A professor of engineering science at Oxford University who used his professional descriptive and analytical skills to examine the hundreds of prehistoric stone circles in England and Scotland. He later extended his surveys to include the many ancient megalithic sites in Brittany. Thom carefully mapped the positions of the stones in more than a hundred of the circles and then tried to explain how and why they came to be the way they are. A common pattern was a combination of a precise semicircle with a distorted semicircle; these inaccurate semicircles formed six groups that could be reproduced by simple geometrical methods. Two groups appeared to be attempts at constructing a circle, using a value of 3 for pi, instead of the more accurate value we use today: 3.14159.Thom also showed that the distorted semicircles in one group were ellipses and concluded that the Britons of 4,000 years ago must have had a better knowledge of geometry than we had hitherto assumed. He also found that they had had a standard measure of length, a unit he called the Megalithic Yard, equal to 82.9 centimeters (2.72 feet).
Thom made careful measurements of the alignment of many of these ancient monuments, in particular of Stonehenge, and checked them against calculations of astronomical data at the time of their construction. The remarkable correlations he was able to obtain led him to suggest that some, at least of the circles, had been early forms of observatories. This idea has been taken up and elaborated by Sir Fred Hoyle in his book Stonehenge (Heinemann, 1977).