This geometry, whose origin is claimed to go back to c.3000 BC, would have used a 366-degree circle rather than a 360-degree circle as we do today.
Megalithic geometry according to Alexander Thom
A stone ring
One of the first persons to associate Megalith builders with geometry was Scottish Professor Alexander Thom (1894–1985). Although he never hypothesised any 366-degree geometry himself, he strongly suspected the Megalith builders had great astronomical and geometrical knowledge that they used when building the stone rings of Britain: “geometrical shapes [were] used for the rings, i.e. circles, flattened circles, egg shapes, ellipses, and other more complicated designs.”2
According to him, these stone arrangements had astronomical alignments, and they used Pythagorean triangles and “geometrical figures which had as many dimensions as possible arranged to be integral multiples of their unit of length.”3
After several decades of research Thom arrived at the conclusion that the Megalithic builders used a standard unit of measurement which he dubbed the Megalithic Yard. According to him, the length of this unit was 2.72 Imperial feet or 82.96 cm.4
A geometry linked to the Earth's circumference?
According to Alan Butler this geometry was based on the Earth's polar circumference. The Megalithic degree is the 366th part of it, i.e. 40,008 / 366 = 109.31 km; the Megalithic arcminute is the 60th part of the Megalithic degree, i. e. 109.31 / 60 = 1.82 km; the Megalithic arcsecond is the 6th part of the Megalithic minute, i.e. 1.82 / 6 = 0.3036 km; if this Megalithic arcsecond is in turn divided into 366 equal segments, the length arrived at is 0.8296 m, which is the presumed length of the Megalithic Yard, the supposedly ancient unit of measurement independently discovered by professor Alexander Thom in the 1950’s.
This is precisely this apparent coincidence that prompted Butler to think that the Megalith builders could have been cognizant with an Earth-based 366-degree geometry.
A 366-day calendar?
Detail from the Phaistos Disc
According to Butler, 366-degree geometry is linked to the Phaistos Disc, an earthen artifact discovered in Crete in 1908, which could have been a Minoan calendar based on a 366-day year, the Phaistos Disc having 30 divisions on one side and 31 on the other: indeed, a calendar alternating 30-day months and 31-day months would result in a 366-day year.
This calendar would have been working on 40-year cycles, because a 366-day calendar gets exactly one month late in 40 years.
Thom had also hypothesized that the Megalithic Yard was divided into 40 Megalithic inches,5 a number that indirectly echoes the 40-year cycles of the presumed Minoan calendar in ancient Crete.
Butler also noticed that the division of the same Megalithic arcsecond into 1000 equal parts yielded a length of 30.36 cm, which is curiously that of the Minoan foot, a unit of length used in the Cretan palaces built roughly at the same time as the Phaistos Disc was made, and independently discovered by Canadian archaeologist J. Walter Graham.
A Megalithic pendulum?
A pendulum
In another book coauthored with English writer Christopher Knight6 Butler wrote that the length of a pendulum giving 366 beats (or 183 periods) during one 366th of a day (which is approximately the time it takes to the Earth to spin one Megalithic degree) is one half-Megalithic Yard long. From that the authors suggested that such a pendulum could well have been used by these ancestral people to accurately reproduce the length of their yard.
Such a pendulum has never been discovered to this day. It should be noted, however, that in a book