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The first day of summer officially kicks off today at 7:28 a.m. ET, the beginning of the summer solstice and the longest day of the year—at least in the Northern Hemisphere.

The summer solstice is a result of the Earth‘s north-south axis being tilted 23.5 degrees relative to the sun. The tilt causes different amounts of sunlight to reach different regions of the planet.

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 thewinter solstice, the shortest day of the year.

As a result, at high noon on the first day of summer, the sun appears at its highest point in the sky—its most directly overhead position—in the Northern Hemisphere.

That doesn’t mean the sun will be exactly overhead at noon for everyone, said James Bell, an astronomer at Cornell University in New York State.

It depends on the viewer’s latitude. “The sun is only shining down directly overhead [at noon] at the Equator. It’s still at a low angle if you’re up in Alaska,” Bell explained.

No matter where you are in the Northern Hemisphere, the path of the sun across the sky—which rises in the lead-up to the first day of summer, then begins descending over the rest of the summer—seems not to change for the few days before and after the summer solstice.

In reality, the sun’s position is still changing, but at a slower rate.

Today is the perfect day for the summer site to launch at Mom-Stuff.com. (more…)