Perihelion and aphelion

Apsis (Greek: ἁψίς; plural apsides /ˈæpsɪdiːz/, 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).The perihelion and aphelion points of the inner planets of the Solar SystemThe perihelion and aphelion points of the outer planets of the Solar SystemDistances of selected bodies of the Solar System from the Sun. The left and right edges of each bar correspond to the perihelion and aphelion of the body, respectively, hence long bars denote high orbital eccentricity. The radius of the Sun is 0.7 million km, and the radius of Jupiter (the largest planet) is 0.07 million km, both too small to resolve on this image.UT TIMEZONEUT TIMEZONEYEARS YEARSYEARS YEARS Apsis (Greek: ἁψίς; plural apsides /ˈæpsɪdiːz/, 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). 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).