PRECESSION (IIL. prffcessio, advance, pre cedence. from Lat. prtreedcre. to go before). The points in which the equator intersects the ecliptic, called the equinoctial points, do not remain sta tionary, hut retrograde slowly, i. e. move from east to west. This motion is called the precession of the equinoxes (q.v.). The word 'precession' is used because if on one day one of the equi noctial points arrive at the meridian of a place simultaneously with a fixed star, it will next day arrive at the meridian sooner than the star. or will prccide it in transit. The amount of this movement is about 50" each year, and the equinoctial points will therefore require 25,800 years to make a complete circuit in the heavens. This movement of the equinoxes also explains the want of coincidence between the signs of the zodiac and those of the ecliptic. See AntEs.
If the earth were truly spherical and homo gi neon:; or if it were composed of spherical layers each of uniform density; or, more general ly. if it were such that the resultant of the gravi tational attractions exerted on all its parts by any other body should always pass through a single definite point in its mass, its diurnal rotation would not be affected by the attractions of any other bodies. If originally rotating about a
principal axis of inertia, it would forever revolve about it, and the direction of the axis would re main fixed in space. To put this in more popu lar language. the pole star (q.v.) would always be the same star. But, although the earth ro tates about an axis almost exactly coinciding with its axis of figure. the attraction of various bodies, especially the sun and moon. on the oblate protuberant portion at the equator, tends to give it a rotation about an axis in the plane of the equator; and the combination of these two rotations gives rise to a shifting of the instan taneous axis of rotation in the earth and also in space. If this attracting force were constant. the pole of the equator would move about the pole of the ecliptic in a circle. But, owing to a disturbance called inflation (q.v.), produced by the unequal attraction of the moon, this motion is waved and not exactly circular. Thus the plane of the equator does not remain fixed, and consequently the points in which the equator intersects the ecliptic will also vary, and this causes the precession of the equinoxes. See X.kft THEORY.