What is "apsis"

Apse \Apse\ ([a^]ps), n.; pl. Apses ([a^]p"s[e^]z). [See Apsis.]

1. (Arch.)

   1. A projecting part of a building, esp. of a church, having in the plan a polygonal or semicircular termination, and, most often, projecting from the east end. In early churches the Eastern apse was occupied by seats for the bishop and clergy. Hence:

   2. The bishop's seat or throne, in ancient churches.

2. A reliquary, or case in which the relics of saints were kept.

   Note: This word is also written apsis and absis.

"perigree of the moon, perihelion of a planet" (plural apsides), 1650s, from Latin apsis "arch, vault" (see apse).

n. 1 (context architecture English) A recess or projection, with a dome or vault, at the east end of a church; an apse. 2 (context astronomy English) Either of the points in the elliptical orbit of a planet or comet where it is closest or furthest from the sun; perihelion or aphelion; an apside

1. n. a domed or vaulted recess or projection on a building especially the east end of a church; usually contains the altar [syn: apse]

2. [also: apsides (pl)]

[[Image:Apogee (PSF).png|thumb|275px|The apsides indicate the nearest and furthest points of an orbiting body around its host.

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An apsis (; plural apsides , Greek: ἁψίδες) 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- and ap-, or apo- 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 .
• For any orbits around a center of mass, there are the terms pericenter and apocenter. Periapsis and apoapsis (or apapsis) are equivalent alternatives.

A straight line connecting the pericenter and apocenter 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 74% 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).