What does apsis mean?
An apsis is one of the two extreme points of an orbiting body's path: the moment of closest approach to the body being orbited, called periapsis, and the moment of farthest separation, called apoapsis. In practice, astronomers usually use more specific terms—perihelion and aphelion for orbits around the Sun, perigee and apogee for orbits around Earth—but 'apsis' serves as the general, system-neutral term. The two apsides together define the line of apsides, the major axis of an elliptical orbit, and gradual shifts in this line, known as apsidal precession, are important in celestial mechanics; Mercury's advancing perihelion famously provided early evidence for Einstein's general relativity. Derived from the Greek word for a fastening or arch, the term is shared with 'apse', the arched end of a church. It appears almost exclusively in scientific writing and remains unfamiliar outside astronomy and aerospace contexts.
nounEither of the two extreme points in an orbit: the point where an orbiting body is closest to the body it orbits (periapsis) or farthest from it (apoapsis).
- The point in an orbit at which the orbiting body is closest to or farthest from the body it orbits (periapsis or apoapsis).
"The comet reaches its perihelion—the apsis nearest the Sun—in early January."
"The satellite's apoapsis carries it nearly 36,000 kilometres above the equator."
"Gravitational perturbations cause slow rotation of the line of apsides over centuries."
The plural follows the Greek/Latin pattern, pronounced /ˈæpsɪdiːz/. The two apsides together are referred to as the 'line of apsides'.
"Over many orbits, tidal forces slowly rotate the binary pair's line of apsides."
Every orbit has two magic moments—closest approach and farthest retreat—and astronomers call them apsides.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of apsis
Apsis comes from the ancient Greek 'hapsis', meaning a fastening, joining, or arch, derived from the verb 'haptein' (to fasten). Latin adopted the word as 'absis'/'apsis', and it entered English in the early 17th century, initially in architectural usage before being applied to orbital mechanics. The same Greek root gives us 'apse', the vaulted semicircular end of a church, and 'synapse', the junction between nerve cells.
Related word forms
How apsis is actually used
A technical term used chiefly in astronomy, celestial mechanics, and orbital engineering; rare in everyday speech. The plural 'apsides' is standard. Specific instances have dedicated names formed with prefixes, such as perihelion/aphelion for orbits around the Sun and perigee/apogee for orbits around Earth.
Easily confused with apsis
An apse is the semicircular, vaulted east end of a church, while an apsis is a point of greatest or least distance in an orbit; both derive from the same Greek root.