Astronomy Essentials

September equinox: All you need to know

A woman standing on an ocean pier, throwing a net toward a reddish sun in a red sky.
View at EarthSky Community Photos. | Across a wide swath of the U.S. now, the sky looks hazy, and the sun looks redder than usual, due to the wildfires in the U.S. West. Nancy Ricigliano on Long Island, New York, caught this image on September 16, 2020, and wrote: “This is a result from the smoke from the wildfires that have reached so many places, including New York. This photo was taken at Jones Beach, Pier 10. Normally at this time, you can’t even take a picture of the sun without a filter.” Thank you, Nancy.

Equinox comes on September 22

The equinox will arrive on September 22, 2021, at 19:21 UTC. That’s when the sun will be exactly above Earth’s equator, moving from north to south. Although the equinox happens at the same moment worldwide, your clock time will depend on your time zone. For time zones in the continental U.S., this equinox comes during the daylight hours on September 22 (3:21 p.m. EDT, 2:21 p.m. CDT, 1:21 p.m. MDT and 12:21 p.m. PDT). Translate UTC to your time.

At the equinox, days and nights will be approximately equal in length. For us in the Northern Hemisphere, the sun is rising later now, and nightfall comes sooner. We’re enjoying the cooler days of almost-autumn.

Meanwhile, south of the equator, spring is about to begin. Their autumn equinox happens in March, or when we in the northern hemisphere have our spring equinox.

Earth perfectly upright with vertical axis, laft 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. But that doesn’t mean that day and night are exactly equal in length. Two factors cause more day than night at an equinox. Image via Wikipedia.

What is an equinox?

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. 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.

Our ancestors built the first observatories to track the sun’s progress. One example is at Machu Picchu in Peru, where the Intihuatana stone, shown below, has been shown to be a precise indicator of the date of the two equinoxes and other significant celestial periods. The word Intihuatana, by the way, literally means for tying the sun.

Stacked slabs of rock with one vertical rock sticking up from the center.
The Intihuatana stone – also called the Hitching Post of the Sun – at Machu Picchu in Peru. It was used to track the sun throughout the year. Photo via

Equinox an astronomical event

Today, we know each equinox and solstice is an astronomical event, caused by Earth’s tilt on its axis and ceaseless orbit around the sun.

Because Earth doesn’t orbit upright, but is instead tilted on its axis by 23 1/2 degrees, Earth’s Northern and Southern Hemispheres trade places throughout the year in receiving the sun’s light and warmth most directly.

At equinox, Earth’s axis points neither toward or away from sun

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.

Earth’s two hemispheres are receiving the sun’s rays about equally around equinox-time. The sun is overhead at noon as seen from the equator. Night and day are approximately equal in length.

The name equinox comes from the Latin aequus (equal) and nox (night).

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

Symmetrical oak tree starting to turn orange against partly cloudy blue sky.
Autumn in Sweden via EarthSky Facebook friend Jörgen Norrland Andersson.

Signs of the equinox in nature

Where should I look to see signs of the equinox in nature? The knowledge that summer is gone – and winter is coming – is everywhere now, on the northern half of Earth’s globe.

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. Birds and butterflies are migrating southward, too, along with the path of the sun.

Shorter days and longer nights

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.

A darkened skyline, with smoke from a chimney blowing sideways and a single star, Fomalhaut, 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!

Sun rises due east and sets due west

Does the sun rise due east and set due west at the equinox? Generally speaking, yes, it does. And that’s true no matter where you live on Earth, 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. 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 noon as seen from Earth’s equator, as the illustration below shows.

Diagram of hemsipherical dome with lines of latitude and longitude.
Illustration of the sun’s location on the celestial equator, every hour, on the day of an equinox, via Tau’olunga/ Wikimedia Commons.

Sun on celestial equator

That’s why the sun rises due east and sets due west for all of us. The sun is on the celestial equator, and the celestial equator intersects all of our horizons at points due east and due west.

This fact makes the day of an equinox a good day for finding due east and due west from your yard or other favorite site for watching the sky. Just go outside around sunset or sunrise 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 orbit around the sun, carrying the sunrise and sunset points southward.

Sunrise, flying birds, 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!

Bottom line: Enjoy the September 2021 equinox – a seasonal signpost in Earth’s orbit around the sun!

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

Year’s fastest sunsets at equinox

Sun over Earth’s equator at equinox

September 17, 2021
Astronomy Essentials

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