apsis

apsis (pl. apsides), point in the orbit of a body where the body is neither approaching nor receding from another body about which it revolves. Any elliptical orbit has two apsides. At the perigee the moon or other satellite is as close as it ever gets to the earth, and it begins to move away; at the apogee it is as far away as it gets, and it begins to move closer. Similarly, in the orbit of the earth or another planet around the sun, the perihelion is the point of closest approach and the aphelion is the point of farthest recession. In the orbit of the stars in a binary star system, the periastron is the point of closest approach and the apastron the point of farthest recession. A line connecting the two apsidal points of an elliptical orbit (e.g., the aphelion and perihelion) is called the line of apsides; it is the major axis of the ellipse. This line may precess because of gravitational influences of other bodies or relativistic effects.

apsis (pl. apsides) Either of two points in an object's orbit. The closest point to the primary body is known as the periapsis, and the furthest the apapsis. The apsides of the Earth's orbit are its perihelion and aphelion; in the Moon's orbit they are its perigee and apogee. The line of apsides connect these points.

ap·sis / ˈapsis/ • n. (pl. -si·des / -siˌdēz/ ) either of two points on the orbit of a planet or satellite that are nearest to or furthest from the body around which it moves. DERIVATIVES: ap·si·dal / ˈapsidl/ adj.