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 th…