The megalithic yard is a suggested ancient unit of length that is about 2.72 feet (0.829 meters). Some experts think this unit may have been used to build large stone structures. Alexander Thom proposed this idea after studying 600 such sites in England, Scotland, Wales, and Brittany. He also suggested a larger unit called the megalithic rod, which equals 2.5 megalithic yards or about 6.77625 feet on average. Thom divided these units further into smaller parts, such as the megalithic inch, which is 2.073 centimeters (0.816 inches). One megalithic rod is made up of 100 megalithic inches, and one megalithic yard is made up of 40 megalithic inches. Thom used a special math method to test his findings and found them meaningful. However, others have questioned his analysis, suggesting that the megalithic yard might actually represent the average length of a human step.

Other units

Thom suggested that "There must have been a headquarters from which standard rods were sent out, but whether this was in these islands or on the Continent, the present investigation cannot determine."

Margaret Ponting has suggested that objects like a marked bone found during excavations at Dail Mòr near Callanish, the Patrickholme bone bead from Lanarkshire, and the Dalgety bone bead from Fife in Scotland show some signs of being measuring rods based on the megalithic yard in Britain. An oak rod from the Iron Age fortified settlement at Borre Fen measured 53.15 inches (135.0 cm) and had marks dividing it into eight parts of 6.64 inches (16.9 cm). Archaeologist Euan Mackie noted that five-eighths of this rod, which is 33.2 inches (84 cm), is "very close to a megalithic yard." A measuring rod found in a Bronze Age burial mound in Borum Eshøj, East Jutland, by P. V. Glob in 1875 measured 30.9 inches (78 cm). Keith Critchlow suggested this rod may have shrunk 0.63 inches (1.6 cm) from its original length of one megalithic yard over 3000 years.

Thom compared his megalithic yard to the Spanish vara, a pre-metric measurement in Iberia that was 2.7425 feet (0.8359 m). Mackie observed similarities between the megalithic yard and a unit of measurement calculated from a long, marked shell found in Mohenjo Daro and ancient measuring rods used in mining in the Austrian Tyrol. He also noted similarities with other units, such as the ancient Indian gaz and the Sumerian šu-du3-a. Along with John Michell, Mackie pointed out that the megalithic yard is the diagonal of a rectangle measuring 2 by 1 Egyptian remens. Jay Kappraff noted that the megalithic yard is similar in length to the ancient Indus short yard, which was 33 inches (0.84 m). Anne Macaulay reported that the megalithic rod is equal in length to the Greek fathom (2.072 meters or 6.80 feet) based on studies by Eric Fernie of the Metrological Relief in the Ashmolean Museum, Oxford.

Reception

Thom's ideas were first dismissed or considered unlikely by many archaeologists. Clive Ruggles, quoting astronomer Douglas C. Heggie, explained that both traditional and Bayesian statistical reviews of Thom's findings showed that the evidence supporting the megalithic yard was weak. Even if the yard existed, the uncertainty in its exact measurement was much larger than the 1mm precision Thom claimed. This suggests that Thom's evidence could be explained by people using pacing to measure distances, with the "unit" reflecting an average step length.

David George Kendall shared this view, arguing that pacing would lead to greater differences in measurements across sites. He believed statistical analysis could determine if sites were measured by pacing or a standardized unit. In a study for the Royal Academy, Kendall found evidence of a consistent unit in Scottish stone circles but not in English ones, and he said more research was needed. Statistician P. R. Freeman reached similar conclusions, noting that two other units also fit the data as well as the megalithic yard.

Douglas Heggie also questioned Thom's claims, stating that his analysis found little proof of a highly accurate measuring unit. In his book Rings of Stone: The Prehistoric Stone Circles of Britain and Ireland, Aubrey Burl described the megalithic yard as "a chimera, a grotesque statistical misconception." Most researchers now agree that there is limited evidence for a standardized measuring unit, but they believe it was not as uniform as Thom suggested.

Arguments for a geometric derivation

Some experts, like John Ivimy and Mackie, have suggested that the megalithic yard might be connected to two ancient Egyptian units of measurement: the remen, which is about 1.2 feet, and the royal cubit, which is about 1.72 feet. These units helped measure land in Egypt. Griffith found that the area of a square made with the royal cubit is twice the area of a square made with the remen. Petri also said the remen is equal to 20 digits.

A square with a side length equal to the diagonal of a square that is one remen long has an area equal to one square royal cubit. Ten thousand of these square royal cubits make up a land measure called the setat. John Ivimy observed that the ratio between the megalithic yard (MY) and the royal cubit (Rc) is approximately the square root of 5 to the square root of 2. This means the megalithic yard is equal to the square root of 5 times the remen, or the length of a rectangle that is 2 times as long as it is wide, with each side equal to the remen.

One weakness in this idea is that the builders of megalithic monuments would need to know about the remen and royal cubit to create their yard. However, since these monuments are much older than the Egyptian pyramids, this argument might be based on incorrect timing.

Recent studies by John Michell, John Neal, and Richard and Robin Heath suggest that the megalithic yard is connected to a system that relates to Earth's measurements and the lunar cycle.