Professor Thom was an Lecturer on Engineering at Oxford. He defined a measurement used on most if not all stone circles called the Megalithic Yard.
All stone circles he studied showed integer multiples of this measurement. He also observed that all stone circles are not circles at all but are elliptical and were using Pythagorean triangles long before Pythagoras.
He observed that certain standing stones were placed to mark the Winter Solstice (literally Sun Stop) and also he observed that some were for Lunar measurements to define the "Lunar Standstill" which takes place every 18.6 years. Neither the sun nor the moon stands still, obviously; what stops, momentarily, is the change in declination.