What does aphelia mean?
Aphelia is the plural of aphelion, an astronomical term denoting the points in a body's orbit around the Sun at which it is farthest from it. The word derives from Greek roots meaning 'away from' (apo-) and 'sun' (helios), making its meaning transparent to those familiar with classical prefixes. Aphelia matter because they mark the slowest, coldest stretches of any solar orbit: a planet or comet moving through its aphelion travels at minimum speed, governed by Kepler's laws of planetary motion. The term appears almost exclusively in astronomy writing, textbooks, and scientific journalism about comets, asteroids, and spacecraft trajectories. Its counterpart is perihelion, the closest approach to the Sun. For Earthlings, aphelia have a tangible consequence: Earth passes its aphelion in early July, contributing subtly to the slightly longer duration of Northern Hemisphere summer compared with Southern Hemisphere summer. Though technical in register, the word rewards anyone curious about the mechanics of the sky.
nounAphelia is the plural of aphelion: the points in a celestial body's orbit that are farthest from the Sun.
- The plural of aphelion: the points in an orbit around the Sun that are farthest from it.
"Comets with highly elliptical orbits reach their aphelia far beyond the outer planets."
"Halley's Comet reaches one of its aphelia some 5 billion kilometres from the Sun before beginning its long fall back inward."
"Astronomers calculated that the dwarf planet's aphelia lie well beyond Neptune's orbit."
Both forms are correct; 'aphelia' follows the Greek/Latinate pattern while 'aphelions' is the anglicized plural. Note that 'aphelia' itself is already a plural and should not be further inflected.
"Each planet's orbit has exactly one aphelion, but comparing the aphelia of different worlds reveals how eccentric their paths are."
Every comet that swings past the Sun must eventually turn around at its aphelion — the loneliest, most frozen point of its journey, sometimes farther out than Pluto.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of aphelia
The word comes from the Greek elements apo-, meaning 'away from', and hēlios, meaning 'sun'. It was formed on the model of apogee, the corresponding farthest point from Earth, as astronomers extended ancient Greek terminology for orbits around our planet to heliocentric orbits after the acceptance of the Copernican system in the 16th and 17th centuries. Cognate terms include perihelion ('near the sun'), apogee, and perigee, all built from the same pair of directional prefixes.
Related word forms
How aphelia is actually used
A strictly technical term used in astronomy and orbital mechanics; rare in everyday language. Both 'aphelia' and 'aphelions' are accepted plurals of aphelion, with 'aphelia' being the traditional Latinate plural. Always used in the plural only when discussing multiple bodies' orbits or multiple orbital extremes.
Easily confused with aphelia
Aphelion is the singular form referring to one farthest point from the Sun, while aphelia is simply its plural.
Perihelion (and perihelia, its plural) refers to the point in an orbit closest to the Sun, the opposite of aphelion and aphelia.