It's Officially Summer!

Today, June 21, 2011, is the first day of summer—the summer solstice.  It is the longest day of the year, when the Sun reaches its most northern point in the sky , directly above the Tropic of Cancer, at local noon (1:16pm ET). After this date, the days start getting shorter.

While some consider the summer solstice as the beginning of summer, many actually regard it as midsummer, though the definition varies between different regions and cultures. In the Southern Hemisphere, today actually marks the 2011 winter solstice. These seasons will again be flipped between the hemispheres on December 22, when the second solstice of the year occurs (winter for Northern Hemisphere, and summer for Southern Hemisphere).

The four seasons are determined by changing sunlight (not heat!)—which is determined by how our planet orbits the Sun and the tilt of its axis.