In 4000-3000 BCE, a slight variation of the megalithic yard was introduced by megalith builders. The original megalithic yard seems very likely to have emerged out of a day-inch count for three solar years which, when subtracted for a day-inch count for three lunar years, yields a difference of thirty two and five eighths inches. A set of similar lengths emerged later into historical metrology, the most famous being the Spanish vara which was varied around a slightly larger figure of 33 inches. Somewhere in-between, the accuracy of the first megalithic yard (accurate over the three year period and equalling 32.625 inches), was enhanced to match the "true" figure over the Metonic period of 19 years = 235 lunar months = 247 lunar orbits = 228 mean solar months. Nineteen lunar years, of 12 lunar months, have 228 lunar months within them and this means a difference of seven months in nineteen years.

The pure astronomical megalithic yard, as differential between solar and lunar years, is therefore 19/7 which in metrological terms would be 19/7 feet. The resulting megalithic yard is then less than that obtained over just three years (as at Le Manio) as the Metonic is a more accurate whole number coincidence of solar years and lunar months and the actual ratio between the solar and lunar years. This unit was seemingly chosen to differ slightly from 19/7 feet to fit the range rational variations that made up ancient metrology. The base foot chosen for this new Astronomical Megalithic Yard (AMY) was the Drusian foot of 27/25 (1.08) feet, an appropriate choice since the lunar orbit to month ratio is 1.08085 feet (29.53059 days over 27.32166 days). The Drusian foot was enlarged by 176/175 (a standard type of micro increase found throughout metrology) to become a foot with a formula 19.008/7 feet (of the standard English foot defined as the number one by those responsible for this prehistoric exercise in rational fractionalisation we moderns received as our historic measures).

However, the lunar month was slightly shorter in 4000 BCE than it is today and the solar year slightly longer. The value chosen in that fourth millenium was therefore not perfectly accurate but ever since it has been becoming more accurate from an astronomical point of view. The easiest way to see this fact is by calculating the difference between the actual lunar month of an epoch and the length of a month for which the AMY would be wholly accurate, in that epoch, as below.

It is one of those strange coincidences that the modern culture, capable of studying ancient metrology but with modern tools and skills, is the one living in the circumstances in which the AMY becomes very accurate relative to the astronomical bodies to which its numerical ratio, to the English foot, was supposed to match. The reason I looked into this was to gain some purchase on what might have been the historical order of development for the AMY relative to the structures for which there is a case that it was employed such as Stonehenge's Aubrey circle of post holes. It seems unlikely that Carnac generated this unit by 4000 BCE and also that it was generated directly from astronomical counting as was the case with Carnac's megalithic yard. Meanwhile, ancient historical professionals have so far apparently failed to study and employ metrological standards to glean extra dimensions of understanding from the past.