Astronomy Essentials

2023 September equinox: All you need to know

September equinox: 4 satellite views of Earth, with different seasonal tilts.
Satellite views of Earth on the solstices and equinoxes. From left to right, a June solstice, a September equinox, a December solstice and a March equinox. Read more about these images, which are via NASA Earth Observatory.

The equinox is here! Autumn for the Northern Hemisphere. Spring for the Southern Hemisphere.

What is it? The September equinox marks the sun’s crossing above Earth’s equator, moving from north to south.
When is the next one? The next September equinox will fall at 6:50 UTC September 23, 2023 (1:50 a.m. CDT on September 23 for central North America; translate UTC to your time).
Note: On this equinox, the sun crosses above Earth’s equator, moving from north to south. So, for everyone across the globe, days and nights are approximately equal in length. The name equinox comes from the Latin aequus (equal) and nox (night).

Read: Why aren’t day and night exactly equal on the 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, two factors cause more day than night at an equinox. Image via NASA.

What is an equinox?

The earliest humans spent more time outside than we do. They used the sky as both a clock and a calendar. Indeed, 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.

Today, we know each equinox and solstice is an astronomical event. It’s caused by Earth’s tilt on its axis and ceaseless orbit around the sun. For example, Earth is tilted by 23 1/2 degrees. And that means Earth’s Northern and Southern Hemispheres trade places throughout the year in receiving the sun’s light and warmth most directly.

Spring equinox and fall equinox

We have an equinox twice a year – spring and fall – when the tilt of the Earth’s axis and Earth’s orbit around the sun combine in such a way that the axis is inclined neither away from nor toward the sun. In fact, Earth’s two hemispheres are receiving the sun’s rays about equally around equinox time. The sun is overhead at solar noon as seen from the equator. As a result, night and day are approximately equal.

Of course, Earth never stops moving around the sun. So for this reason, these days of approximately equal sunlight and night will change quickly.

Photos of the sun creating an infinity symbol shape with a view of Taipei, Taiwan, in the foreground.
View at EarthSky Community Photos. | Mei-Ying Lee in Taipei, Taiwan, captured this solar analemma during an entire year, from September 2020 to September 2021 (equinox to equinox), and wrote: “This sun analemma photo of Taipei is composed in Startrails by stacking 30 images taken at 4:30 p.m. on different days with 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!

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. That’s because, generally speaking, 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. Therefore, that 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.

So with this in mind, go outside around sunset or sunrise, and notice the location of the sun on the horizon with respect to familiar landmarks. Indeed, 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 orbit around the sun, carrying the sunrise and sunset points southward.

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 knowledge that summer is gone – and winter is coming – is everywhere now, on the northern half of Earth’s globe. Indeed, if you live in the Northern Hemisphere, you can easily notice the later dawns and earlier sunsets. Also, notice the arc of the sun across the sky each day. You’ll find it’s shifting toward the south. In like manner, birds and butterflies are migrating southward, too, along with the path of the sun.

In addition, 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 are responding with glorious autumn leaves, or a last burst of bloom before winter comes.

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

Read: Year’s fastest sunsets at equinox

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

September equinox images from EarthSky’s community

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 – on September 16, 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!
Symmetrical oak tree starting to turn orange against partly cloudy blue sky.
Autumn in Sweden via EarthSky Facebook friend Jörgen Norrland Andersson.

Bottom line: The September equinox is here! It’ll arrive at 06:50 UTC on September 23, 2023. 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.

September 22, 2023
Astronomy Essentials

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