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An apsis (Greek: ἁψίς; plural apsides , Greek: ἁψῖδες) is an extreme point in an object's orbit. The word comes via Latin from Greek and is cognate with apse. For elliptic orbits about a larger body, there are two apsides, named with the prefixes peri- (from περί (peri), meaning 'near') and ap-, or apo- (from ἀπ(ό) (ap(ó)), meaning 'away from') added to a reference to the thing being orbited. For a body orbiting the Sun, the point of least distance is the perihelion (), and the point of greatest distance is the aphelion (). The terms become periastron and apastron when discussing orbits around other stars. For any satellite of Earth including the Moon the point of least distance is the perigee () and greatest distance the apogee. For objects in Lunar orbit, the point of least distance is the pericynthion () and the greatest distance the apocynthion (). Perilune and apolune are also used. For any orbit around a center of mass, there are the terms periapsis and apoapsis (or apapsis). Pericenter and apocenter are equivalent alternatives. A straight line connecting the periapsis and apoapsis is the line of apsides. This is the major axis of the ellipse, its greatest diameter. For a two-body system the center of mass of the system lies on this line at one of the two foci of the ellipse. When one body is sufficiently larger than the other it may be taken to be at this focus. However whether or not this is the case, both bodies are in similar elliptical orbits each having one focus at the system's center of mass, with their respective lines of apsides being of length inversely proportional to their masses. Historically, in geocentric systems, apsides were measured from the center of the Earth. However, in the case of the Moon, the center of mass of the Earth–Moon system, or Earth–Moon barycenter, as the common focus of both the Moon's and Earth's orbits about each other, is about 75% of the way from Earth's center to its surface. In orbital mechanics, the apsis technically refers to the distance measured between the centers of mass of the central and orbiting body. However, in the case of spacecraft, the family of terms are commonly used to refer to the orbital altitude of the spacecraft from the surface of the central body (assuming a constant, standard reference radius).

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