The official first day of fall, otherwise known as the autumnal equinox, is this weekend in the Northern Hemisphere. According to Encyclopedia Britannica, it is one of only two days, the other being the vernal equinox, when the sun is directly above the Equator, essentially dividing night and day into equal portions.

The exact time of the fall equinox changes slightly each year. This year, the equinox arrives Sept. 22 at 8:43 a.m. Eastern time.

The definition of the equinox as being a time of equal day and night is a convenient oversimplification. For one thing, it treats night as simply the time the sun i

During an equinox, places around the globe experience nearly equal amounts of daytime and nighttime. The second occurrence this year happens on Sunday.

If you get up early enough, you can experience an equinox sunrise. The autumnal equinox heralds the arrival of fall in the northern hemisphere.