The Nabta Playa, is arguable the oldest archeoastronomical site in the world, pre-dating Stonehenge by over 1,000 years.
Located in Egypt, the site is .8 miles wide by 1.8 miles long, and includes “10 slabs some 9 feet high, 30 rock-lined ovals, nine burial sites for cows, each under a pile of 40 to 50 rocks weighing up to 200 or 300 pounds apiece, and a “calendar circle” of stones.”
Because the site is located near the Tropic of Cancer, the position of the midday sun is at its zenith 3 weeks before and after the summer solstice, which prevents objects from casting shadows, and the vertical stones in the circle correspond to the zenith sun during the summer solstice. For ancient cultures, this astronomical date would have been significant.
Source 1, 2