An apsis, plural apsides (pronounced /ˈæpsɨdiːz/), 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 περὶ, peri, around. The point of farthest excursion is called the apoapsis (ἀπό, apó, "from", which becomes ἀπ-, ap- or ἀφ-, aph- before an unaspirated or aspirated vowel, respectively), 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.
Derivative terms are used to identify the body being orbited. The most common are perigee and apogee, referring to orbits around the Earth (Greek γῆ, gê, "earth"), and perihelion and aphelion, referring to orbits around the Sun (Greek ἥλιος, hēlios, "sun"). During the Apollo program, the terms pericynthion and apocynthion were used when referring to the moon.
These formulae 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 (conservation of angular momentum) and the conservation of energy, these quantities are constant for a given orbit:
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