Apsides

An apsis is one of two extreme points in an orbit around something. The furthest point is the apoapsis or apocentre; the closest point is the periapsis or pericentre.

The minimum orbital speed occurs at the apoapsis; the maximum orbital speed occurs at the periapsis.

Words

The plural of apsis is apsides.1 There are many -apsis words in English which refer to distances in the orbit around some body (though most of them are not commonly used). These include:

apoapsis periapsis orbiting
apogee perigee the Earth
aphelion perihelion the Sun
apocynthion pericynthion the Moon
apolune perilune the Moon
apohermion perihermion Mercury
apocythe pericythe Venus
apoareion periareion Mars
apojove perijove Jupiter
apochron perichron Saturn
apodemeter peridemeter Ceres
apastron periastron any star other than the Sun
apogalacticon perigalacticon the centre of a galaxy
  1. Pronounced /ˈæpsɪdiːz/