equinox

noun

equi·​nox ˈē-kwə-ˌnäks  ˈe- 

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" or "even," 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

Around the spring and fall equinoxes, conditions can become more favorable for the solar wind to interact with Earth’s magnetosphere. — Michele Laufik, Martha Stewart, 8 Sep. 2026

The 52-piece instrumental Astronomia albums were released sequentially on the spring equinox, summer solstice, fall equinox, and winter solstice of 2021. — Lily Moayeri, SPIN, 8 Sep. 2026

The equinox occurs when the sun’s center passes through the imaginary line from the north to the south. — Alexiah Syrai Olsen, Idaho Statesman, 3 Sep. 2026

This year, the first day of fall, also known as the autumnal equinox, is set to happen later in September. — Greta Cross, USA Today, 1 Sep. 2026

Word History

Etymology

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

First Known Use

15th century, in the meaning defined at sense 1

Rhymes for equinox

• aftershocks
• chatterbox
• chickenpox
• firebox
• goldilocks
• hollyhocks
• interlocks
• orthodox
• paradox
• tomahawks
• balks
• blocks

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Kids Definition

equinox

noun

equi·​nox ˈē-kwə-ˌnäks  ˈek-wə- 

: either of the two times each year about March 21 and September 23 when the sun appears overhead at the equator and day and night are everywhere of equal length

Etymology

Latin equinoxium (same meaning), derived from earlier Latin aequi- "equal" and noct-, nox "night" — related to nocturnal