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"Apogee", "Aphelion", "Perigee" and "Perihelion" redirect here. For the literary journal, see Perigee: Publication for the Arts. For Edenbridge's Album, see Aphelion (album). For the architectural term, see Apse. For other uses, see Apogee (disambiguation) and Perihelion (disambiguation).

An apsis (Greek ἁψίς, gen. ἁψίδος), plural apsides (pron.: /ˈæpsɨdiːz/; Greek: ἁψίδες), is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system. Historically, in geocentric systems, apsides were measured from the center of the Earth.

The point of closest approach (the point at which two bodies are the closest) is called the periapsis or pericentre, from Greek περί, peri, around, and κέντρον, kentron, centre. The point of farthest excursion is called the apoapsis, apocentreor apapsis (ἀπ(ό), ap(ó), "from") (the last of these terms, 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.

Derivative terms are used to identify the body being orbited. The most common are perigee pron.: /ˈpɛrɨdʒiː/ and apogee pron.: /ˈæpɵdʒiː/, referring to orbits around the Earth (Greek γῆ, gê, "earth"), and perihelion pron.: /ˌpɛrɨˈhiːliən/ (Greek meaning, "near the Sun") and aphelion pron.: /əˈfiːliən/ (Greek meaning, "away from the Sun"), referring to orbits around the Sun (Greek ἥλιος, hēlios, "sun"). 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.^[1]

Contents [hide] 1 Mathematical formulae 2 Terminology 3 The perihelion and aphelion of the Earth 4 Planetary perihelion and aphelion 5 See also 6 References 7 External links

[edit]Mathematical formulae

These formulas characterize the periapsis and apoapsis of an orbit:

• Periapsis: maximum speed  at minimum (periapsis) distance 


• Apoapsis: minimum speed  at maximum (apoapsis) distance 

while, in accordance with Kepler's laws of planetary motion (based on the conservation of angular momentum) and the conservation of energy, these two quantities are constant for a given orbit:

• specific relative angular momentum