1911 Encyclopædia Britannica/Precession of the Equinoxes

PRECESSION OF THE EQUINOXES, in astronomy, the term assigned to the progressive motion of the equinox, because it takes place in a direction from east towards west, opposite to that in which planets move, and in which longitudes are measured. The equinox being defined as the point of intersection of the equator and ecliptic, its motion arises from the fact that both of these great circles are in continuous though slow motion. The motion of the ecliptic is due to the action of the planets on the earth, which produces a slow progressive change in the position of the plane of the earth's orbit, and therefore of the ecliptic. This motion takes place round a diameter of the celestial sphere as an axis or nodal line, which intersects the sphere in two points, which are at present in longitudes about 173° and 353°. The direction of the motion around this axis is such that from the limits 353° through 0° to 173°, which includes the vernal equinox, the motion is towards the south, while, in the remainder of the circle, it is towards the north. At the present time the rate of the motion is 46.7" per century. In consequence of the smallness of the angle, 7°, which the axis of motion makes with the line of the equinoxes, its effect on the precession is quite small, now amounting to only 0.14” per annum. Owing to its cause this small part of the precession is called “ planetary.”

The motion of the equator is due to the combined action of the sun and moon on the equatorial protuberance of the earth (see Astronomy). Owing to its cause this largest part of the precession is called “ luni-solar.” Its fundamental law is that the mean celestial pole at each instant (see Nutation) moves at right angles to the circle joining it to the pole of the ecliptic as that instant. Hence if the pole of the ecliptic were fixed, the celestial pole would revolve around it in a circle at a constant distance equal to the obliquity of the ecliptic. Owing, however, to the slow change in the position of the pole of the ecliptic, the motion is only approximately in a circle, and the obliquity varies slowly from century to century. At the present time the rate of motion measured on a great circle is about 20″ per year; that is to say both the pole and the plane of the equator move through this angle annually. But when measured around the pole of the ecliptic as a centre the motion is about 2.5 times this or, at present, 50.37″ annually. This is the present amount of the luni-solar precession which, if it remained constant, would carry the pole completely round in a period of 25,730 years. But the exact period varies slightly, owing to the motion of the pole of the ecliptic. The combined effect of the luni-solar and planetary precession or the total motion of the equinox is called the general precession. Its annual amount during our time is 50.2564+0.02220″ T, T being the time reckoned from 1900 in centuries.