Today is the First Day of Fall – Autumn Equinox Explained

by Brad Dison

Today, September 22, 2021, at precisely 3:20 p.m. is the Autumn Equinox which marks the First Day of Fall.  But what is Autumn Equinox?

Autumn Equinox, also called Autumnal Equinox, is the astronomical beginning of the fall season in the Northern Hemisphere.

Equinox derives the combation of two Latin words, aequus, which means “equal” and nox, “night.”  On the equinox, the time length of day and night are roughly equal.  After the Autumn Equinox, days become shorter than nights. 

The full moon nearest to the Autumn Equinox is called the Harvest Moon.  It’s name derived from farmers who used the light from this full moon to finish their harvest. 

Pretty soon, leaves will begin to change color due to shortened daylight hours and photosynthesis.  

Can you see it?  When the Sun shines on the main pyramid at Chichen Itza, Mexico at the exact moment of the equinox, a giant “snake of sunlight” seemingly slithers down the pyramids stairs.


  1. The Old Farmer’s Almanac. “Autumnal Equinox 2021: The First Day of Fall Facts.” September 16, 2021.

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