Editing Apsis (section)

==General description==
[[File:Periapsis_apoapsis.png|thumb|upright=1.15|The two-body system of interacting [[elliptic orbit]]s: The smaller, satellite body (blue) orbits the [[primary (astronomy)|primary body]] (yellow); both are in elliptic orbits around their [[center of mass|common center of mass]] (or [[barycenter]]), (red +).<br /> ∗Periapsis and apoapsis as distances: the smallest and largest distances between the orbiter and its host body.]]

There are two apsides in any [[elliptic orbit]]. The name for each apsis is created from the prefixes ''ap-'', ''apo-'' ({{ety||''ἀπ(ό)'', (ap(o)-)|away from}}) for the farthest or ''peri-'' ({{ety||''περί'' (peri-)|near}}) 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.<ref>{{Cite web |title=Earth-Moon Barycenter - SkyMarvels.com |url=https://www.skymarvels.com/gallery/Vid%20-%20Earth-Moon%20Barycenter.htm |access-date=2024-04-23 |website=www.skymarvels.com}}</ref> If, compared to the larger mass, the smaller mass is negligible (e.g., for satellites), then the [[orbital elements|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 [[barycenter]] of the 2-body system and the center of mass of the 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).

[[Image:Angular Parameters of Elliptical Orbit.png|thumb|upright=1.15|[[Kepler orbit|Keplerian]] [[orbital elements]]: point ''G'', 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.]]