Apsis

Astronomy / Math and Science
An apsis (Greek ἁψίς, gen. ἁψίδος), plural apsides (/ˈæpsɨdiːz/; Greek: ἁψίδες), is a point of least or greatest distance of a body in an elliptic orbit about a larger body. For a body orbiting the  Sun the point of least distance is the perihelion (/ˌpɛrɨˈhiːliən/ and the point of greatest distance is the aphelion (/æpˈhiːliən/). For any satellite of Earth including the Moon the point of least distance is the perigee (/ˈpɛrɨdʒiː/) and greatest distance the apogee . More generally, the prefixes peri- (from περί (peri), meaning "near") and ap-, or apo-, (from ἀπ(ό) (ap(ó)), meaning "away from") can be added to center (of mass) giving pericenter and apocenter. The words periapsis and apoapsis (or apapsis) are also used for these.

A straight line connecting the pericenter and apocenter is the line of apsides. This is the major axis of the ellipse, its greatest diameter. For a two-body system the center of mass of the system lies on this line at one of the two foci of the ellipse. When one body is sufficiently larger than the other it may be taken to be at this focus. However whether or not this is the case, both bodies are in similar elliptical orbits each having one focus at the system's center of mass, with their respective lines of apsides being of length inversely proportional to their masses. Historically, in geocentric systems, apsides were measured from the center of the Earth. However in the case of the Moon, the center of mass of the Earth-Moon system or Earth-Moon barycenter, as the common focus of both the Moon's and Earth's orbits about each other, is about 74% of the way from Earth's center to its surface.

In orbital mechanics, the apsis technically refers to the distance measured between the centers of mass of the central and orbiting body. However, in the case of spacecraft, the family of terms are commonly used to describe the orbital altitude of the spacecraft from the surface of the central body (assuming a constant, standard reference radius).