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 ); see origin at equi-, nocti-, -ium

Compare meaning

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

• equinox vs. solstice

Explanation

The equinox is one of the two times in a year when the sun crosses the plane of the earth's equator, and day and night are of equal length. “During the spring equinox you can balance eggs on end, because of the equal balance between night and day.” There are two equinoxes every year, one in the spring (the vernal equinox) and one in the fall (the autumnal equinox), each usually on the first day of spring and the first day of fall. These days are often celebrated and even worshipped in some traditions, although an equinox is actually a point in time — not an entire day.

Example Sentences

Easter is always on the first Sunday after the first full Moon that follows the spring equinox.

From BBC • Apr. 4, 2026

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

The date of the astronomical equinox also tends to naturally fluctuate between 20 and 21 March.

From BBC • Mar. 29, 2026

This time of year - close to the spring equinox - day length increases and the Sun moves higher in the sky.

From BBC • Mar. 18, 2026

Most of the time it happens around the spring equinox of the year you turn fifteen.

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