Officially, the Autumn or Fall Equinox starts September 22 at 4:44pm EDT. We've all probably felt that Fall has begun simply by the chill in the air the last few days. Daylight hours will become shorter and nights will become longer in the Northern hemisphere. In the Southern hemisphere Spring begins today.
The earth's axis is usually titled 23 degrees except for twice a year; during the Spring and Fall Equinox when the earth is more upright and both the Northern and Southern hemispheres receive the same amount of light and dark for a short period of time. As we head toward winter here in the Northern hemisphere, the earth will begin to tilt again and we will experience shorter days and longer nights until the Spring Equinox begins in the spring on March 20, 2014.
The Sun will be rising and setting due west and east for those of you who are sky watchers. The shortest day of the year will be December 21 which is the official beginning of Winter.