (apparent path of the sun around the earth), so that the sun comes to them in its annual revolution a little earlier than it otherwise would. The cause of this motion is the attraction of the sun and moon upon the protuberant part of the earth about the equator — the equatorial diameter of the earth being 26 miles longer than the polar diameter. The moment of force thus exerted tends to tilt the earth, which, like a top, “precesses” instead of falling over. As this disturbing force on the earth is very small relatively to its mass, this turning of its axis about the pole of the ecliptic takes place at about the rate of 50.1″ per annum, and it therefore requires 25,868 years for the equinoxes to describe a complete circle on the ecliptic. See Earth, Pole-Star and Top.