Did you know that yesterday, June 21 at 1:04 A.M. EDT (Eastern Daylight Time) was the official start of Summer 2012?
This year's summer solstice takes place a day earlier than it's been for the past three years, due to the fact that 2012 is a leap year—this February got an extra day, to keep our calendar year of 365 days in sync with the astronomical year, which is about 365.24 days.
The solstices are the results of Earth's north-south axis being tilted 23.4 degrees relative to the ecliptic, the plane of our solar system. This tilt causes different amounts of sunlight to reach different regions of the planet during Earth's year-long orbit around the sun.
Visit the National Geographic website to learn more: http://news.nationalgeographic.com/news/2012/06/120620-summer-solstice-2012-longest-day-sun-earth-space-science/