2023 September equinox: All you need to know
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).
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.
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.
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.
September equinox images from EarthSky’s community
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.