A:Autumn starts in the northern hemisphere on September 22 or 23, depending on the year. In 2008, the autumnal equinox is September 22 at 15:44 UTC (11:44 AM on the east coast of the continential US and 8:44 on the west coast). You can find a chart of times until 2020 at the top right of the Wikipedia article on equinoxes.
