apsides

pl. [APSE.] The plural of the form APSIS (q.v.). I. Generally . II. Technically ( Astron .): The two points in the elliptic orbit of a planet where it is at the greatest and at the least distance respectively 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 elliptic orbits around the sun, which is situated in one of the foci, pa 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 apojove and perijove. Line of the apsides: 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. Let ADB be A D B the orbit of the moon, of which the eccentricity has been purposely exaggerated, and let c be the earth; then a and Bare the two lunar apsides. Progre ion of the moon's apsides : A slow movement in the position of the apsides of the Revolution of the moon's apsides : The movement of the apsides around the entire circumference of the ellipse, which takes place in 3232 5753 mean solar days, or about nine years. Libration in planetary apsides : A movement sometimes forward and sometimes backward in the apsides of Venus and Mercury, from perturbations caused by other heavenly bodies. ǎp'-sis, s. [APSE.]