The 2013 Summer Solstice is here. And with it, Summer is official in the U.S. and many other cultures. Any barbecues or swimming that may have occurred already were just a prelude to the real thing, slated to arrive immediately. In fact, it's already upon us.

The Summer Solstice, the longest day of the year, will occur on June 21, 2013 at 1:04 AM EDT. But the solstice will begin on Thursday, June 20 for much of the Western US, according to the Clark Planeterium.

And this weekend, there's also a Supermoon. The 2013 Supermoon will be June 22-23 and mark the biggest, brightest Moon of the entire year. Backyard astronomers are anticipating the celestial event.

The summer solstice occurs when Earth's axis is at its "maximum axial tilt", meaning the most tilted toward the sun. This means the longest day of the year occurs in the northern hemisphere; the sun rises at its most northeasterly point along the horizon and sets at its most northwesterly point.

The Solstice occurs when the Sun passes over the Tropic of Cancer, which runs through Mexico, India, and the Sahara Desert. For all points North of the Tropic of Cancer, Friday will mark the sun's highest point in the sky. For the Northern Hemisphere, Friday will be the longest day of the year, peaking at over 16 hours and 10 minutes in Paris and 13 hours and 16 minutes in Mumbai.

Sadly, today also means winter is imminent - the days only get shorter and darker from here. As the planet continually whirls toward the winter solstice, the sun's path will angle south until it crosses the equator on September 22, the date of the autumnal equinox.

But that's a long way away. For the present, anyway, summer is upon us. Some think the Solstice marks the beginning of summer, particularly in the U.S. But European Solstice celebrations often involve bonfires, and celebrate Midsummer's Day, meaning, well, mid-summer.