The first day of fall, also known as the autumnal equinox, is Sunday, Sept. 22, at 8:44 a.m. ET. While the equinox is known as the astronomical start of the season, there is another method used to measure the change in seasons. The meteorological beginnings of seasons are based on the annual temperature cycle and the 12-month calendar.

See how our seasons come from Earth's tilt and yearly loop around the sun, which changes the ice cover, vegetation, and sunlight across the planet.