Apsis

This article is about the astronomical term. In architecture, apsis is also a synonym for apse.

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

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In astronomy, an apsis (plural apsides) is the point of greatest or least distance of the elliptical orbit of a celestial body from its 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 rather than the Latin ones, since "peri" and "apo" are Greek and it is considered bad form to mix Greek and Latin roots.
(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".