The autumnal equinox occurs today, September 22nd, at 344 pm CDT. The autumnal equinox marks one of two times during the year, when the tilt of the earth's axis is neither towards nor away from the Sun. The term equinox originates from the Latin aequus--meaning "equal"--and nox--meaning "night." At about the time of the equinox, the night and day have approximately equal length. The other equinox occurs at the beginning of Spring. After the Fall equinox, the nights become increasingly long in Middle Tennessee, with a corresponding decrease in daylight hours. This continues to be the case until the Winter Solstice, in December--at which point the daylight hours will gradually start to increase again.
Seasons are caused by the fact that the Earth is tilted on its axis by 23.5°. As the Earth journeys around the sun, it's general inclination toward or away from the sun is constantly changing. The Northern Hemisphere is maximally tilted toward the sun in June (when the Northern Hemisphere experiences the beginning of Summer), and the Southern Hemisphere is maximally tilted toward the sun in December (when the Southern Hemisphere experiences the beginning of Summer). During the autumnal equinox and Vernal (Spring) equinox, there is no tilt toward nor away from the sun at all.