What is an equinox?

An equinox occurs twice a year when the Sun crosses the celestial equator, making day and night approximately equal in length (each about 12 hours) at every latitude on Earth. The word "equinox" comes from Latin aequus (equal) and nox (night).

The two equinoxes

Vernal (spring) equinox: around March 19–21. The Sun moves from the southern to the northern celestial hemisphere. Marks the start of astronomical spring in the Northern Hemisphere.
Autumnal (fall) equinox: around September 21–24. The Sun moves from the northern to the southern celestial hemisphere. Marks the start of astronomical autumn in the Northern Hemisphere.

Why dates vary

Equinox dates shift slightly each year due to the elliptical shape of Earth's orbit and the Gregorian calendar's leap year corrections. Over the 400-year Gregorian cycle, the vernal equinox falls between March 19 and March 21.

Solstice vs. equinox

Equinoxes and solstices are the four astronomical markers of the seasons. At the equinoxes (March and September), day and night are approximately equal in length everywhere on Earth. At the solstices (June and December), one hemisphere experiences its longest day and the other its shortest. The June solstice is the longest day in the Northern Hemisphere (around June 20–21); the December solstice is the shortest (around December 21–22).

Cultural significance

Equinoxes have marked the beginning of new seasons across many cultures for millennia:

Nowruz: Persian New Year celebrated at the vernal equinox; one of the world's oldest festivals.
Ostara: modern Pagan spring festival aligned with the March equinox.
Mabon: Pagan autumn harvest celebration at the September equinox.
Autumnal Equinox Day: a national holiday in Japan (Shūbun no Hi).
Stonehenge alignments: the monument aligns with sunrise and sunset at both equinoxes.

Astronomical explanation

Earth's rotational axis is tilted 23.5° relative to its orbital plane around the Sun. This tilt is what causes the seasons - not Earth's distance from the Sun. At the vernal equinox, the Sun crosses the celestial equator moving northward. At this moment, every location on Earth receives approximately 12 hours of daylight. The slight variations from exactly 12 hours are due to atmospheric refraction and the finite angular diameter of the Sun.