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PIE word
During an equinox (sense 1) at noon according to Central European Time (ignoring daylight saving time), the Sun’s rays illuminate the Earth as shown in this diagram.
A diagram showing where the equinoxes (sense 3) occur in space relative to the Earth and the Sun. As the Sun appears to move around the Earth (actually, it is the Earth that moves around the Sun), its apparent path (the ecliptic; yellow circle) first intersects with the Earth’s equatorial plane (blue disc) in March at the green dot marked “first point of Aries”, and then again in September at the green dot marked “first point of Libra”. In the Northern Hemisphere, the March equinox is called the spring equinox as it traditionally marks the start of spring, while the September equinox is the autumn equinox that marks the start of autumn. It is the other way around in the Southern Hemisphere.

From Middle English equinox, equinoxe, equynox (one of the two periods in the year when the day and night are of equal length, equinox; either the zodiac sign Aries or Libra, in which the sun crosses the celestial equator),[1] from Old French equinoce, equinoxe (modern French équinoxe), or from its etymon Medieval Latin ēquinoxium, ēquinoctium, from Latin aequinoctium (equinox), from aequus (equal) + nox (night) (ultimately derived from Proto-Indo-European *nókʷts (night)) + -ium (suffix forming abstract nouns).[2][3] The Latin word, ultimately adopted in Middle English and modern English, displaced Old English efnniht (modern English evennight).

The plural form equinoctes treats equinox as if it were a Latin word; the accusative, nominative, or vocative plural of Latin nox (night) is noctēs.



equinox (plural equinoxes or (rare) equinoctes)

  1. One of the two occasions in the year when the length of the day and night are equal, which occurs when the apparent path of the Sun (the ecliptic) intersects with the equatorial plane of the Earth; this happens on a day between March 19 and 21 (spring in the Northern Hemisphere, and autumn in the Southern Hemisphere), and on another day between September 21 and 24 (autumn in the Northern Hemisphere, and spring in the Southern Hemisphere); hence, the exact time when the intersection occurs.
    Synonym: (rare) evennight
    • 1692 December 15, Richard Bentley, A Confutation of Atheism from the Origin and Frame of the World. The Third and Last Part. [], London: [] H[enry] Mortlock [], published 1693, →OCLC, pages 25–26:
      [T]he Months of March and September, the tvvo Æquinoxes of Our year, are the moſt vvindy and tempeſtuous, the moſt unſettled and unequable of Seaſons in moſt Countries of the VVorld.
    • 1793 September 22, Edward Williams, “Ode on Converting a Sword into a Pruning Hook”, in Poems, Lyric and Pastoral. [], London: [] J[ohn] Nichols; [], published 1794, →OCLC, footnote *, pages 160–161:
      The four grand and ſolemn Bardic days are, of ancient uſage, the tvvo equinoxes, and the tvvo ſolſtices; the nevv and full moons are alſo, ſubordinately, ſolemn Bardic days: []
    • 1838 July, Ralph Waldo Emerson, “Milton”, in J[ames] E[lliot] Cabot, editor, The Natural History of the Intellect (Emerson’s Complete Works; XII), Riverside edition, London: The Waverley Book Company, published 1893, →OCLC, page 155:
      [H]e [Milton] believed, his poetic vein only flowed from the autumnal to the vernal equinox; and, in his essay on Education, he doubts whether, in the fine days of spring, any study can be accomplished by young men.
    • 1842, Alfred Tennyson, “Will Waterproof’s Lyrical Monologue”, in Poems. [], volume II, London: Edward Moxon, [], →OCLC, page 194:
      Live long, nor feel in head or chest / Our changeful equinoxes, / Till mellow Death, like some late guest, / Shall call thee from the boxes.
    • 1848, Charles Richard Weld, chapter V, in A History of the Royal Society, with Memoirs of the Presidents. [], volume I, London: John W[illiam] Parker, [], →OCLC, page 100:
      21. Report the experiments, if conveniently they may, at both the solstices and equinoctes. / 22. Observe accurately the time of the sun’s rising on the top of the hill and below, and note the difference.
    • 1854, John Williams, “Pro-consul. b.c. 55.”, in The Life of Julius Cæsar, London; New York, N.Y.: G[eorge] Routledge & Co., [], →OCLC, page 195:
      For [Julius] Cæsar says, that on the night of the fourth day after his landing there was a full moon. He had before mentioned that the summer was far spent, and the æquinox not come, hence, the full moon must have been either in July or August.
    • 2005, Clive [L. N.] Ruggles, “Equinoxes”, in Ancient Astronomy: An Encyclopedia of Cosmologies and Myth, Santa Barbara, Calif.: ABC-CLIO, →ISBN, page 148:
      The word equinox is generally taken to refer to the days when, at every point on the earth, day and night are of equal length. But this definition of the equinox is a bit misleading. Since it gets light before the sun rises and remains light after the sun sets, the actual period of darkness at the equinox will be substantially less than twelve hours, the exact amount depending on latitude and how one defines the boundary between twilight and night. [] In practice, one cannot determine the equinox by measuring the length of time between sunrise and sunset.
    • 2006, John T. Koch, “calendar, Celtic”, in Celtic Culture: A Historical Encyclopedia, volume I, Santa Barbara, Calif.: ABC-CLIO, →ISBN, §4 (The Seasons), page 332, column 1:
      [] Midsummer’s Day falls near the beginning of summer meteorologically, but was the midpoint of summer in the traditional calendar. Though Midsummer’s Day celebrations are common in the modern Celtic countries, there is no evidence that the ancient Celts celebrated either the solstices or the equinoctes.
  2. (also figuratively) The circumstance of a twenty-four hour time period having the day and night of equal length.
  3. (astronomy) One of the two points in space where the apparent path of the Sun intersects with the equatorial plane of the Earth.
  4. (obsolete)
    1. (rare) A gale (very strong wind) once thought to occur more frequently around the time of an equinox (sense 1), now known to be a misconception; an equinoctial gale.
    2. (astronomy) A celestial equator (great circle on the celestial sphere, coincident with the plane of the Earth's equator (the equatorial plane)); also, the Earth's equator.
      Synonym: (obsolete) equinoctial line
      • 1697, William Dampier, chapter IV, in A New Voyage Round the VVorld. [], London: [] James Knapton, [], →OCLC, page 90:
        [T]hey [seals] are over all the American Coaſt of the South Seas, from Terra del Fuego, up to the Equinoctial Line: but to the North of the Equinox again, in theſe Sea, I never ſavv any, till as far as 21 North Lat[itude].

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  1. ^ ē̆quinox, n.”, in MED Online, Ann Arbor, Mich.: University of Michigan, 2007.
  2. ^ Compare “equinox, n.”, in OED Online Paid subscription required, Oxford, Oxfordshire: Oxford University Press, March 2022.
  3. ^ equinox, n.”, in Dictionary.com Unabridged, Dictionary.com, LLC, 1995–present, reproduced from Stuart Berg Flexner, editor in chief, Random House Unabridged Dictionary, 2nd edition, New York, N.Y.: Random House, 1993, →ISBN.

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Borrowed from French équinoxe, from Latin aequinoctium.


  • IPA(key): /ˌeː.k(ʋ)iˈnɔks/
  • Hyphenation: equi‧nox
  • Rhymes: -ɔks


equinox m (plural equinoxen)

  1. equinox
    Synonyms: dag-en-nachtevening, equinoctium, nachtevening

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