Astronomy Essentials

2024 September equinox: All you need to know

Orangish full Harvest Moon in the distance sandwiched between high rise buildings.
View at EarthSky Community Photos. | Kent Kirkley of Dallas, Texas, captured this image on September 9, 2022, and wrote: “September Harvest Full Moon rise, downtown Dallas, Texas.” Thank you, Kent!

We celebrate the September equinox as the first day of autumn for the Northern Hemisphere, and first day of spring for the Southern Hemisphere.

What is it? It’s a milestone in Earth’s orbit around the sun. At an equinox, the sun appears directly above Earth’s equator. At the September equinox, it’s crossing from north to south.
When is it? The September equinox will fall at 12:44 UTC (7:44 a.m. CDT) on September 22, 2024.
Note: The name equinox comes from the Latin aequus (equal) and nox (night). At the equinox, days and nights are said to be equal in length for everyone across the globe. And it’s true. They are approximately equal.

Read more: Why aren’t day and night exactly equal on the equinox?

Four images of half-Earth from space, 2 upright and 2 tilted.
Satellite views of Earth on the solstices and equinoxes. We are at the September equinox now. Read more about these images, which are via NASA Earth Observatory.

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.

And, indeed, 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, right half in shadow.
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).

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. And 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.

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.

Constellation chart showing Piscis Austrinus and Fomalhaut.
A representation of Piscis Austrinus and its bright star Fomalhaut, via Torsten Bronger/ Wikimedia Commons (GFDL).

September equinox images from EarthSky’s community

September equinox: 29 photos of the sun creating an infinity symbol shape over a cityscape in twilight.
View at EarthSky Community Photos. | Mei-Ying Lee in Taipei, Taiwan, captured this solar analemma (it shows the sun’s path over a year), from September 2020 to September 2021 (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.” Thank you, Mei-Ying!
Sunrise, flying birds, sun peeking out over very top of palm tree.
View at EarthSky Community Photos. | Peter Lowenstein in Mutare, Zimbabwe, captured this glorious sunrise photo – a near-spring sunrise for him – in September 2020. He wrote: “Rising sun behind palm tree with circling swallows and crows.” Thank you, Peter!
A darkened skyline, with smoke from a chimney blowing sideways and a single star above.
View larger. | From the Northern Hemisphere, Fomalhaut is sometimes called the Autumn Star. It’s also called the Loneliest Star because no other bright stars shine near it in the sky. Photo by EarthSky friend Tony Gieracki. Thank you, Tony!
Trees with brilliant red leaves and a gap showing gray rain clouds behind.
View at EarthSky Community Photos. | Sharon Kizer, who is mother to EarthSky’s Kelly Kizer Whitt, took this image of fiery maples and rain clouds on October 9, 2022, in Madison, Wisconsin. It illustrates some of the vivid reds of autumn. Thank you, Sharon!

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

Read more: Equinox shadows trace a straight line from west to east

September 22, 2024
Astronomy Essentials

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