Apsis

Apsis is a term used in celestial mechanics to denote the points of greatest and least distance between an orbiting body and the body it orbits. The two principal apsides are periapsis, the closest approach, and apoapsis, the farthest distance. When the central body is the Sun, the terms perihelion and aphelion are used; for other central bodies the generic terms periapsis and apoapsis apply. The concept is relevant for planets, moons, comets, and spacecraft, and it can be used for both bound (elliptical) or unbound (parabolic or hyperbolic) trajectories.

Geometrically, the apsides lie on the major axis of the orbit and define the line of apsides. In an ellipse, the distance from the central focus to the orbit is minimal at periapsis and maximal at apoapsis. In non-elliptical trajectories, the notion still describes the closest approach, but an opposite apsis may not exist in repeated cycles.

Apsidal precession refers to the rotation of the line of apsides over time, caused by perturbations from other bodies or by relativistic effects. A famous instance is Mercury’s perihelion precession, which was a key test of general relativity. Accurate knowledge of apsides is important in mission planning, orbital dynamics, and the study of exoplanet orbits.

Etymology traces apsis to ancient languages meaning a point or edge of an orbit. The term provides a concise way to describe the extremes of an orbiting body's distance from its primary.