equi·​nox | \ ˈē-kwə-ˌnäks, ˈe- \

Definition of equinox

1 : either of the two points on the celestial sphere where the celestial equator intersects the ecliptic
2 : either of the two times each year (as about March 21 and September 23) when the sun crosses the equator and day and night are everywhere on earth of approximately equal length

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Equinox and the Seasons

Equinox descends from aequus, the Latin word for "equal," and nox, the Latin word for "night"—a fitting history for a word that describes days of the year when the daytime and nighttime are equal in length. In the northern hemisphere, the vernal equinox marks the first day of spring and occurs when the sun moves north across the equator. (Vernal comes from the Latin word ver, meaning "spring.") The autumnal equinox marks the first day of autumn in the northern hemisphere and occurs when the sun crosses the equator going south. In contrast, a solstice is either of the two moments in the year when the sun's apparent path is farthest north or south from the equator.

Examples of equinox in a Sentence

Recent Examples on the Web

The 2018 autumnal equinox is on September 22, and the next full moon occurs two days later on September 24. Megan Friedman, Country Living, "What Is a Harvest Moon? All About the Lunar Spectacle That Means Fall Is Here," 20 Sep. 2018 In 2009 and 2010, during the equinox, Cassini observed a sudden and short-lived brightening on Titan. Chris Lee, Ars Technica, "The deserts of Titan have organic dust storms," 4 Oct. 2018 Nowadays, thousands of people flock to Stonehenge each year to celebrate the solstices and equinoxes, many of them decked out in traditional pagan garb. Justin Grieser, Washington Post, "Summer solstice is Thursday — 5 things to know about the longest day of the year," 20 June 2018 During the equinox, the North Pole points neither toward the sun nor away from it. Avery Thompson, Popular Mechanics, "This Is the Exact Minute That Fall Will Begin," 21 Sep. 2016 However, with the spring equinox right around the corner, that means it's...Aries season. Allure, "Bite Beauty's Astrology Lipstick for Leo Is an Attention-Grabbing Glittery Gold," 19 July 2018 The fall -- or autumnal -- equinox falls on or around Sept. 22 each year. Leigh Morgan, AL.com, "Spring equinox 2018: Winter is finally, officially, over," 20 Mar. 2018 Thursday, September 22, is the date of the autumn equinox, the first official date of fall. Avery Thompson, Popular Mechanics, "This Is the Exact Minute That Fall Will Begin," 21 Sep. 2016 Cassini found its most stunning ring surprises during the time surrounding the August 2009 equinox. Carolyn Porco, Scientific American, "Cassini at Saturn: A Retrospective," 1 Oct. 2017

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'equinox.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of equinox

14th century, in the meaning defined at sense 1

History and Etymology for equinox

Middle English, from Anglo-French or Medieval Latin; Anglo-French equinocce, from Medieval Latin equinoxium, alteration of Latin aequinoctium, from aequi- equi- + noct-, nox night — more at night

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Last Updated

8 Jan 2019

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Time Traveler for equinox

The first known use of equinox was in the 14th century

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English Language Learners Definition of equinox

: a day when day and night are the same length


equi·​nox | \ ˈē-kwə-ˌnäks, ˈe-kwə-\

Kids Definition of equinox

: either of the two times each year (as in spring around March 21 and in fall around September 23) when the sun's center crosses the equator and day and night are everywhere of equal length

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More from Merriam-Webster on equinox

Rhyming Dictionary: Words that rhyme with equinox

Spanish Central: Translation of equinox

Nglish: Translation of equinox for Spanish Speakers

Britannica English: Translation of equinox for Arabic Speakers

Britannica.com: Encyclopedia article about equinox

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