Equinox

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An equinox is the moment when the Sun appears directly above the Earth’s equator. On those days, the Sun rises roughly in the east and sets roughly in the west, and day and night are about the same length.

Equinoxes happen twice a year: around March 20 (the vernal or spring equinox in the Northern Hemisphere) and around September 22 (the autumnal or fall equinox). In the Southern Hemisphere, the seasons are opposite even though the dates are the same.

At the equinox the Sun’s path crosses the celestial equator, and the tilt of Earth’s axis is such that neither hemisphere is tilted toward or away from the Sun. The date shifts slightly over the years because our calendar must stay in sync with the Earth's orbit.

Day and night are not exactly equal on the equinox. The Sun is a disk, light is refracted by the atmosphere, and the length of days changes quickly around this time, so the precise moment of equal daylight is a little different depending on where you are. The special moment when day and night are exactly equal is called the equilux, and it happens only roughly and at different places.

Culturally, the vernal equinox often marks the start of spring in the Northern Hemisphere, while the autumnal equinox marks autumn. Various cultures celebrate springtime festivals around these dates, such as Nowruz in Iran and nearby regions.

The dates of the equinoxes drift slightly over long periods due to how our calendar and leap years interact with the Earth's orbit.