Thom began accurately surveying and carefully measuring
megalithic sites throughout Britain, the initial results of which
he published in 1955 in the Journal of the Royal Statistical
Society. After his retirement, he published two more articles in
1962 and 1964.
His discovery of what he called the megalithic yard (2.72 feet, or
0.83 metres), a unit of measurement he found at many prehistoric
megalithic sites, is a key feature of his book Megalithic Sites in
Britain, published in 1967, for which he had surveyed some 300
megalithic circles, alignments, and isolated standing stones. This
was followed in 1971 by another book, Megalithic Lunar
Observatories.
Besides the megalithic yard, Thom also categorised stone circles,
showing there were six geometric types: true circles, ellipses, two
sorts of egg-shaped circles, and two sorts of flattened circles. In
important ways, Thom's work converged with that of Gerald Hawkins
at Stonehenge, which also linked it with astronomy and mathematics.
Whereas Hawkins focused on Stonehenge, Thom was able to demonstrate
that a large number of other megalithic sites were also oriented to
the sun and the moon.
Although admired for his impeccable science and respected for the
thoroughness and intellectual rigour of his research, Thom's
findings remain controversial among scholars.
