When does autumn 2010 begin?

Answer 1

The answer to this depends on which definition of autumn is being used.

In the northern hemisphere, the astronomical autumn begins September 23 at the time of the autumnal equinox. This date is sanctioned by the International Astronomical Union and can vary by a day or two from one year to the next.

The climatological autumn begins September 1. This date is sanctioned by the World Meteorological Organization for the northern hemisphere and never changes.

The ecological autumn begins locally whenever the leaves change colour and start to fall. This date varies from one region to another and can shift from one year to the next.