2010 Summer Solstice - The first day of summer officially kicked off today at 7:28 a.m. ET, the beginning of the summer solstice and the longest day of the year in the Northern Hemisphere.
The summer solstice is a result of Earth's north-south axis being tilted 23.5 degrees relative to the sun, experts say. This tilt causes different amounts of sunlight to reach different regions of the planet over the course of the year. Today the North Pole is tipped closer to the sun than on any other day of 2010. The opposite holds true for the Southern Hemisphere, for which today is the winter solstice, the shortest day of the year.