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Apsis
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Apsis

This article is about the astronomical term. In architecture, apsis is also the plural of apse.

Apogee is also the name of a major video game publisher.


In astronomy, an apsis (plural apsides) is the point of greatest or least distance of the elliptical orbit of a celestial body from a centre of attraction (the centre of mass of the system). The point of closest approach is called the periapsis and the point of farthest approach is the apoapsis. A straight line drawn through the periapsis and apoapsis is the line of apsides, sometimes called the major-axis of the ellipse. It's simply a line drawn through the longest part of the ellipse.

According to Kepler's laws of planetary motion, as a body approaches the periapsis it will increase in velocity and as a body approaches the apoapsis it will decrease in velocity. This is because the segment of line running from the centre of mass to the object sweeps a fixed amount of area per unit of time; as the segment lengthens, the angular velocity diminishes and vice-versa.

Related and increasingly specific terms are often used to identify the body being orbited. For example, the terms apogee and perigee refer to an orbit around the earth, Aphelion and Perihelion refer to an orbit around the sun, and Apastron and Periastron refer to an orbit around a star. Other terms are less commonly or rarely used:

Body Closest approach Farthest approach
Star Periastron Apastron
Black hole Perimelasma Apomelasma
Sun Perihelion Aphelion (1)
Mercury Perihermion Aphermion (2)
Venus Pericytherion Apocytherion
Earth Perigee Apogee
Moon Periselene Aposelene (3)
Mars Periareion Apoareion
Jupiter Perizene Apozene (4)
Saturn Perikrone Apokrone
Uranus Periuranion Apuranion
Neptune Periposeidion Apoposeidion
Pluto Perihadion Aphadion (5)
The terms are formed from the Greek roots for the planet names.
(1) Pronounced "Ap-helion", not "Aff-elion".
(2) Pronounced "Ap-hermion", not "Aff-ermion".
(3) Perilune/Apolune are to be avoided. Pericynthion/Apocynthion are sometimes used for artificial bodies.
(4) Perijove/Apojove are to be avoided.
(5) Pronounced "Ap-hadion", not "Aff-adion".