The arrival of fall often announces itself with crisp breezes and changing leaf colors. But autumn, as the season is also known, doesn't officially begin until the fall equinox, which falls this year on Wednesday. Here is everything you need to know about the fall equinox.

In the northern hemisphere, the fall, or autumnal, equinox occurs every year in September, usually on the 22nd, 23rd or 24th. It is the moment when the sun crosses what's known as the celestial equator, the imaginary line over the equator, that signifies when night and day are the same length.

Because the Earth's axis is tilted at a slight angle, during all other days of the year, except for the spring, or vernal, equinox, night and day are not equal in length. The word "equinox" is derived from Latin, in which "equi" refers to equal and "nox," night. The counterpart of the equinox is the solstice, of which there are two, during the summer and winter. In the northern hemisphere, the summer solstice is the longest day of the year; the winter solstice is the shortest.

The exact time of this year's autumnal equinox, when the sun passes over the equator is 4:21 a.m. Wednesday. 



Because the fall is also harvest time, many festivals and traditions celebrate the arrival of autumn, from the Jewish Sukkot to the Chinese Mid-Autumn Festival. Nathaniel Hawthorne saw something special in fall; “I cannot endure to waste anything so precious as autumnal sunshine by staying in the house," he wrote in The American Notebooks.

But not everyone sees fall as so joyful and celebratory. As Ernest Hemingway wrote 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."