Today, June 21, is the summer solstice in the Northern Hemisphere, which means it’s the longest day of the year in 2011. The summer solstice occurs when the sun is the farthest north in the sky, directly above the Tropic of Cancer. According to USA Today, the exact moment of the solstice today will occur at 1:16 p.m. ET.
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).
Scientifically speaking, the two solstices in a year are the days during which the earth’s tilt towards the sun is the most extreme. For the solstice in June today, the tilt is towards in the sun in the Northern Hemisphere (summer solstice) and away from the sun in the Southern Hemisphere (winter solstice).