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.

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."