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Science And Nature

Why the equinox ushers in the arrival of fall

What’s an Equinox?

What exactly are equinoxes? What can cause these astronomical events? Learn how they influence the times of year and hours of daylight on each planet.

Published September 16, 2022

4 min read

Every half a year, once in March and again in September, an equinox splits Earths day almost in two, giving us about 12 hours of daylight and 12 of night.

On September 22, 2022, the autumnal equinox will signal the coming of fall for the Northern Hemisphere. Then, on March 20, 2023, nature will once more bring us the vernalequinox, enough time of year that ushers in spring 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?

Stonehenge is definitely a favorite destination for the equinoxes. Druids and pagans gather here to celebrate the total amount of light and dark on the planet.

Stonehenge is definitely a favorite destination for the equinoxes. Druids and pagans gather here to celebrate the total amount of light and dark on the planet.

Photograph by Donald Slack / Alamy Stock Photo

Our world normally orbits sunlight on an axis thats tilted 23.5 degrees, and therefore the hemispheres trade off getting ultimately more warmth from sunlight. Two times per year, Earths orbit and its own axial tilt combine so the sun sits right above Earths Equator, casting the dividing line between your light and dark elements of the earththe so-called terminator, or twilight zonethrough the North and South Poles.

The terminator doesnt perfectly divide the earth into dark and light; Earths atmosphere bends sunlight by 37 miles (60 kilometers), which equals half of a degree. Which means half of the earth is still a bit more lit compared to the other, even on an equinox.

Earth isnt the only real planet that experiences equinoxes: Every planet inside our solar system has them. In 2009, the Cassini probe in orbit around Saturn captured an equinox on the ringed planet. As on the planet, equinoxes occur every half-year on Saturn, but that equals 15 years on the planet, making Cassini’s photo session a distinctive event.

Marking the equinox

Ancient cultures have tracked the equinoxes in various ways on the millennia. From constructed monuments, like pyramids, to stone engravings that acted as calendars, to churches that incorporated sunlight to their architecture, civilizations marked the passage of sunlight and the times of year with great accuracy.

(Listen: In this bout of our podcast Overheard, we discuss Nowruz, a historical Persian holiday marking the brand new year right now of the vernal equinox.)

Some cultures continue steadily to celebrate the equinox today, just like the Lakota Tribe of the U.S. Midwest. The Lakota connect the planet earth with the sky by making tobacco from the red willow tree, which matches the Dried Willow constellation, where in fact 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 other people who would like to interact gather to witness the sunrise on the ancient stones.

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