Apsis
An
apsis (Greek ἁψίς, gen. ἁψίδος), plural
apsides (/ˈæpsɨdiːz/; Greek: ἁψίδες), is a point of greatest or least distance of a body in an elliptic orbit about a larger body. For a body orbiting the Sun the greatest and least distance points are called respectively
aphelion and
perihelion (/æpˈhiːliən/, /ˌpɛrɨˈhiːliən/), whereas for any satellite of Earth including the Moon the corresponding points are
apogee and
perigee (/ˈpɛrɨdʒiː/). The generic suffix, independent of the particular central body, can be either
apsis or
centre, hence
apoapsis,
apocentre or
apapsis (from ἀπ(ό)
(ap(ó)), meaning "from"), and
periapsis or
pericentre (from περί
(peri), meaning "around"). During the Apollo program, the terms
pericynthion and
apocynthion (referencing Cynthia, an alternative name for the Greek Moon goddess Artemis) were used when referring to the Moon. A straight line connecting the periapsis and apoapsis is the
line of apsides. This is the major axis of the ellipse, its greatest diameter.