SOUE News Issue 8

Professor Alexander Thom and Megalithic Astronomy: A Revival of Interest

David Witt

Professor Thom was Head of the Department of Engineering Science from 1944 to 1961. His engineering career had been in aeronautics and aerodynamics, but he was a man of great versatility and practical skills, and from the 1930s onward he developed a passionate curiosity about the arrays of standing stones, mostly in Scotland and the North of England, but also the much more studied ones of Stonehenge and Avebury. He attributed the origin of this interest to a visit to the stones of Calanais (Callanish) in 1933, when cruising his yacht in the Hebrides. The Pole star being visible, he spotted that some stones defined an avenue aligned almost north-south. But when the stones had been erected, several thousand years earlier, the Pole star had been in a quite different place, as a result of the "precession of the equinoxes". So how did the builders establish which way was north?

Over subsequent years, and particularly in retirement, he surveyed hundreds of these arrays, mostly either circles or egg-shaped rings, but sometimes more subtle arrangements. From statistical analysis of his measurements, he concluded that they had been erected by people who used a linear measurement unit of 0.829 m (2.72 ft), and that they preferred to lay things out using, wherever possible, integral numbers of this unit. He termed this unit the "megalithic yard", and concluded that multiples and sub-multiples of it were also used for various purposes.

He believed that many of these stone arrays had been erected for astronomical purposes, and looked for specific instances of this. Observation of the azimuth of the rising or setting sun seemed to have been used to determine the dates of solstices and equinoxes, and hence define a calendar, of obvious use to a farming community. But the inhabitants of Northern Scotland were a sea-faring community too, and therefore much interested in tides and tidal currents, which depend on the moon. This alone would justify study of the moon, but there was the added incentive that in northern latitudes with wide horizons and no outdoor artificial lighting, the rather odd motions of the moon would have been much more noticeable than they are to modern city-dwellers, so might inspire study for its own sake. A desire to predict eclipses might have been part of it too. Thom claimed that at numerous sites there were "alignments" between standing stones and prominent features on the skyline, to indicate the extreme positions at which the moon rose or set. They don't indicate them now, because of astronomical changes in the intervening 3500-4000 years, particularly in the angle between the Earth's equatorial plane and the plane of its orbit round the Sun, now about 23.4°, but then about half a degree more. This can sometimes mean that a modern observer would have to stand hundreds of metres from where the megalithic observer would, in order to see the same thing. So Thom had to do extensive calculations to make his case.

An attempt to predict eclipses would, Thom thought, have required years of observations and some quite advanced "calculations" on the part of his Megalithic observers. These are explained in one of his books, Megalithic Lunar Observatories1, and would have involved two levels of interpolation between daily records of setting or rising azimuth, effectively "samples" of a continuous process. There are some curious fan-shaped arrays of stones in Caithness, which Thom suggested may have been effectively "slide rules" to do these calculations. And only every nine years could one make meaningful observations! But they did have several hundred years to do it, if, as Thom assumed, there was a "caste" of people devoted to the task. Julius Caesar recorded of the Celts in about 50 BC that their Druids enrolled young men for 20-year courses in which they learnt much about the heavenly bodies (and much else), but committed it all to memory rather than wrote it down. But that was quite a different race of people, who came on the scene much later.

Thom never claimed to be an archaeologist, but when he published his findings2, the archaeologists obviously had to take note, though most of them were not equipped to understand or criticise Thom's mathematics. Initial reaction seems to have been of great interest and admiration, but gradually scepticism grew. Thom's work seemed to imply a much higher level of sophistication on the part of these early people than was consistent with previous research results. To which it could be replied that if they could erect these enormous stones, build seaworthy boats and navigate the treacherous seas around Northern Scotland, then they were no fools, and the size of their brains was the same as ours. But unlike the ancient civilisations around the Mediterranean, no written records survive, even if they ever had any. But those whose culture does not include a written language develop their brains in other ways. The early bards of around 800 BC who preserved the Homeric sagas, Iliad and Odyssey (24 books in each!), did so purely by memorising them.

Some who extended or repeated Thom's work have concluded e.g.:

  1. that the use of the megalithic yard was certainly not universal - some areas show no trace of it;
  2. that Thom's choice of astronomic "alignments" was over-selective3. If you choose this stone and that cleft on the horizon, then they seem to indicate a relevant direction. But that stone and that other cleft indicate nothing at all. Perhaps the pair you chose was just a random occurrence.

To me it does seem that Thom may have occasionally claimed that a conclusion was established beyond doubt, when it might have been safer to say "it does look very much as if ...". He clearly felt an empathy with the people whose work he was studying, as if he could instinctively see what they were aiming at. Others from different backgrounds might fail to see it at all.

More recently various people have been trying to re-assert the significance of Thom's findings. Edmund Sixsmith, an ex-Cambridge civil engineer, is one of them, with whom I have been in recent correspondence. He has just published an article on Thom in the statistical magazine Significance4, and has another on the way. He claims that Thom "has been airbrushed from accepted wisdom". A work published in 2007 was Robin Heath's Alexander Thom - Cracking the Stone Age Code. The author's background, like Thom's, is technical rather than archaeological, and I think his book is not entirely error-free (is any?) but the one that most irritated me, his mis-spelling of Barnes Wallis' surname, is irrelevant to the larger question. A lot of work has clearly gone into it. He gives a good account both of Thom's work and of the subsequent controversy. Thom's views are now apparently highly unfashionable among professional archaeologists - if you believe them you are most unlikely to get promoted, rather like a biologist who suggests that acquired characteristics can perhaps be inherited by the next generation, or that someone designed us.

But others have tried to link the stone-circle erectors and their megalithic yard to all sorts of curious speculations about the past, varying from the just believable to the highly unlikely5. One that I myself find extremely unlikely is the suggestion that the megalithic yard was defined in terms of the length of a pendulum that had a specified periodic time. For a start, the period of a pendulum depends on how its mass is distributed (point masses on the end of light rods are more appropriate to A-Level questions than to the real world); and then, how did they measure the periodic time to adequate precision?

Units similar to the megalithic yard do seem to have been used in other places, and there are some tantalising possible links to some other units. For instance, you could define a litre as the volume enclosed by a cube of side 0.1 m. A cube of side 0.1 megalithic yard has a volume quite close to one English pint! But who first defined the pint, and when? And there is that curious unit, the "perch", which pupils once had to learn about in school, but probably not these last 50 years. A linear perch is 5.5 Imperial yards, and a "square perch" (5.5 × 5.5 yards) is a 160th part of an acre, and was much used in land measurement until quite recently. But why ever would anyone define one unit as being 5.5 times the length of another one? But if you replace imperial yards by megalithic yards, the ratio is quite close to six. Did someone in the distant past redefine the yard, but leave the perch the same, so as not to upset the farmers and landowners?

Thom seems to have left a fascinating controversy behind him. If anyone cares to grab a battle-axe and join in, they might have an interesting time.

The standing stones of Calanais
The standing stones of Calanais on the Isle of Lewis that inspired Professor Thom 76 years ago (photograph by Sarah Witt)

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