# Perihelion and Aphelion refer to orbits around the Sun.

A diagram of Keplerian orbital elements.

In astronomy, an apsis, plural apsides 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 pericentre and the point of farthest excursion is the apoapsis (Greek ap, from, which becomes ap before a vowel, and af before rough breathing), apocentre 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 orbits around the Earth, and perihelion and aphelion, referring to orbits around the Sun (Greek helios sun).

Formulae

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

(each the same for both points, like they are for the whole orbit, in accordance with Kepler's laws of planetary motion (conservation of angular momentum) and the conservation of energy)

where:

• is the Semi-major axis.
• is the eccentricity.
• is the specific relative angular momentum.
• is the specific orbital energy.
• is the standard gravitational parameter.

Properties:

Note that for conversion from heights above the surface to distances, the radius of the central body has to be added, and conversely.

The arithmetic mean of the two distances is the semi-major axis . The geometric mean of the two distances is the Semi-minor axis .

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).

Terminology

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.

BodyClosest approachFarthest approach
Galaxy Perigalacticon Apogalacticon
star Periastron Apastron
black hole Perimelasma/Perinigricon Apomelasma/Aponigricon
Sun Perihelion Aphelion
Earth Perigee Apogee
Moon Periselene/Pericynthion/Perilune Aposelene/Apocynthion/Apolune
Jupiter Perizene/Perijove Apozene/Apojove
Saturn Perikrone/Perisaturnium Apokrone/Aposaturnium

Since "peri" and "apo" are Greek, it is considered by some 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. The '-cynthion' form was the version used in the Apollo Project, following a NASA decision in 1964.
• 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)).

Why not also search for...

• Eccentric anomaly.
• elliptic orbit.
• perigee.

Go To Print Article

 Universe - Galaxies and Stars: Links and Contacts
 the web this site