Astronomy Essentials

2024 September equinox: All you need to know

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September 22, 2024

Happy equinox! We celebrate the September equinox as the first day of autumn for the Northern Hemisphere, and the first day of spring for the Southern Hemisphere. Watch a video about the September equinox, above.

The September equinox is a seasonal milestone in Earth’s yearly orbit around the sun. At an equinox, the sun appears directly above Earth’s equator. At the September equinox, the sun is moving from north to south as it crosses above the equator. It’s bringing summer to the Southern Hemisphere and winter to the Northern Hemisphere.

The 2024 September equinox will fall at 12:44 UTC (7:44 a.m. CDT) on September 22, 2024. On this day, days and nights are approximately (but not exactly) equal in length for everyone across the globe. The word equinox comes from the Latin aequus (equal) and nox (night), that is, equal night.

Keep reading to learn more about this important day.

Drawing of a yellow sun in space, with an oval shape around it (an orbital path). Two titled Earths are located opposite each other on the orbital path. — Happy equinox! This artist’s concept of the September and March equinoxes isn’t to scale. But it illustrates the fact that every equinox is a milestone in Earth’s orbit around the sun. And it shows that, at the equinoxes, Earth’s Northern and Southern Hemispheres receive nearly equal amounts of daylight. Image via NASA/ GSFC/ Genna Duberstein.

Earth’s tilt causes it

The earliest humans spent more time outside than we do. They used the sky as both a clock and a calendar. And they could easily see that the sun’s path across the sky, the length of daylight and the location of the sunrise and sunset all shift in a regular way throughout the year.

The equinoxes and solstices happen because Earth tilts on its axis by 23 1/2 degrees. Because of the Earth’s tilt, the Northern and Southern Hemispheres trade places in receiving the sun’s light and warmth most directly. The solstices indicate our greatest (or least) tilt toward the sun, in either hemisphere. The equinoxes fall midway between the solstices.

Earth’s two hemispheres receive the sun’s rays about equally around equinox time. But Earth never stops moving in orbit around the sun. And these days of approximately equal daylight and night will change quickly as we move toward the December solstice. Maybe you’ve noticed that? The length of daylight changes more quickly from day to day around the equinoxes than around the time of the solstices.

September equinox: Earth perfectly upright with vertical axis, left half sunlit in yellow with 3 arrows toward Earth, annotated sun rays, right half in shadow, with 5 latitudes annotated. — Around the time of an equinox, Earth’s Northern and Southern Hemispheres are receiving the sun’s rays equally. However, that *doesn’t* mean that day and night are exactly equal in length. In fact, 2 factors cause more day than night during an equinox. Image via Wikipedia (CC BY-SA 2.0).

A good day to find due east and due west

The day of an equinox is a good day for finding the directions due east and due west from your favorite place to watch the sky. The sun rises due east and sets due west at the equinoxes. It’s true no matter where you live on Earth. Why? Because we all see the same sky.

Everywhere on Earth, except at the North and South Poles, you have a due east and due west point on your horizon. And each point marks the intersection of your horizon with the celestial equator, the imaginary line above the true equator of the Earth.

At the equinoxes, the sun appears overhead at local solar noon as seen from Earth’s equator, as the illustration below shows. The sun is on the celestial equator. The celestial equator intersects due east and due west for everyone around the globe. So the sun rises and sets due east and due west at the equinox.

So go outside around sunset or sunrise on the day of an equinox. Notice the location of the sun on the horizon with respect to familiar landmarks. If you do this, you’ll be able to use those landmarks to find those cardinal directions in the weeks and months ahead, long after Earth has moved on in its ceaseless orbit around the sun.

Equinox sun: Diagram of dome with lines of latitude and longitude and red dots around base. — Illustration of the sun’s location on the celestial equator, every hour, on the day of an equinox, via Tau’olunga/ Wikimedia Commons (CC BY-SA 2.5).

Signs of the September equinox in nature

The signs that summer is gone – and winter is coming – are everywhere now on the northern half of Earth’s globe. In the Northern Hemisphere, dawn comes later, and sunset earlier. Also, notice the arc of the sun across the sky. It’s shifting southward now. And birds and butterflies are migrating south, along with the path of the sun.

The shorter days are bringing cooler weather. A chill is in the air. In New York City and other fashionable places, some people have stopped wearing white. Creatures of the wild are putting on their winter coats.

All around us, trees and plants are ending this year’s cycle of growth. Perhaps they’re responding with glorious autumn leaves, or a last burst of bloom before winter comes.

In the night sky, Fomalhaut – our Autumn Star – is making its way across the heavens each night.

Guy Ottewell chart of eclipse night - September 17-18, 2024 - showing both Saturn and Fomalhaut, as well as a bent lines in light blue with annotations. — In most years, around the September equinox, we point out the bright star Fomalhaut. This star typically appears bright and solitary in the southern sky in autumn, as seen from northern part of the globe. From our part of the world, Fomalhaut is sometimes called the Autumn Star. But in 2024, Fomalhaut isn’t alone. The planet Saturn is near it! Watch for the twin beacons of Fomalhaut and Saturn in the evening sky around the equinox. This chart also shows the location of the eclipsed moon – near this planet, and this star – on the night of September 17-18, 2024. Chart via Guy Ottewell’s blog.

Star chart outlining a blob-like shape with 1 star, Fomalhaut, labeled. — Here’s Fomalhaut’s constellation Piscis Austrinus the Southern Fish. When you look towards this part of the sky, you’re looking out our galaxy’s south window, into intergalactic space. Fomalhaut is bright! But the rest of the stars of Piscis Austrinus are tough to see without a dark sky. Chart via EarthSky.

September equinox images from EarthSky’s community

Autumn trees in the distance, reflecting in a body of water with a log laying across the lake. — Paul C. Peh captured this image in New York on October 23, 2022, and wrote: “This photo was taken during our vacation in the late fall around the Hudson Valley. The foliage and the scenery were simply beautiful beyond words. We found the fall colors, and it was way better than we had expected. Happy Autumnal Equinox Day, everyone!” Thank you, Paul!

Series of images in orange shades of sunrises and sunsets on the September equinox. — View at EarthSky Community Photos. | Peter Lowenstein captured these images and wrote: “Equinox sunsets before and after equinox sunrise on 23 September. The instant of the Southern Hemisphere spring equinox in 2023 was at 6.50 UTC on 23 September. This means it was possible to record two equinox sunsets one on the 22nd a few hours before and another a few hours after the equinox sunrise on the 23rd. These are shown in the accompanying composite of three 9 video frame montages.” Thank you, Peter!

29 photos of the sun creating an infinity symbol shape over a cityscape in twilight. — View at EarthSky Community Photos. | Meiying Lee in Taipei, Taiwan, captured this solar analemma showing the sun’s path over a year, from equinox to equinox, and wrote: “This sun analemma photo of Taipei is composed … by stacking 30 images taken at 4:30 p.m. on different days with a sun filter. They were shot from September 22, 2020, to September 23, 2021, from the same viewing platform on an eastern hill of Taipei. The foreground is the city view of Taipei as seen at the location, with the tallest building being Taipei 101, a famous Taipei landmark.” Thank you, Mei-Ying!

Equinoxes and Earth’s seasons

Seasons diagram. — The tilt of the Earth’s axis affects the amount of sunlight we each receive, on our various locations on the globe, as Earth travels around the sun. At the equinoxes, the sun in shining most evenly across Earth. Image via NASA.

Bottom line: The September equinox is here! It’ll arrive at 12:44 UTC on September 22, 2024. The sun will be exactly above Earth’s equator, moving from north to south. Autumn for the Northern Hemisphere. Spring for the Southern Hemisphere. Here’s all you need to know.

Read: Year’s fastest sunsets at equinox

Posted

September 22, 2024

Astronomy Essentials

Deborah Byrd

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About the Author:

Our Editor-in-Chief Deborah Byrd works to keep all the astronomy balls in the air between EarthSky's website, YouTube page and social media platforms. She's the primary editor of our popular daily newsletter and a frequent host of EarthSky livestreams. Deborah created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Prior to that, she had worked for the University of Texas McDonald Observatory since 1976, and created and produced their Star Date radio series. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. In 2020, she won the Education Prize from the American Astronomical Society, the largest organization of professional astronomers in North America. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.

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