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Concept
Apsis
Summary
An apsis (; ˈæpsɪˌdiːz ) is the farthest or nearest point in the orbit of a planetary body about its primary body.
The line of apsides is the line connecting the two extreme values.
For example, for orbits about the Sun the apsides are called aphelion (farthest) and perihelion (nearest). The Moon's two apsides are the farthest point, apogee, and the nearest point, perigee, of its orbit around the host Earth. The Earth's two apsides are the farthest point, aphelion, and the nearest point, perihelion, of its orbit around the host Sun. The terms aphelion and perihelion apply in the same way to the orbits of Jupiter and the other planets, the comets, and the asteroids of the Solar System.
There are two apsides in any elliptic orbit. The name for each apsis is created from the prefixes ap-, apo- () for the farthest or peri- () for the closest point to the primary body, with a suffix that describes the primary body. The suffix for Earth is -gee, so the apsides' names are apogee and perigee. For the Sun, the suffix is -helion, so the names are aphelion and perihelion.
According to Newton's laws of motion, all periodic orbits are ellipses. The barycenter of the two bodies may lie well within the bigger body—e.g., the Earth–Moon barycenter is about 75% of the way from Earth's center to its surface. If, compared to the larger mass, the smaller mass is negligible (e.g., for satellites), then the orbital parameters are independent of the smaller mass.
When used as a suffix—that is, -apsis—the term can refer to the two distances from the primary body to the orbiting body when the latter is located: 1) at the periapsis point, or 2) at the apoapsis point (compare both graphics, second figure). The line of apsides denotes the distance of the line that joins the nearest and farthest points across an orbit; it also refers simply to the extreme range of an object orbiting a host body (see top figure; see third figure).
In orbital mechanics, the apsides technically refer to the distance measured between the center of mass of the central body and the center of mass of the orbiting body.
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