The first day of fall has arrived. Here's how the equinox marks the changing of seasons.

  • In Science
  • 2022-09-22 10:17:44Z
  • By Business Insider
A view of Earth taken by NASA
A view of Earth taken by NASA's Earth Polychromatic Imaging Camera.Reuters  
  • The 2022 fall or autumnal equinox happens Thursday, September 22.

  • The fall equinox happens when the sun's warming rays square up perfectly with the earth.

  • During an equinox at Earth's equator, the sun appears almost directly overhead.

Thursday is this year's fall equinox.

This astronomical event, which happens every year, means the day is about as long as the night.

Also called the September or autumnal equinox, this event means the fall starts officially and summer ends as the Northern hemisphere heads towards colder and darker days.

For those in the southern hemisphere, the milestone marks the beginning of spring.

There are two reasons for these all-important seasonal shifts: the Earth's tilted axis and the planet's orbit around the sun.

How the fall equinox works

In June and December, the Earth hits its solstices - the longest and shortest days of the year.

And twice a year, in March and September, the Earth reaches its equinoxes, meaning the days and the nights are roughly the same lengths of time.

Earth equinoxes and solstices graphic
Earth equinoxes and solstices graphic  

This is why: the sun rises and falls on our planet once per day because our planet also rotates around its axis.

But it takes about a year -around 365 days, 5 hours, 59 minutes, and 16 seconds to be more precise - for the Earth makes its way around the sun, per NASA.

The Earth's axis isn't completely squared up with the plane of its orbit around the sun. Instead, its axis is slightly tilted to one side, about 23.5 degrees (for now).

That means the rays from the sun hit different parts of the world at different angles, depending on the time of the year.

When the Earth is tilted towards the sun, during the summer, more sunlight can hit the Earth and it can warm up the atmosphere. But when the Earth is tilted away, during the winter, less sunlight hits the Earth, and the atmosphere is cooler.

During an equinox, however, the Earth squares up perfectly with the sun's warming rays. That means that the most intense sun rays hit the Earth straight on the equator, and the day and the night last about the same amount of time.

Earth during equinox
Earth during equinox  

This year, this will happen at precisely 9:03 p.m. ET.

If you were to stand directly on the equator at the moment the equinox peaks, the day would last about 12 hours and six-and-a-half minutes. The sun would appear more or less directly overhead, and your shadow would also be as small as it would ever be.

But this moment doesn't last, since the Earth makes its way around the sun at a speed of roughly 66,600 mph.

An animation below provides an overview of how this works:


Uneven seasons

Our planet's orbit around the sun isn't a perfect circle. It is actually slightly elliptical, and its center of gravity is slightly offset from the sun, so the time it takes to cycle through the seasons isn't perfectly divvied up.

About 89 days and 19 hours after the fall equinox, the Earth will reach its winter solstice - when the most direct sunlight strikes the Southern Tropic (or Tropic of Capricorn). Another 89 days later, the spring or vernal equinox will occur.

Then it's another 93 days and 18 hours to the summer solstice - when the most direct rays of the sun reach their northernmost latitude, called the Northern Tropic (or Tropic of Cancer) - and another 92 days and 16 hours to get back to the fall equinox.

Druids, pagans and others gather around Stonehenge on September 23, 2017.Matt Cardy/Getty Image
Druids, pagans and others gather around Stonehenge on September 23, 2017.Matt Cardy/Getty Image  

Celebrating equinox

For some, equinox is more than a day that comes with the return of the pumpkin spice latte. It has marked the arrival of the harvest season for many throughout human history and several cultures still mark the event today per USA Today.

One example is at Stonehenge in the UK, where pagans and druids gather around the monolithic structure to watch the sun rise over the stones.

Dave Mosher contributed to a previous version of this article


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