What Is the Autumnal Equinox?

The autumnal equinox is an astronomical event that marks the start of autumn (or “fall”). In the Northern Hemisphere, the autumnal equinox occurs in September; in the Southern Hemisphere, it occurs in March. 

What Is an Equinox?

During an equinox, the Sun crosses 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.

For those of us in the Northern Hemisphere, when the Sun crosses the equator going from north to south, this marks the autumnal equinox; when it crosses from south to north, this marks the vernal equinox. In the Southern Hemisphere, it’s the reverse.

After the autumnal equinox, days become shorter than nights as the Sun continues to rise later and nightfall arrives earlier. This ends with the winter solstice, after which days start to grow longer once again. 

The word “equinox” comes from Latin aequus, meaning “equal,” and nox, ”night.” On the equinox, day and night are roughly equal in length.

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!

Fall Weather

It is the summer’s great last heat,
It is the fall’s first chill: They meet.
–Sarah Morgan Bryan Piatt

Another definition of fall is “nights of below-freezing temperatures combined with days of temperatures below 70 degrees Fahrenheit (21°C)”. From here on out, the temperatures begin to drop.

Fall Foliage

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 edge actually 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). 

Q: Can You Balance An Egg On the Equinox?

A: There’s an old-wives’ 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 possible not only on the equinox. Egg balancing truths.

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!

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.  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.