Solstice Science: How Humans Celebrate Official Start of Summer
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Today (June 21) marks the summer solstice, the official start of summer in the Northern Hemisphere.

This year, the solstice will occur at 12:39 p.m. EDT (1639 GMT), when the sun is directly over the Tropic of Cancer. The summer solstice is also the longest day of the year for places in the Northern Hemisphere, which means daylight will get progressively shorter each day until the winter solstice in December.

For countries north of the Tropic of Cancer, the summer solstice takes place when the Earth's tilt toward the sun is at a maximum, and the sun is directly over countries located across the Tropic of Cancer, such as Mexico, Egypt, India, and southern China. After the winter solstice, the shortest day of the year, the sun rises higher and higher in the sky, culminating in the summer solstice when it reaches its highest point. [Image Gallery: Stunning Summer Solstice Photos]

The summer solstice is recognized all over the world, "although we don't pay much attention to it. We think more about summer being between the Fourth of July and Labor Day," said Anthony Aveni, a professor of astronomy and anthropology at Colgate University in Hamilton, New York. But for ancient humans, the sun was one of the first things they recognized and subsequently used to keep time, Aveni said.

In medieval Germany, people of the Mosel Valley would set a big hoop of straw on fire, dubbing it the "fiery wheel." The hoop was then rolled down a hill through vineyards, toward the water. "The idea being [that] fire is associated with purity and fertility," Aveni told Live Science. "This is the time of year when the landscape is fertile."

In northern Germany, a prehistoric site called Externsteine was likely built to celebrate the summer solstice, said Michael York, an independent scholar of cultural astronomy and astrology. Externsteine is a well-known rock formation consisting of five massive sandstone pillars. At the top of one of the pillars is a roofless chapel with a small alter carved out of the rock. A hole on the alter lights up at sunrise on the summer solstice, York said.

Another giant monument, Britain's Stonehenge, may have been created to celebrate the summer solstice, according to Aveni.

"Stonehenge was intended to be a sun temple from its very inception. It seems to have been arranged deliberately so that a viewer who stands in the center of the great stone ring could capture the rising sun's disk in the Heel Stone gateway to the northeast on June 21 — the day the sun reached its northern standstill," Aveni wrote in his book, "The Book of the Year: A Brief History of Our Seasonal Holidays" (Oxford University Press, 2004).

And although the summer solstice likely held more importance for ancient populations, humans have kept many solstice traditions through the years. In his book, Aveni writes, "yesterday's solstice fires and fire-jumping rituals, though doused by our inherited puritanical mind-set, still lives on in our Fourth of July fireworks. And rites once rife with feminine magic still persist in the month of brides (May)."

Elizabeth Goldbaum is on Twitter. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.