Her father returned from a visit to the ruins enthralled with the magnificent stacks of stones. He passed on the enthusiasm to his daughter. Challener and her father built scale models and toyed with them for years, trying to figure out how the various celestial bodies would have aligned with the rocks. But the sun shadows just never panned out, and so over the years Challener decided that the megalith was mostly about the moon. This diagram shows the moon and sun's apparent motion about the Earth.
Lunar eclipses occur when the sun is in one node and the moon at the other.
This allignment can only occur during a full moon. Challener began by studying when ancient eclipses would have been visible in southern England. I looked at the times between lunar eclipses and did it really the old-fashioned way, which is the way they might have done it. I just noticed a pattern. The trick that complicates this explanation, however, is that more than one lunar eclipse would have been visible from Stonehenge every 47 months.