What does apsides mean?
Apsides are the two turning points of an elliptical orbit: the point where an orbiting body comes nearest to the object it circles, and the point where it swings farthest away. In the Earth–Sun system these extremes are called perihelion and aphelion; around any central body they are termed periapsis and apoapsis. Astronomers often speak of the 'line of apsides', the imaginary straight line connecting the two points through the central body. This line is not always fixed — it can slowly rotate, a motion known as the precession of the apsides, famously observed in Mercury's orbit and explained by Einstein's general relativity. Though the word is confined largely to astronomy and celestial mechanics, it captures a concept fundamental to understanding how planets, moons, comets, and satellites move through space.
nounThe plural of apsis: the two points in an orbiting body's path that lie closest to and farthest from the body around which it orbits.
- The two points of an orbit at which the orbiting body is closest to (periapsis) and farthest from (apoapsis) the body it circles.
"The line joining a planet's two apsides slowly rotates over time, a phenomenon known as the precession of the apsides."
"Comets on elongated orbits swing dramatically between their two apsides."
"Newton devoted considerable attention to explaining the motion of the Moon's apsides."
'Apsides' is itself the standard plural of 'apsis'; there is no further plural form.
"Both apsides of the comet's orbit lie well within the inner solar system."
The tiny drift of Mercury's apsides was one of the first confirmations of Einstein's general relativity — Newton's gravity alone couldn't explain it.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of apsides
The word traces back to the Greek 'hapsis' (or 'apsis'), meaning an arch or vault, which passed into Latin as 'apsis'. In architecture the same root gave rise to the apse, the vaulted recess at the end of a church. Astronomers adopted the term by analogy, since each extreme of an orbit resembles the springing point of an arch, and English scientific usage of 'apsis' and its plural 'apsides' became established in the early modern period as celestial mechanics developed.
Related word forms
How apsides is actually used
A technical term used almost exclusively in astronomy, celestial mechanics, and orbital physics. It is always plural in form, being the plural of 'apsis'; the singular refers to either extreme point individually.
Easily confused with apsides
Apses are the semicircular vaulted recesses at the ends of churches, while apsides are the extreme points of an orbital path.