In astronomy, an apsis (plural apsides IPA: ˈæpsɪdiːz) is the point of greatest or least distance of the elliptical orbit of a celestial body from its center of attraction, which is generally the center of mass of the system.
The point of closest approach is called the periapsis or pericenter and the point of farthest excursion is the apoapsis (Greek ἀπο, from), apocenter or apapsis (the latter term, although etymologically more correct, is much less used). A straight line drawn through the periapsis and apoapsis is the line of apsides. This is the major axis of the ellipse, the line through the longest part of the ellipse.
Related terms are used to identify the body being orbited. The most common are perigee and apogee, referring to Earth orbits, and perihelion and aphelion, referring to orbits around the Sun (Greek ‘ἥλιος hēlios sun).
There are formulae used to derive apsis and periapsis:
- Periapsis: maximum speed at minimum distance (periapsis distance)
- Apoapsis: minimum speed at maximum distance (apoapsis distance)
where one easily verifies
- is the semi-major axis
- is the eccentricity
- is the specific relative angular momentum
- is the specific orbital energy
- is the standard gravitational parameter
Note that for conversion from heights above the surface to distances, the radius of the central body has to be added, and conversely.
The geometric mean of the two speeds is , the speed corresponding to a kinetic energy which, at any position of the orbit, added to the existing kinetic energy, would allow the orbiting body to escape (the square root of the sum of the squares of the two speeds is the local escape velocity).
The words "pericentre" and "apocentre" are occasionally seen, although periapsis/apoapsis are preferred in technical usage.
Various related terms are used for other celestial objects. The '-gee', '-helion' and '-astron' and '-galacticon' forms are frequently used in the astronomical literature, while the other listed forms are occasionally used, although '-saturnium' has very rarely been used in the last 50 years. The '-gee' form is commonly (although incorrectly) used as a generic 'closest approach to planet' term instead of specifically applying to the Earth. The term peri/apomelasma (from the Greek root) was used by Geoffrey A. Landis in 1998 before peri/aponigricon (from the Latin) appeared in the scientific literature in 2002.
|Body||Closest approach||Farthest approach|
Since "peri" and "apo" are Greek, it is considered by purists  more correct to use the Greek form for the body, giving forms such as '-zene' for Jupiter and '-krone' for Saturn. The daunting prospect of having to maintain a different word for every orbitable body in the solar system (and beyond) is the main reason why the generic '-apsis' has become the almost universal norm.
- In the Moon's case, in practice all three forms are used, albeit very infrequently. The '-cynthion' form is, according to some, reserved for artificial bodies, whilst others reserve '-lune' for an object launched from the Moon and '-cynthion' for an object launched from elsewhere.
- For Venus, the form '-cytherion' is derived from the commonly used adjective 'cytherean'; the alternate form '-krition' (from Kritias, an older name for Aphrodite) has also been suggested.
- For Jupiter, the '-jove' form is occasionally used by astronomers whilst the '-zene' form is never used, like the other pure Greek forms ('-areion' (Mars), '-hermion' (Mercury), '-krone' (Saturn), '-uranion' (Uranus), '-poseidion' (Neptune) and '-hadion' (Pluto)).