In mid-September each year, we greet the fall season with the arrival of the Autumnal Equinox. This is the moment when the Sun crosses the Equator, and those of us living in the Northern Hemisphere begin to see more darkness than daylight. Regardless of whether it has been chilly for weeks or there are still balmy summer-like temperatures, this day 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 (at least in terms of light). Even after three months of shortening days, though, we still see 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.
In December, we will experience the Winter Solstice, 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.
From the Vernal Equinox, the days will continue to grow longer, until we reach the Summer Solstice again, and the whole cycle begins anew!