Following nearly 40 years of research, Norman Stockdale and Peter Harris conclude that a standard unit of length was used in the construction of megalithic monuments, but not the same unit proposed long ago by Professor Thom. Thom had been Professor Emeritus of Engineering at Oxford, and an expert in astro-navigation and surveying, and drew plans for hundreds of ancient monuments prior to publishing his book, 'Megalithic Sites in Britain', in 1967.
     Middle to late-Neolithic stone circles date from about 3,400 to 2,700 BCE, and mainly found in the northern half of the United Kingdom and Ireland. Late Neolithic to early Bronze Age circles, from 2,700 BCE to about 2,000 BCE, are scattered throughout the United Kingdom, with concentrations in Cornwall and Brittany. The most prolific period for stone circle building was in the middle Bronze Age, from 2,000 BCE to 1,200 BCE, and chiefly concentrated in Scotland. Few circles were erected after that, and none are known from later than 1,000 BCE.
     Despite this separation in time, the complex geometries of some of the flattened circles, egg-shaped rings and ellipses built throughout the entire period were identical and occur in all regions. Alignments to the key stations of the moon through its 18.6-year cycle are commonly met alongside those to the solar and equinoctial sunrises and sunsets. Harris says that while we cannot think of all the structures solely as 'observatories', the measures incorporate key astronomical data into monument design.
     Aubrey Burl believed there is support for the idea that societies in late Neolithic and early Bronze Age Britain used local measuring lengths, such as the Perth Yard and the Cork Yard.
     Professor Thom proposed a length of 2.72 Imperial feet (83 centimetres), which he called the Megalithic Yard, concluding that there were concentrations of values at 10, 20, 30, and 40 Megalithic Yards in the diameters of stone circles, and perimeters with multiples of 12.5 Megalithic Yards. His measurements also show a high percentage of very similar perimeter lengths.
     Thom looked in detail at the mathematical background of megalithic planning and building of non-circular stone rings, identifying a few basic types: flattened circles, egg-shaped rings, and ellipses. Stockdale and Harris believe the effort put into creating circles of varying shapes shows that the perimeter must be significant, and contend the proportions incorporate lunar and solar values, not pure numerical values. They believe a standard unit a fraction less than 36 centimetres was widely used from approximately 3000 years BCE, which they call the Megalithic Foot, subdivided into 56 equal parts, which they call Megalithic Inches.

Edited from RILKO Journal (Winter 2015)