aphelion

English

WOTD – 6 July 2023

PIE word
*h₂epó

PIE word
*sóh₂wl̥

From New Latin aphēlium (whence English aphelium, now displaced) + -ion (used in the names of other apsides). Aphelium was formed from Ancient Greek ἀπο- (apo-, prefix meaning ‘away, from, off’) + ἥλῐος (hḗlĭos, “the sun”) (ultimately from Proto-Indo-European *sóh₂wl̥ (“the sun”)), modelled after New Latin apogaeum (“apogee”).^[1]

The plural form aphelia is from aphelion + -a (plural form of the suffix -on).

(Received Pronunciation) IPA^(key): /əˈfiː.lɪ.ən/, /æpˈhiː.lɪ.ən/
- Audio (Southern England): (file)
(General American) IPA^(key): /əˈfi.li.ən/, /əˈfil.jən/, /æpˈhi.li.ən/, /æpˈhil.jən/
Hyphenation: ap‧hel‧i‧on
Homophone: Ophelian (one pronunciation)
Rhymes: -iːliən

aphelion (plural aphelia or (deprecated) aphelions)

(astronomy, also figuratively) The point in the elliptical orbit of a comet, planet, or other astronomical object, where it is farthest from the Sun.
Antonym: perihelion
- 1837, William Whewell, “The Inductive Epoch of Newton—Discovery of the Universal Gravitation of Matter, according to the Law of the Inverse Square of the Distance”, in History of the Inductive Sciences, from the Earliest to the Present Times. […], volume II, London: John W[illiam] Parker, […]; Cambridge, Cambridgeshire: J. and J. J. Deighton, →OCLC, book VII (History of Physical Astronomy), page 170:
  [I]t follows from the theory of gravity, that the aphelia of Mercury, Venus, the Earth, and Mars, slightly progress.

point in the elliptical orbit of a planet where it is farthest from the Sun