Megalithic yard

The megalithic yard (sometimes abbreviated to MY) is a theoretical unit of prehistoric measurement first suggested by the Scottish engineer, Alexander Thom in 1955.

Thom undertook an initial statistical analysis of 46 of the Neolithic and Bronze Age stone circles, in the British Isles. By plotting his measurements on a graph, he noted that many of the diameters of the stone circles came clumped together in groups, there were several examples with close to 22 foot diameters, another group measuring c. 44 feet across and another measuring c. 55 feet. A best fit for these results implied a common factor of 5.43 feet which he believed could have served as a manageable unit for measuring out figures on the ground. Thom went on to survey more than 300 sites, becoming increasingly convinced of the yard's existence.

Thom halved his original best fit to 2.72 feet as he argued that the circle builders would have set out their monuments using a radius from a central point rather than using a diametrical measurement.

From the data he gathered, Thom argued that the megalithic sites were all built by ancient architects using a standard measurement unit equating to 0.829m or a little less than an imperial yard. Arguing for a common small unit of measurement from larger quantities such as this is known as a quantum hypothesis. Thom posited that wooden 'measuring rods' of this length or multiples of it would have been used for surveying purposes and that they must have been produced from a central place in order to maintain the consistency he had observed. He believed that Avebury could have been such a place. By using such rods laid out on the ground along with rope and a plumb line, Thom demonstrated that even complex eliptical shapes can be precisely measured out using basic geometry including Pythagorean triangles.

Analytical methods employed by the British statisticians S.R. Broadbent and D.G. Kendall indicate that the 1955 dataset has a 1% significance meaning that such a best fit would only occur in 1 in 100 random datasets. Other archaeo-statisticians consider Thom's original 5.43 feet, the so-called megalithic fathom to provide a more persuasive argument for a standard prehistoric measurement unit. Thom and others have also claimed to have found evidence for megalithic feet, inches and cubits.

The megalithic yard theory is an attractive one as it has several pleasing properties. It can be used to produce elipses with circumferences an integral multiple of their internal measurements. This led Thom to later introduce the megalithic rod equalling 2.5 megalithic yards as he calculated a ratio of 1:2.5 between radius and perimeter in the oval-shaped monuments he examined.

Such well-thought out measuring has therefore been claimed as the earliest example of ancient mathematics, although using a circle of 366 degrees rather than the 360 adopted by the later Babylonians. Its use has also been identified by some in other British megalithic structures and attempts have been made to link it with the engineering methods used to build the Pyramids. The significance of 366 in the context of the solar year has not been overlooked by some archaeoastronomers.

Later analysis of Thom's data has raised numerous questions regarding his approach however and professional archaeologists treat his ideas with scepticism. Thom's measurements of some circles have been found to be up to 0.3m out and for other broken or sub-circular monuments he studied, the precise diameter is open to question anyway. Local variations have been identified in measurement data from different parts of the British Isles suggesting that there was no centrally decreed 'yard' and it has been argued that body measurements such as the cubit would have been more likely to have been used. Anthropological studies of modern stone-using tribes have also been employed to suggest that Neolithic Britons would not have had a numbering system complex enough to create advanced geometric forms using such surveying techniques and that elliptical enclosures are the results of attempts to mark out circles by eye or to align a long axis on astronomical features. The lack of serious corroborating evidence from continental Europe should also be mentioned.

Until such a time as a Neolithic measuring rod is excavated, the theory remains unproved.

External links

• Explaining the fundamental flaws in the foundation for the alleged common unit of length of "Megalithic Sites in Britain" (http://myweb.tiscali.co.uk/davel/MY.htm)