equinox

Etymology

PIE word *nókʷts 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”), 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). 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.

noun

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.
   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: […] 22 September 1793, 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
   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. 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
   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. 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
   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. 2005, Clive [L. N.] Ruggles, “Equinoxes”, in Ancient Astronomy: An Encyclopedia of Cosmologies and Myth, Santa Barbara, Calif.: ABC-CLIO, page 148
   […] 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. 2006, John T. Koch, “calendar, Celtic”, in Celtic Culture: A Historical Encyclopedia, volume I, Santa Barbara, Calif.: ABC-CLIO, §4 (The Seasons), page 332, column 1
2. (also figurative) 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.
      The Equinox drove the sand into their faces or round their legs, as they dived among the sheep-haunted hollows. 1 September 1920, Rudyard Kipling, “The Satisfaction of a Gentleman”, in The London Magazine, London: Hunt and Clarke,[…], →OCLC; republished on the website of The Kipling Society, accessed 7 September 2022, archived from the original on 2021-12-22
   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.