In late September each year, we greet the fall season with the arrival of the Autumnal Equinox. In 2017, it arrives on Friday, September 22 at precisely 4:02 p.m. EDT. Regardless of what’s happening outside your window, whether it’s been chilly or blazing hot temperature-wise, this is the official start of autumn.
So what does that mean? Essentially, our hours of daylight — the period of time each day between sunrise and sunset — have been growing slightly shorter each day since the Summer Solstice in June, which is the longest day of the year (in terms of daylight). Even after three months of shortening days, though, we still are seeing more light than darkness over the course of a day.
The Autumnal Equinox marks the turning point, when darkness begins to win out over daylight. For the next three months, our hours of daylight will continue to grow shorter. At this equinox, day and night are approximately equal in length. The name “equinox” comes from the Latin word aequus, meaning equal, and nox, meaning night. An equinox occurs twice a year (Autumnal and Vernal).
Everywhere you look, you can see the visible changes as nature prepares for winter: birds are flying south, temperatures are getting cooler, leaves are changing colors, and animals’ coats are thickening, to name a few.
The days are growing shorter until we reach the Winter Solstice on December 21 at 11:28 a.m. EST, which will mark the shortest day of the year in terms of hours of daylight.
After the Winter Solstice, the days will begin to grow longer again. It will take another three months, until the Vernal Equinox (also called the Spring Equinox), for the periods of daylight and darkness to reach equilibrium once again.