Why the equinox ushers in the arrival of spring

Every six months, once in March and again in September, an equinox splits Earth’s day almost in half, giving us about 12 hours of daylight and 12 of night.

On March 19, 2024, the vernal equinox will signal the coming of spring for the Northern Hemisphere. Then, in September, nature will once again bring us the autumnal equinox, the time of year that ushers in fall in the North. Those dates are swapped for the vernal and autumnal equinoxes in the Southern Hemisphere. (Here's why each season begins twice.)

Why do equinoxes happen?

Our planet normally orbits the sun on an axis that’s tilted 23.5 degrees, meaning that the hemispheres trade off getting more warmth from the sun. Two times a year, Earth’s orbit and its axial tilt combine so that the sun sits right above Earth’s Equator, casting the dividing line between the light and dark parts of the planet—the so-called terminator, or twilight zone—through the North and South Poles.

The terminator doesn’t perfectly divide the planet into dark and light; Earth’s atmosphere bends sunlight by 37 miles (60 kilometers), which equals half a degree. That means one half of the planet is still a little more lit than the other, even on an equinox.

Earth isn’t the only planet that experiences equinoxes: Every planet in our solar system has them. In 2009, the Cassini probe in orbit around Saturn captured an equinox on the ringed planet. As on Earth, equinoxes occur every half-year on Saturn, but that equals 15 years on Earth, making Cassini's photo session a unique event.

Marking the equinox

Ancient cultures have tracked the equinoxes in different ways over the millennia. From constructed monuments, like pyramids, to stone engravings that acted as calendars, to churches that incorporated the sun into their architecture, civilizations marked the passing of the sun and the seasons with great accuracy.

Some cultures continue to celebrate the equinox today, like the Lakota Tribe of the U.S. Midwest. The Lakota connect the earth with the sky by making tobacco from the red willow tree, which matches the Dried Willow constellation, where the sun rises on the spring equinox. They smoke this sacred tobacco in a ceremony marking the return of longer days.

And at Stonehenge's equinox celebrations in England, druids, pagans, and anyone else who wants to join in gather to witness the sunrise over the ancient stones.