AUTUMNAL EQUINOX 2021: THE FIRST DAY OF FALL


FACTS & FOLKLORE ABOUT THE FALL EQUINOX

By The EditorsSeptember 16, 2021

Autumn in Colorado
Pixabay

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 (192286)

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 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. 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. 

Harvest moon with people standing in front

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!

almanac.com