Apsides

Collier's New Encyclopedia (1921)

the plural of APSE or APSIS; in ast astronomy, the two points in the elliptic orbit of a planet where it is at the greatest and the least distance respec tively from the body around which it revolves. The moon moving in an elliptic orbit around the earth, which is situated in one of the foci, is at what was anciently called its higher apse when it is in apogee, and at its lower one when it is in perigee. Similarly, the primary planets, including the earth and comets, moving in in elliptic orbits around the sun, which is situated in one of the foci, pass through their higher apse when they are in aphelion, and their lower one when in perihelion. It is the same with the satellites of Jupiter when they are in the apojove ap and perijove. The line of the apsides is the line connecting the two apsides of a primary or secondary planet. Were it not for a motion of the apsides, it would exactly coincide with the major or longer axis of the ellipse. The progression of the moon's apsides is a slow movement in the position of the apsides of the moon, produced by the perturbing attraction of other heavenly bodies. It is about three degrees of angular motion, in one revolution of the moon, and in the same direction as her progression in her orbit. The apsides of the primary planets are also, to a certain extent, perturbed. The revolution of the moon's apsides is the movement of the apsides around the entire circumference of the ellipse, which takes place in 3,232.5753 mean solar days, or about nine years. A libration in planetary apsides is a movement sometimes forward and sometimes backward in the apsides of Venus and Mercury, from perturbations caused by other heavenly bodies.