What does aphelion mean?
Aphelion is the point in the orbit of any body travelling around the Sun — a planet, asteroid, or comet — at which that body is farthest from the Sun. Its counterpart is perihelion, the point of closest approach, and together the two terms describe the extremes of an elliptical orbit. Because planetary orbits are ellipses rather than perfect circles, every orbiting body has both an aphelion and a perihelion; Earth, for instance, reaches its aphelion in early July, when it lies roughly three million miles farther from the Sun than it does in January. The word derives from Greek roots meaning 'away from' and 'sun', and its plural form is 'aphelia'. Though primarily a term of astronomy, aphelion also appears in popular science writing and journalism, often to explain the counterintuitive fact that Earth is farthest from the Sun during the northern summer — a reminder that seasons are governed by axial tilt rather than orbital distance.
nounThe point in the orbit of a planet, comet, or other body around the Sun at which it is farthest from the Sun.
- The point in a solar orbit at which a body is farthest from the Sun.
"Earth reaches its aphelion in early July, when it is about three million miles farther from the Sun than at its closest approach."
"Mars passes through its aphelion at a distance of roughly 249 million kilometres from the Sun."
"Comet Hale–Bopp will not return to aphelion for thousands of years after its 1997 perihelion passage."
The plural follows the classical Greek pattern, like 'perihelia'; the singular 'aphelion' is more common in most contexts.
"The orbits of the inner planets have their aphelia at different points around the Sun."
Earth is actually farthest from the Sun in July — during northern summer — because seasons come from Earth's tilt, not its distance from the Sun.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of aphelion
Aphelion comes from the Greek elements 'apo-', meaning 'away from', and 'hēlios', meaning 'sun'. It was formed in modern scientific Latin on the model of the older term 'perihelion', which denotes the nearest point to the Sun, and entered English usage in the seventeenth century as astronomers developed precise terminology for orbital mechanics. It shares this 'apo-' root with related astronomical terms such as apogee (farthest point from Earth) and apoapsis (the general term for the farthest point of any orbit).
How aphelion is actually used
A technical term used mainly in astronomy and related sciences; it appears in popular science writing but is rare in everyday conversation. The plural 'aphelia' follows the classical pattern of Greek-derived nouns.
Easily confused with aphelion
Perihelion is the point in an orbit closest to the Sun, whereas aphelion is the point farthest from it.