March 20, 2012, is a date that most of us recognize as symbolic of changing seasons. As we welcome spring, people south of the equator are actually gearing up for the cooler temperatures of autumn.

Far from being an arbitrary indicator of the changing seasons, March 20 (March 21 in some years) is significant for astronomical reasons. On March 20, 2012, at precisely 1:14 A.M. EDT (March 20, 05:14 Universal Time), the Sun will cross directly over the Earth's equator. This moment is known as the vernal equinox in the Northern Hemisphere. For the Southern Hemisphere, this is the moment of the autumnal equinox.