Sunday, September 22, at 4:44 p.m., is the Autumnal Equinox for 2013. 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. In December (on December 21, 2013, at 12:11 p.m., to be exact) it will be the Winter Solstice, the shortest day of the year in terms of hours of daylight.
After the Winter Solstice, the days will begin to grown longer again. It will take another three months, until the Vernal Equinox (March 20, 2014, at 12:57 p.m.) 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 on June 21, 2014, at 6:51 a.m., and begin the whole cycle anew!