equinox

American  

[ee-kwuh-noks, ek-wuh-] / ˈi kwəˌnɒks, ˈɛk wə- /

noun

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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equinox British  

/ ˈɛkwɪˌnɒks, ˈiːkwɪˌnɒks /

noun

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

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equinox Scientific  

/ ē′kwə-nŏks′ /

1. Either of the two points on the celestial sphere where the ecliptic (the apparent path of the Sun) crosses the celestial equator.

2. ◆ 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.

3. ◆ The point at which the Sun's path crosses the celestial equator moving from north to south is called the autumnal equinox.

4. 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.

5. Compare solstice

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equinox Cultural  

1. 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.

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Usage

What is an equinox? An equinox is one of the two times of the year when the amount of daylight and nighttime hours are just about of equal length. The two equinoxes occur around March 20–21 and September 22–23. The equinoxes occur on these days, but an equinox is not the whole day—it is the moment when the sun is directly above Earth’s equator. The word equinox can also refer to the position of the sun at this moment. This can also be called the equinoctial point. The equinoxes are traditionally considered to mark the start of spring and fall. In the Northern Hemisphere, the vernal equinox (or spring equinox) occurs in March and the autumnal equinox occurs in September. In the Southern Hemisphere, it’s the reverse. In contrast, a solstice is one of the two times of the year when the positioning and tilt of Earth relative to the sun results in the most amount of daylight time or the least amount of daylight time in a single day. There are two solstices during the year: one that occurs around June 20–22 (usually June 20 or 21) and one that occurs around December 20–23 (usually December 21 or 22). In the Northern Hemisphere, the summer solstice occurs in June and the winter solstice occurs in December. In the Southern Hemisphere, it’s the reverse. Example: Many ancient cultures recognized and observed the equinoxes as times that marked the change of the seasons.

Etymology

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 ); equi-, nocti-, -ium

Compare meaning

How does equinox compare to similar and commonly confused words? Explore the most common comparisons:

• equinox vs. solstice

Example Sentences

This ensures the holiday lands on the first Sunday after the first full moon occurring on or after the spring equinox.

From Barron's • Apr. 2, 2026

Towards the end of March, Iranians celebrated Nowruz, the Persian new year festival that marks the spring equinox and is often a time when families get together.

From BBC • Mar. 31, 2026

On Friday - also the spring equinox - temperatures peak again in Scotland and Northern Ireland where it could be their warmest day of the year so far.

From BBC • Mar. 19, 2026

The season then runs until the vernal or spring equinox which in 2026 is Friday 20 March.

From BBC • Nov. 23, 2025

It isn’t anywhere close to the spring equinox yet, when rites usually happen, but maybe that’s what Dr.

From "The Wrong Way Home" by Kate O’Shaughnessy