American Indians who built Fort Ancient used the sun and moon to measure time. By observing the rising location of these celestial objects they knew when to gather together for religious or social events or when to plant crops. They used sunrise locations to predict annual events and moonrise location to measure intervals of several years' duration. At the southwest corner of a nearly perfect square made by four mounds, three astronomical alignments have been demonstrated. On the summer solstice, usually June 21, the sun rises in the gap just north of Rt. 350. The maximum northern moonrise occurs approximately 9.3 years after the minimum northern moonrise, with the complete lunar cycle taking 18.6 years, or one generation.