The research done by the astrophysicist Thomas G. Brophy suggests that these monoliths might tell much more. The calendar circle itself is made up of one doorway that runs north-south, a second that runs northeast-southwest marking the summer solstice, and six center stones. Brophy's theory proposes first that three of the center stones match the belt of Orion at its minimum tilt and the other three match the shoulder and head stars of Orion at their maximum tilt. This cycle repeats approximately every 25,000 years, following the precession of the equinoxes. The last minimum of Orion's belt occurred between 6400 BC and 4900 BC, matching the radio-carbon dating of campfires around the circle.