Here comes the changing of the leaves. Fall is set to officially begin Thursday with the Autumnal Equinox.

To be exact, the Northern Hemisphere enters fall at exactly 10:21 a.m. EDT, according to the Old Farmer's Almanac. The day — which has roughly an equal amount of nighttime and daytime — marks the official end of summer, a bummer to beach bums but a welcome change for fans of sweater weather. 

In Latin, the word equinox is composed of two words meaning "equal" and "night." This hints at the fact that during the equinox the Sun crosses the so-called "celestial equator," an imaginary line over the equator that signifies when night and day are equal to one another. 

The autumnal equinox occurs in September every year, most often on the 22nd, 23rd or 24th. There is also a spring, or vernal equinox, every year. The solstices — one during summer, another during winter — are the counterparts of the equinox. The summer solstice is the longest day of the year in the northern hemisphere, while the winter is the shortest day. 

Scientists do point out that night and day aren't exactly equal on the day of the equinox. The split can be off by a few minutes and it shifts the further you move from the equator. 

"The equinox is defined as the time of an event. It’s really not when the day and the night are of equal length, although that’s what we think of — it’s really that moment is when the sun is on the equator at local noon," Matthew Holman, an astrophysicist at Harvard University, told National Geographic last year.

There are a number of traditions timed to the coming of autumn, which traditionally meant it was time to harvest crops. Holidays include everything from the Jewish Sukkot to the Chinese Mid-Autumn Festival.

"No spring nor summer beauty hath such grace as I have seen in one autumnal face," English poet John Donne once wrote. 

Others, like Ernest Hemingway — who co-opted a Donne line for a book title — weren't fans of the season. Wrote Hemingway in "A Moveable Feast": "You expected to be sad in the fall. Part of you died each year when the leaves fell from the trees and their branches were bare against the wind and the cold, wintery light."