An equinox is one of two days each year when daytime and nighttime are nearly equal in length all over Earth.

Earth tilts on its axis as it orbits the Sun, which is why we have seasons. During most of the year, one hemisphere gets more sunlight than the other: summer in the north means winter in the south. But twice a year, around March 20th and September 22nd, Earth reaches a position where neither hemisphere tilts toward or away from the Sun. On these days, the Sun rises directly in the east and sets directly in the west, and almost everyone on the planet gets about twelve hours of daylight and twelve hours of darkness.

The spring equinox (also called the vernal equinox) marks the beginning of spring in the Northern Hemisphere, when days start getting longer than nights. The autumn equinox (or autumnal equinox) marks the beginning of fall, when nights start growing longer than days. In the Southern Hemisphere, these seasons are reversed: their spring begins in September, their autumn begins in March.

Many cultures have celebrated equinoxes for thousands of years. Ancient people built monuments like Stonehenge partly to track these astronomical events, which helped them know when to plant crops or prepare for winter.