Apsis

The apsides refer to the farthest (1) and nearest (2) points reached by an orbiting planetary body (1 and 2) with respect to a primary, or host, body (3).
∗The line of apsides is the line connecting positions 1 and 2.
∗The table names the (two) apsides of a planetary body (X, "orbiter") orbiting the host body indicated:
(1) farthest(X) orbiter(3) host(2) nearest
apogeeMoonEarthperigee
apojoveGanymedeJupiterperijove
aphelionEarthSunperihelion
aphelionJupiterSunperihelion
aphelionHalley's CometSunperihelion
apastronexoplanetstarperiastron
apocentercomet, e.g.primarypericenter
apoapsiscomet, e.g.primaryperiapsis
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For example, 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.
The two-body system of interacting elliptic orbits: The smaller, satellite body (blue) orbits the primary body (yellow); both are in elliptic orbits around their common center of mass (or barycenter), (red +).
Periapsis and apoapsis as distances: The smallest and largest distances between the orbiter and its host body.
Keplerian orbital elements: point F, the nearest point of approach of an orbiting body, is the pericenter (also periapsis) of an orbit; point H, the farthest point of the orbiting body, is the apocenter (also apoapsis) of the orbit; and the red line between them is the line of apsides.

Apsis (Greek: ἁψίς; plural apsides z/, Greek: ἁψῖδες; "orbit") denotes either of the two extreme points (i.e., the farthest or nearest point) in the orbit of a planetary body about its primary body (or simply, "the primary"). The plural term, "apsides," usually implies both apsis points (i.e., farthest and nearest); apsides can also refer to the distance of the extreme range of an object orbiting a host body. For example, the apsides of Earth's orbit of the Sun are two: the apsis for Earth's farthest point from the Sun, dubbed the aphelion; and the apsis for Earth's nearest point, the perihelion (see top figure). (The term "apsis", a cognate with apse, comes via Latin from Greek).[1]

Typically, there are two apsides in any elliptic orbit. Each is named by selecting the appropriate prefix: ap-, apo- (from ἀπ(ό), (ap(o)-), meaning 'away from'), or peri- (from περί (peri-), meaning 'near') — then joining it to the reference suffix of the "host" body being orbited. (For example, the reference suffix for Earth is -gee, hence apogee and perigee are the names of the apsides for the Moon, and any other (man-made) satellites of the Earth. The suffix for the Sun is -helion, hence aphelion and perihelion are the names of the apsides for the Earth and for the Sun's other planets, comets, asteroids, etc., (see table, top figure).)

According to Newton's laws of motion all periodic orbits are ellipses, including: 1) the single orbital ellipse, where the primary body is fixed at one focus point and the planetary body orbits around that focus (see top figure); and 2) the two-body system of interacting elliptic orbits: both bodies orbit their joint center of mass (or barycenter), which is located at a focus point that is common to both ellipses, (see second figure). For such a two-body system, when one mass is sufficiently larger than the other, the smaller ellipse (of the larger body) around the barycenter comprises one of the orbital elements of the larger ellipse (of the smaller body).

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 barycenters of the central body and orbiting body. However, in the case of a spacecraft, the terms are commonly used to refer to the orbital altitude of the spacecraft above the surface of the central body (assuming a constant, standard reference radius).

Terminology

The words "pericenter" and "apocenter" are often seen, although periapsis/apoapsis are preferred in technical usage.

  • For generic situations where the primary is not specified, the terms pericenter and apocenter are used for naming the extreme points of orbits (see table, top figure); periapsis and apoapsis (or apapsis) are equivalent alternatives, but these terms, frequently, also refer to distances—that is, the smallest and largest distances between the orbiter and its host body, (see second figure).
  • For a body orbiting the Sun, the point of least distance is the perihelion (n/), and the point of greatest distance is the aphelion (n/);[2] when discussing orbits around other stars the terms become periastron and apastron.
  • When discussing a satellite of Earth, including the Moon, the point of least distance is the perigee (/), and of greatest distance, the apogee, (from Ancient Greek: Γῆ (), "land" or "earth").
  • There are no natural satellites of the Moon. For man-made objects in lunar orbit, the point of least distance may be called the pericynthion (n/) and the greatest distance the apocynthion (n/); or perilune and apolune are sometimes used.[3]

Etymology

The words perihelion and aphelion were coined by Johannes Kepler[4] to describe the orbital motions of the planets around the Sun. The words are formed from the prefixes peri- (Greek: περί, near) and apo- (Greek: ἀπό, away from), affixed to the Greek word for the sun, (ἥλιος, or hēlíou).[5]

Various related terms are used for other celestial objects. The suffixes -gee, -helion, -astron and -galacticon are frequently used in the astronomical literature when referring to the Earth, Sun, stars, and the galactic center respectively. The suffix -jove is occasionally used for Jupiter, but -saturnium has very rarely been used in the last 50 years for Saturn. The -gee form is also used as a generic closest-approach-to "any planet" term—instead of applying it only to Earth.

During the Apollo program, the terms pericynthion and apocynthion were used when referring to orbiting the Moon; they reference Cynthia, an alternative name for the Greek Moon goddess Artemis.[6] Regarding black holes, the terms perimelasma and apomelasma (from a Greek root) were used by physicist and science-fiction author Geoffrey A. Landis in a 1998 story;[7] which occurred before perinigricon and aponigricon (from Latin) appeared in the scientific literature in 2002,[8] and before peribothron (from Greek bothros, meaning hole or pit) in 2015.[9]

Terminology summary

The suffixes shown below may be added to prefixes peri- or apo- to form unique names of apsides for the orbiting bodies of the indicated host/(primary) system. However, only for the Earth and Sun systems are the unique suffixes commonly used. Typically, for other host systems the generic suffix, -apsis, is used instead.[10][failed verification].

Host objects in the Solar System with named/nameable apsides
Astronomical host object Sun Mercury Earth Moon Mars Ceres Jupiter Saturn
Suffix -⁠helion -⁠hermion -⁠gee -⁠lune[3]
-⁠cynthion
-⁠selene[3]
-⁠areion -⁠demeter[11] -⁠jove -⁠chron[3]
-⁠kronos
-⁠saturnium
Origin
of the name
Helios Hermes Gaia Luna
Cynthia
Selene
Ares Demeter Zeus
Jupiter
Cronos
Saturn
Other host objects with named/nameable apsides
Astronomical host
object
Star Galaxy Barycenter Black hole
Suffix -⁠astron -⁠galacticon -⁠center
-⁠focus
-⁠apsis
-⁠melasma
-⁠bothron
-⁠nigricon
Origin
of the name
Lat: astra; stars Gr: galaxias; galaxy Gr: melos; black
Gr: bothros; hole
Lat: niger; black