Another possible explanation for the holes, suggested by Richard Atkinson, is that they were excavated in turn in some unknown ritual involving a procession around the inside of the monument. Others have pointed out the significance of the 28-day human menstrual cycle and argued that the holes may have been fertility indicators. Alexander Thom calculated that the circle of holes had been laid out in a circumference of 131 of his megalithic rods although this number has no known significance. A recent CAD study of the holes has shown that a 56-sided polygon can be generated by the simple use of square and circle geometry. Aubrey Burl also notes that the azimuth of the Heel Stone, beyond the Aubrey Circle, marks the midpoint in the swing of the Moon between major and minor standstill points, at 51.3 degrees.