Today is the summer solstice that marks the longest day of the year at least in the Northern Hemisphere. This year, the summer solstice comes a day in advance because 2012 is a leap year. The timing of the summer solstice changes every year, but there’s usually a huge difference in a leap year because an extra day is missed in January to match the astronomical calendar.
During the summer solstice, the Earth's north-south axis are tilted by 23.4 degrees relative to the ecliptic, the plane of our solar system. Owing to the tilt, there is a difference in the amount of sunlight that reaches Earth, hence the temperatures soar, but it’s not the hottest day of the year. Some people mistakenly believe that the summer solstice is also the hottest day of the year.
At noon on the summer solstice, the sun can be seen at the highest point in the sky especially in the Northern Hemisphere. James Bell, a Cornell University astronomer, explained this concept by saying, “That doesn't mean the sun will be exactly overhead at noon for everyone, It depends on the viewer's latitude the sun will shine down directly overhead at noon only along the Tropic of Cancer, an imaginary line that circles the planet at about the latitude of Cuba.”
During the summer solstice, oceans and the atmosphere serve as heat absorbers and once they absorb sun’s radiations they release it after several weeks, which is why the hottest days usually occur in July and August and the temperatures begin to normalize after these two months.
According to astronomer Robert Howell from University of Wyoming, "If you think about turning up an oven, it takes it a long time to heat up. And after you turn it off, it takes a while for it to cool down. It's the same with the Earth."
The summer solstice, which is also known as midsummer, has been celebrated by various cultures across the globe. The Egyptians built the Great Pyramids in such a manner that the sun becomes visible from the Sphinx exactly between two of the Pyramids on the summer solstice.
There is a lot of interesting information available about the summer solstice on the National Geographic website.