Apsis

An apsis (from Ancient Greek ἁψίς (hapsís) 'arch, vault' (third declension); pl. apsides AP-sih-deez) is one of the two extreme points in the orbit of a planetary body about its primary body, either the farthest point (apoapsis) or the nearest point (periapsis). The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two apsides. Apsides pertaining to orbits around different bodies have distinct names to differentiate themselves from other apsides. Apsides pertaining to geocentric orbits, orbits around the Earth, are at the farthest point called the apogee, and at the nearest point the perigee, as with orbits of satellites and the Moon around Earth. Apsides pertaining to orbits around the Sun are named aphelion for the farthest and perihelion for the nearest point in a heliocentric orbit. Earth's two apsides are the farthest point, aphelion, and the nearest point, perihelion, of its orbit around the host Sun. The terms aphelion and perihelion apply in the same way to the orbits of Jupiter and the other planets, the comets, and the asteroids of the Solar System.