![]() ![]() 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). 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). 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. 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. 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 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).)Īccording 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 example, the reference suffix for Earth is -gee, hence apogee and perigee are the names of the apsides for the Moon, and any other artificial satellites of the Earth. Each is named by selecting the appropriate prefix: ap-, apo- (from ἀπ(ό), (ap(o)-) 'away from'), or peri- (from περί (peri-) 'near')-then joining it to the reference suffix of the "host" body being orbited. There are two apsides in any elliptic orbit. 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 Earth's two apsides are the farthest point, aphelion, and the nearest point, perihelion, of its orbit around the host Sun. *The table names the (two) apsides of a planetary body (X, "orbiter") orbiting the host body indicated: (1) farthestįor example, the Moon's two apsides are the farthest point, apogee, and the nearest point, perigee, of its orbit around the host Earth. *The line of apsides is the line connecting positions 1 and 2. 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).
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