The first day of fall is today, Sunday, which is marked by the Autumnal Equinox 2013.
The equinox is the precise instant when the sun is located in the sky directly over Earth’s equator.
That instant is at 4:44 p.m. EDT.
“The Autumnal Equinox marks the first day of Autumn in the northern hemisphere. It is also the day the Sun disappears from view for 6 months (i.e. sets) as seen from the North Pole, and the day the Sun rises into the sky to be visible for 6 months as seen from the South Pole,” according to the University of Massachusetts Department of Astronomy.
The Autumnal Equinox and the Vernal Equinox (first day of spring) are the two days of the year when the Sun is above the horizon for exactly half the day and is below the horizon an equal amount of time, according to the nonprofit Pisgah Astronomical Research Institute.
“Thus, the length of daylight is equal to that of the night (neglecting twilight) and this day is termed the equinox from the Latin for ‘equal night,'” the institute explains. “After the equinox in September, called the autumnal equinox, the hours of daylight continue to shorten with the Sun above the horizon for a shorter time each day. This continues until the winter solstice in December (This year at 12:11 p.m. EST on December 21). Following the solstice the days get longer until at the spring or vernal equinox (next at 12:57 p.m. EDT on March 20), the day and night are once again equal in length.”
The word equinox is derived from two Latin words – aequus (equal) and nox (night), according to the National Weather Service.