Autumn is almost here. What to know about the 2021 fall equinox

Dean Olsen
State Journal-Register

Wednesday — specifically 3:20 p.m. Eastern time  — is the first day of fall, known as the autumnal equinox or the September equinox or fall equinox, according to The Old Farmer’s Almanac.

Here are some interesting details about this time of the year.

What is an equinox?

It’s when day and night are roughly equal in length. The earth spins on a tilted axis, and the almanac says the equinox is when the sun crosses the “‘celestial equator’ — an imaginary extension of the earth’s equator line in space. The equinox occurs precisely when the sun’s center passes through that 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.”

The leaves of a sugar maple along Williams Boulevard give way to their fall colors as the first sign that autumn will soon arrive in Springfield, Ill., Monday, September 20, 2021. [Justin L. Fowler/The State Journal-Register]

The seasons that we enjoy, according to, occur because the tilt of the earth means that for half of the year, the North Pole is pointed toward the sun. For the other half of the year, the South Pole gets more light. 

What happens after the autumnal equinox? 

Days become shorter than nights as the sun continues to rise later and nightfall arrives earlier. After the December solstice, days begin to get longer again.

What is the Harvest Moon, and how is it related to the equinox?

The almanac says the full moon closest to the autumnal equinox always is called the Harvest Moon, and the reason is related to farming. Near the fall equinox, according to the almanac, “the full moon rises around sunset for several nights in a row, which traditionally provided farmers with just enough light for them to finish their harvests before the killing frosts of fall set in. Normally, the moon rises about an hour later (than sunset) 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.”

When do the leaves start changing colors?

According to, the shortening of days causes trees to stop producing chlorophyll, the pigment that makes leaves green and is responsible for photosynthesis. Changes in colors are more dependent on light than temperature. Because of several variables, it's difficult to predict when fall colors will peak, but you can track the fall color change across New York.

Fall colors:Check out the latest New York foliage map to find colorful fall leaves this weekend

Meteorologists classify fall in a different way. Why? 

The equinoxes are based on astronomy, involving the specific positioning of the earth and sun, and so the days they occur each year can vary slightly. The meteorological classification is based on ease in maintaining climate-related records, according to Mike Albano, a meteorologist with the National Weather Service office in Lincoln, Illinois.

For meteorologists, fall begins every year on Sept. 1, and the seasons change on the first of the month every three months after that, he said. 

“I wish it were something cooler,” Albano said. “The astronomers have us beat as far as the ‘cool’ angle.”

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