What does apoapsis mean?
Apoapsis is the astronomical term for the point in an orbit at which an orbiting body — a planet, moon, satellite, or spacecraft — is at its greatest distance from the body it circles. It is the counterpart of periapsis, the point of closest approach, and together the two define the extremes of any elliptical orbit. Because orbits differ in what they encircle, English offers specialized variants: apogee for orbits around Earth, aphelion for orbits around the Sun, and so on, with apoapsis serving as the general-purpose term. Used almost exclusively in scientific and technical contexts, the word appears frequently in mission planning and orbital mechanics, as when engineers raise a spacecraft's apoapsis to reach a higher orbit or a transfer trajectory. Its Greek roots — 'apo' meaning away from and 'apsis' meaning arc or orbit — make its meaning transparent once known, and it pairs naturally with Kepler's laws, which explain why objects move slowest at apoapsis and fastest at periapsis.
nounThe point in an orbit around a celestial body that is farthest from that body; the opposite of periapsis.
- {'definition': 'The point in an orbit at which the orbiting body is farthest from the body it orbits.'}
"The spacecraft's apoapsis lies far beyond the orbit of the Moon."
"Mission controllers raised the probe's apoapsis to place it on a trajectory toward Jupiter."
"The satellite travels slowest when it reaches apoapsis, conserving angular momentum as described by Kepler's second law."
The plural follows the Greek-style pattern used for other '-sis' terms (e.g., 'bases', 'apses'); however, many writers avoid it by rephrasing.
"The two orbits have their apoapsides oriented in nearly opposite directions."
Every elliptical orbit has a farthest and closest point — and 'apoapsis' is the umbrella word for that high point, whatever world you're circling.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of apoapsis
The word 'apoapsis' derives from Greek elements: 'apo-', meaning 'away from' or 'off', combined with 'apsis', meaning 'arc', 'orbit', or the point where a heavenly body is nearest or farthest. The term was formed on the model of earlier compounds such as 'apogee' and 'aphelion', which date to the early modern period of astronomical terminology. It belongs to a family of Greek-derived orbital terms including 'perigee' and 'periapsis', which use the opposite prefix 'peri-' ('near').
Related word forms
How apoapsis is actually used
A technical term used chiefly in astronomy, astrodynamics, and orbital mechanics. It is often replaced in general writing by more specific terms such as 'apogee' (Earth) or 'aphelion' (Sun). The plural is 'apoapsides', though many writers simply use 'apoapsis points' or avoid the plural.
Easily confused with apoapsis
Apogee refers specifically to the farthest point of an orbit around Earth, while apoapsis is the general term for the farthest point of an orbit around any body.