(Currier & Ives Lithograph, 1871)
Autumn (still called Fall in American English; in England that usage was once standard, but has now become archaic) is one of the four temperate seasons, the transition between summer and winter. In the temperate zones[?], autumn is the season during which most crops are harvested, and deciduous trees lose their leaves. Astronomically, it begins with the autumnal equinox (around 23 September in the Northern hemisphere, and 21 March in the southern hemisphere), and ends with the winter solstice (around 21 December in the Northern hemisphere and 21 June in the Southern hemisphere). However, meteorologists count instead the whole months of March, April and May in the Southern hemisphere and September, October and November in the Northern hemisphere.
Either definition, as with those of the seasons generally, is flawed because it assumes that the seasons are all of the same length, and begin and end at the same time throughout the temperate zone of each hemisphere.
See also: axial tilt