This week we begin the Autumn season. It happens precisely at 7:57 p.m. Eastern Daylight Time September 22nd. This equinox marks the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere.
On this day, September 22nd, the Sun rises due east and sets due west. There is an equal number of daylight and nighttime hours. The roots of the word equinox means "equal." After that date the Sun begins to rise and set a little more south of east and west every day until the winter solstice in December.
Because the Sun appears to rise and set farther and farther south, it is in the sky less hours and does not reach as high a noonday height in the sky. This of course accounts for our shorter and colder days.