Equinoxes

An equinox is one of two moments in the year when the plane of Earth's equator passes through the center of the Sun. At an equinox the Sun lies directly above the celestial equator, and the Sun’s declination is zero degrees.

There are two equinoxes each year: the vernal (spring) equinox and the autumnal (fall) equinox. They occur roughly on March 20–21 and September 22–23, though the exact dates vary slightly from year to year. The names reflect the seasonal change in temperate regions, with the vernal equinox signaling the start of spring in the Northern Hemisphere and the autumnal signaling the start of autumn. In the Southern Hemisphere, the seasons are opposite.

Day length around the equinox is approximately equal between day and night, which is the origin of the term equinox. In practice, daylight and night are not exactly 12 hours each, especially due to atmospheric refraction and the Sun’s apparent disk. The effect is more pronounced at higher latitudes, while near the equator the lengths are very similar.

Equinoxes are distinct from solstices, which mark the longest and shortest days of the year. The equinoxes arise from the tilt of Earth’s axis relative to its orbit around the Sun, and over long timescales the positions of the equinoxes shift due to the precession of the equinoxes. The concept has been used in calendars and cultural traditions around the world, such as Nowruz in parts of the Middle East and Central Asia.