[ ee-kwuh-noks, ek-wuh- ]

  1. the time when the sun crosses the plane of the earth's equator, making night and day of approximately equal length all over the earth and occurring about March 21 (vernal equinox, or spring equinox ) and September 22 (autumnal equinox ).

  2. either of the equinoctial points.

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Origin of equinox

First recorded in 1350–1400; Middle English, from Medieval Latin equinoxium, for Latin aequinoctium “the time of equal days and nights” (equivalent to aequi- + noct- + -ium ); see origin at equi-, nocti-, -ium

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British Dictionary definitions for equinox


/ (ˈiːkwɪˌnɒks, ˈɛkwɪˌnɒks) /

  1. either of the two occasions, six months apart, when day and night are of equal length: See vernal equinox, autumnal equinox

  2. another name for equinoctial point

Origin of equinox

C14: from Medieval Latin equinoxium, changed from Latin aequinoctium, from aequi- equi- + nox night

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Scientific definitions for equinox


[ ēkwə-nŏks′ ]

  1. Either of the two points on the celestial sphere where the ecliptic (the apparent path of the Sun) crosses the celestial equator.♦ The point at which the Sun's path crosses the celestial equator moving from south to north is called the vernal equinox. The vernal equinox marks the zero point in both the equatorial and ecliptic coordinate systems; horizontal angular distances (right ascension in the equatorial system and celestial longitude in the ecliptic system) are measured eastward from this point. The vernal equinox is also known as the first point of Aries because when first devised some 2,000 years ago this point occurred at the beginning of Aries in the zodiac. Because of the westward precession of the equinoxes, the vernal equinox is now located at the beginning of Pisces.♦ The point at which the Sun's path crosses the celestial equator moving from north to south is called the autumnal equinox.

  2. Either of the two corresponding moments of the year when the Sun is directly above the Earth's equator. The vernal equinox occurs on March 20 or 21 and the autumnal equinox on September 22 or 23, marking the beginning of spring and autumn, respectively, in the Northern Hemisphere (and the reverse in the Southern Hemisphere). The days on which an equinox falls have about equal periods of sunlight and darkness. Compare solstice.

The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Cultural definitions for equinox


[ (ee-kwuh-noks, ek-wuh-noks) ]

The twice yearly times when the lengths of day and night are equal. At equinox, the sun is directly over the Earth's equator. The vernal equinox occurs about March 22 and the autumnal equinox about September 21.

The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.