Summer returns to the northern hemisphere tomorrow. It’s the summer solstice — when the Sun stands farthest north for the entire year. The solstice also brings the year’s longest days.

In centuries long past, the skywatchers of many cultures watched the solstice from special observatories. Some of those observatories were circles of stones. The most famous is Stonehenge in England, but circles of smaller stones were also found in the Americas.

One of the most ancient stone observatories was built in southern Egypt, at a site called Nabta. It was used at least 6500 years ago, and perhaps even earlier — many centuries before Stonehenge.

Anthropologists discovered the site in 1974. Studies in the 1990s confirmed that Nabta probably served an astronomical purpose.

Among other artifacts, the site contains a 12-foot-wide circle of small stones. Two pairs of stones stand across the circle from each other. Looking through the gaps between each pair reveals the point where the Sun rose on the summer solstice thousands of years ago.

This alignment was important to the people of Nabta because monsoons brought a few inches of rain to the region soon after the solstice.

Over the centuries, though, the rains dried up and Nabta was abandoned. But the people of Nabta may have left a legacy. Their culture may have helped stimulate the formation of Egypt’s Old Kingdom — the beginning of the culture that built the great pyramids.

Script by Damond Benningfield