It is becoming apparent that megalithic man possessed and used a considerable knowledge of geometry. As more of his constructions are unravelled, we obtain an increasing appreciation of his attainments. Undoubtedly he also observed the heavenly bodies and used them to tell the time of day or night and to tell the day of the year. To take geometry first, let us look at the various shapes which, in his hands, a ring of stone could take. To understand these rings fully it is necessary to appreciate that he used extensively a very precise unit of length—the megalithic yard (MY). The exact length of this unit has become known to us by an examination of simple circles and flattened circles. When the author produced the first batch of circle diameters there was no universally accepted statistical analysis for the determination of the reliability of a quantum such as the suggested value for the megalithic yard. Then Broadbent produced two papers providing exactly the methods required to find, from a set of measurements, the most probable value of the quantum and the probability level at which it could be accepted [1,2]. This last is very important because Hammersley had shown that almost any random set of (say) diameters will yield an apparent unit of some sort.