Astronomy Essentials

2025 September equinox: All you need to know

Four black and white 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 this image. Images via NASA Earth Observatory.

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 2025 September equinox will fall at 18:20 UTC (1:20 p.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.

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.

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 18:20 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

Posted 
September 21, 2025
 in 
Astronomy Essentials

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