"perigree of the moon, perihelion of a planet" (plural apsides), 1650s, from Latin apsis "arch, vault" (see apse).
| apsis (āp'sĭs) Plural apsides (āp'sĭ-dēz') In the path of an orbiting body, either of the two points at which it is closest to or farthest away from the body it is orbiting. See also apoapsis, periapsis. |
in astronomy, either of the two points on an elliptical orbit that are nearest to, and farthest from, the focus, or centre of attraction. The line of apsides, connecting the two points, is the major axis of the orbit. The point nearest the focus is the pericentre, or periapsis, and that farthest from it is the apocentre, or apoapsis. Specific terms can be used for individual bodies: if the Sun is the centre, the specific terms perihelion and aphelion are generally used; if the Earth, perigee and apogee. Periastron and apastron refer to an orbit around a star, and perijove and apojove refer to an orbit around Jupiter