Variation in the Obliquity of the Ecliptic. — Although it is sufficiently near for all general purposes to consider the inclination of the plane of the ecliptic as invariable; yet this is not strictly the case, inasmuch as it is subject to a small but appreciable change of about 48″ per century. This phenomenon has long been known to astronomers, on account of the increase it gives rise to, in the latitude of all stars in some situations, and corresponding decrease in the opposite regions. Its effect at the present time is to diminish the inclination of the two planes of the equator and the ecliptic to each other; but this dimimition will not go on beyond certain very moderate limits, after which it will again increase, and thus oscillate backwards and forwards through an arc of 1 ° 21 ′: the time occupied in one oscillation being about 10,000 years. One effect of this variation of the plane of the ecliptic—that which causes its nodes on a fixed plane to change—is associated with the phenomena of the precession of the equinoxes, and undistinguishable from it, except in theory.

Precession.—The precession of the equinoxes is a slow but continual shifting of the equinoctial points from East to West.