The stone circle at Nabta is special in several ways. Firstly, it is the only megalithic circle in Egypt, but perhaps more importantly, it was situated on the Tropic of Cancer (presumably deliberately), which makes it one of the first known examples of an astronomic observatory in the world.
The suns rays, on the summer solstice, strike the ground absolutely vertically and no casts no shadow for several minutes.
Located 100 km west of Abu Simbel, in southernmost Egypt, Nabta Playa is a large, internally drained basin, which during the early Holocene ( ca. 11,000 - 5500 calibrated radiocarbon years ago) was a large and important ceremonial centre for prehistoric people. It was intermittently and seasonally filled with water, which encouraged people to come there, and today it contains dozens, and perhaps hundreds, of archaeological sites. People came from many regions to Nabta Playa to record astronomical events, erect alignments of megaliths, and build impressive stone structures.