Alexander Thom is widely acknowledged as one of the founding fathers of modern archaeoastronomy. An engineer by profession, he was also a keen amateur astronomer, and he was aware from an early age of the writings of Sir Norman Lockyer on the possible astronomical significance of ancient stone monuments in his native Scotland and elsewhere in the British Isles. His own active interest in the megalithic monuments of Britain, and particularly in his native Scotland, is said to have been aroused in 1933 by the sight, one evening as he was sailing in the Outer Hebrides looking for an anchorage, of one of the stone circles at Callanish silhouetted against the rising moon. Twenty years and many dozens of site surveys later, he began to publish research papers in which he proposed three ideas that, during the 1960s and 1970s, would shake the archaeological world.
The first of Thom’s ideas was that stone circles were laid out using a precisely defined unit of measurement, which he called the “megalithic yard.” The second, and related, idea was that each circle was laid out according to one of a small number of particular geometrical designs using techniques based upon Pythagorean triangles. The third idea was that a variety of “megalithic sites” incorporated alignments of remarkable precision upon the rising and setting positions of various celestial bodies. Taken together, these showed that “megalithic man,” to use Thom’s own term, was a competent engineer with considerable technical knowledge, whose accomplishments included sophisticated numeration, geometry, and astronomy— three things that would come to be known together as “megalithic science.” Thom’s ideas first appeared in topic form in 1967, in a slim but highly technical volume called simply Megalithic Sites in Britain, which contained the detailed analysis of survey results from over 250 standing stone monuments.
Thom’s second topic, Megalithic Lunar Observatories, published in 1971, set the archaeological world into a turmoil that did not calm down fully until more than a decade later. His claim was that many standing stone monuments were deliberately aligned upon “notches” and other features in distant mountainous horizons, and that these horizon points marked extreme rising and setting points of the moon with incredible precision. To attain that level of precision, prehistoric astronomers would have had to carry out meticulous programs of observation, following them consistently for years if not decades (in other words, over generations). The problem was that these conclusions were completely unpalatable to most prehistorians at the time. Their social models, informed by a variety of archaeological evidence, simply did not fit with such ideas. Yet Thom’s results could not easily be refuted: they were based on careful fieldwork and supported by rigorous statistical analysis.
When detailed reappraisals of Thom’s ideas on “megalithic astronomy” were eventually completed in the 1980s, it emerged that the supposed astronomical sightlines of very high precision could all be explained away as chance occurrences. On the other hand, certain groups of monuments, such as the recumbent stone circles of eastern Scotland and the short stone rows of western Scotland and southwest Ireland, definitely did contain consistent lunar alignments, though at a much lower degree of precision, that could not be ignored. Similarly, although the idea of a precise “megalithic yardstick” being used all over Britain is not sustainable as Thom conceived of it, the idea that there was a widespread practice of laying out stone circles and other monuments using broadly consistent measurements based upon the human body (such as arm lengths or paces) remains a distinct possibility.
In hindsight, even though Thom’s theories have been largely superseded, his work led archaeologists to start thinking about issues and facing problems that they had largely avoided. In particular, Thom’s work motivated others to investigate intentional astronomical alignments at prehistoric monuments and eventually resulted in this area of study becoming archaeo-logically informative and respectable. Finally, Thom pioneered ways of carrying out site surveys using a theodolite, analyzing the data, and presenting the results that are now a fundamental part of field methodology within ar-chaeoastronomy.
