Nabta Playa in Egypt is an ancient stone circle that researchers suspect was used to determine the summer solstice, which signaled rain was on the way.Today, its main stones are in a museum in Aswan to protect them from vandalism. But originally, Nabta Playa consisted of dozens of upright stones, each a few feet high, in the desert about 60 miles (100 km) west of the Nile.In addition to a pile of stones above a central tomb, many of the stones seemed to have been deliberately placed in a circle to align with the rising of certain stars.The circle's six...