In 2021, the autumnal equinox—also called the September equinox or fall equinox—arrives on Wednesday, September 22. This date marks the start of fall in the Northern Hemisphere and spring in the Southern Hemisphere. Read about the signs of fall and the ways we mark the approaching equinox.
Autumn has caught us in our summer wear.
–Philip Larkin, British poet (1922–86)
When Is the Autumnal Equinox?
The fall equinox arrives on Wednesday, September 22, 2021, at 3:20 P.M.EDT in the Northern Hemisphere. The equinox occurs at the same moment worldwide.
Autumnal Equinox Dates
Autumnal Equinox (Northern Hemisphere)
Autumnal Equinox (Southern Hemisphere)
Wednesday, September 22
Saturday, March 20
Thursday, September 22
Sunday, March 20
Friday, September 22
Monday, March 20
Sunday, September 22
Tuesday, March 19
What Is the Autumnal Equinox?
Autumn days come quickly, like the running of a hound on the moor. –Irish proverb
The autumnal equinox—also called the September or fall equinox—is the astronomical start of the fall season in the Northern Hemisphere and of the spring season in the Southern Hemisphere.
What Is an Equinox?
The word “equinox” comes from Latin aequus, meaning “equal,” and nox, “night.” On the equinox, day and night are roughly equal in length. (See more about this below.)
During the equinox, the Suncrosses what we call the “celestial equator”—an imaginary extension of Earth’s equator line into space. The equinox occurs precisely when the Sun’s center passes through this line. When the Sun crosses the equator from north to south, this marks the autumnal equinox; when it crosses from south to north, this marks the vernal equinox.
After the autumnal equinox, days become shorter than nights as the Sun continues to rise later and nightfall arrives earlier. This ends with the December solstice, when days start to grow longer once again.
The Harvest Moon & the Equinox
One of our favorite pieces of trivia surrounding the autumnal equinox involves its relationship with the full Moon. Curiously, the full Moon that occurs nearest to the autumnal equinox is always called the ”Harvest Moon!” Why is that?
Surprise, surprise: it has to do with farming! Around the fall equinox, the full Moon rises around sunset for several nights in a row, which traditionally provided farmers with just enough extra light for them to finish their harvests before the killing frosts of fall set in. Normally, the Moon rises about an hour later each night, but around the time of the fall equinox, the angle of the Moon’s orbit and the tilt of the Earth line up just right and cause the Moon to rise only about 20 to 30 minutes later each night for several nights in a row!
An Astronomical Moon Name
The Harvest Moon is one of only two Moon names that are astronomical terms and aren’t tied to one specific month. Because it’s always the full Moon nearest to the equinox that’s called the “Harvest Moon,” either September or October’s full Moon can take on the name. (The other astronomical Moon name is the Hunter’s Moon, which is the full Moon that directly follows the Harvest Moon. It can occur in either October or November.)
Note that fall foliage isn’t due to current weather conditions. This is a common misconception. Leaves change color because of the amount of daylight and photosynthesis. Learn more about autumn leaves.
Fall Equinox FAQs
Q: Are Day and Night Perfectly Equal on the Equinox?
A: Some say that during an equinox, day and night is equal. Well, not exactly. It depends on where you live.
On the equinox, the center of the Sun is indeed above the horizon for 12 hours. However, “sunrise” is said to begin when the upper edge of the Sun’s disk becomes visible above the horizon (which happens a bit before the center rises) and ends when the entire Sun has set. In this case, daylight is still a bit longer than nighttime.
Not only that, but the Sun is actually visible when it is below the horizon, as Earth’s atmosphere refracts the Sun’s rays and bends them in an arc over the horizon. Yes, you can see the Sun before the edgeactually reaches the horizon! This causes daylight to be longer than 12 hours as well.
However, they are very close to equal (the total lengths may differ by only a few minutes).
Did you know our rise/set tool now provides day length? In Dublin, New Hampshire, USA—home of The Old Farmer’s Almanac—our day length on the equinox is 12:08 hours.
See our Sunrise/set calculator for day length in YOUR area.
Q: Is the Autumnal Equinox Really the First Day of Fall?
A: Based on the astronomical definition of seasons, yes, the autumnal equinox does mark the first day of fall (in the Northern Hemisphere). However, according to the meteorological definition of seasons, which is based on temperature cycles and the Gregorian calendar, the first day of fall is September 1.
Q: Can You Balance An Egg On the Equinox?
A: There’s an old-wife’s tale that you can stand an egg on its end of the equinox. Well, yes, it’s true (and fun to try). But it’s not only on the equinox. See more about equinox facts from Almanac astronomer, Bob Berman.
Signs of Fall
What are your local signs of fall? In many regions of the Northern Hemisphere, the landscape silently explodes with vibrant colors of red, yellow, and orange. The leaves begin to drop off the trees, providing endless hours of jumping into leaf piles for kids and raking them back up for parents!
Trees snapping and cracking in the autumn indicate dry weather.
Fall also brings some wonderful holidays, including Halloween and Thanksgiving, which carry us through the season until temperatures begin to drop, nights begin to get longer, and all the woodland critters start storing up for the long haul of winter.
And don’t forget about the end of Daylight Saving Time, when we “fall” back, setting our clocks back one hour and regaining an hour of precious sleep!
Plants and trees are slowing down, as sunlight decreases, to get ready for the colder season ahead. In the garden, asters and chrysanthemums bloom beautifully as orange pumpkins and corn mazes abound.
Football season is warming up and so is sweater weather.
Also notice the arc of the Sun across the sky each day as it starts shifting south. Birds and butterflies migrate along with the path of our Sun!