Summer solstice will be a cause for celebration this weekend, as the date marks the start of summer for people across the Northern Hemisphere.
On June 21—the longest day of the year—the sun will be at its highest point in the sky.
The Earth's tilt is to thank for this demarcation of seasons: Summer solstice occurs when the North Pole is tilted directly toward the sun. (The Southern Hemisphere experiences its summer solstice in December, when the South Pole is directly tilted toward the sun.)
Signifying growth and life, the event has a long history of being recognized as a critical event in the calendar, and many cultures have developed celebrations and rituals to coincide with it.
Stonehenge—shown here as hundreds of blackbirds take flight—is perhaps the most famous monument associated with summer solstice celebrations.
Standing tall on the Salisbury Plain in southern England, the giant rock formation was constructed by prehistoric worshippers beginning about 5,000 years ago.
Each year, thousands of people flock to Stonehenge to celebrate the summer solstice. Most arrive under the cover of darkness, hoping to witness the sun rising along a plane that stretches from the central Altar Stone through the Slaughter and Heel Stones.
Some of the sandstone slabs at Stonehenge weigh upwards of 40 tons (36 metric tons). They were hauled over a distance of more than 18 miles (30 kilometers) and were erected upright, which—even by today's standards—is quite the feat.
Stonehenge was replicated on the High Plains of Nebraska when Jim Reinders and his family built Carhenge. Instead of giant slabs of sandstone, Carhenge is composed of vintage American automobiles painted gray. It includes all the key features of Stonehenge.
The site was dedicated during the summer solstice of 1987 and celebrations are held there each year on June 21.
The Ehrwald Basin in Austria is known for elaborate fire displays that are lit each year to commemorate the summer solstice. Once the sun has set, 8,000 fires ignite along the valley floor and up the mountain slopes, depicting animals and other images.
The event has its roots in pagan traditions and was recently recognized by UNESCO as an example of "intangible cultural heritage."
The Temple of Kukulkan (shown in the background) in the Maya city of Chichén Itzá in Mexico is another ancient monument built to align with celestial events.
On June 21, the sun bathes the northern and eastern sides of the temple with light, casting a shadow on the western and southern sides.
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