Ap'sides

Ap'sides (Gr. apsis, 'connection'), the two extreme points in the orbit of a planet—one at the greatest, the other at the least distance from the sun. The term apsidies is also applied in the same manner to the two points in the orbit of a satellite—one nearest to, the other farthest from, its primary; corresponding, in the case of the moon, to the perigee and apogee. A right line connecting these extreme points is called the line of apsidies. In all the planetary orbits, this line has no fixed position in space, but makes a forward motion in the plane of the orbit, except in the case of the planet Venus, where the motion is retrograding. This fact in the orbit of the earth gives rise to the Anomalistic Year (q.v.). This advancing motion of the line of apsidies is especially remarkable in the orbit of the moon, where it amounts to $40^\{\backslash circ\}\; 40\text{'}\; 32\text{'}\text{'}\; \backslash cdot\; 2$ annually, an entire revolution thus taking place in rather less than nine years. See MOON, PERTURBATIONS.