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.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.