Ad Quadratum is a convenient and profound technique in which continuous scaling of size can be given to square shapes, either from a centre or periphery. The differences in scale are multiples of the square root of two [sqrt(2)] between two types of square: cardinal (flat) and diamond (pointed).
The word Alignment is used in France to describe its stone rows. Their interpretation has been various, from being an army turned to stone (a local myth) to their use, like graph paper, for extrapolation of values (Thom). That stone rows were alignments to horizon events gives a partial but useful explanation, since menhirs (or standing stones) do form a web of horizon alignments to solstice sun and to the moon’s extreme rising and setting event, at maximum and minimum standstill. At Carnac the solstice sun was aligned to the diagonal of the 4 by 3 rectangle and maximum and minimum standstill moon aligned to the diagonal of a single or double square, respectively.
It seems quite clear today that stone rows at least represented the counting of important astronomical time periods. We have seen at Crocuno that eclipse periods, exceeding the solar year, are accompanied by some rectalinear structures (Le Manio, Crucuno, Kerzerho) which embody counting in miniature, as if to record it, and it has been observed that cromlechs (or large stone kerb monuments) were built at the ends of the long stone rows of Carnac and Erdeven. Sometimes, a cromlech initiated a longer count,with or without stone rows, that ended with a rectangle (Crucuno). The focus on counting time naturally reveals a vernacular quite unique to this region and epoch. We have seen that the Kerzerho alignments were at least a 4 by 3 rectangle which recorded the 235 lunar months in feet along its diagonal to midsummer solstice sunset. After that rectangle there follows a massive Alignment of stone rows to the east,ending after 2.3 km having gradually changed their bearing to 15 degrees south of east. Just above the alignments lies a hillock with multiple dolmens and a north-south stone row (Mané Braz) whilst below its eastern extremity lies the tumulus and dolmen,”T-shaped passage-grave” (Burl. Megalithic Brittany. 196) called Mané Groh.
In 1973, Alexander Thom found the Crucuno rectangle to have been
“accurately placed east and west” by its megalithic builders, and
“built round a rectangle 30 MY [megalithic yards] by 40 MY” and that
“only at the latitude of Crucuno could the diagonals of a 3, 4, 5
rectangle indicate at both solstices the azimuth of the sun rising and setting
when it appears to rest on the horizon.” In a recent article I found metrology was used between the Crucuno
dolmen (within Crucuno) and the rectangle in the east to count 47 lunar months,
since this closely approximates 4 eclipse years (of 346.62 days) which is the
shortest eclipse prediction period available to early astronomers.
About 1.22 miles northwest lie the alignments sometimes called
Erdeven, on the present D781 before the hamlet Kerzerho – after which hamlet
they were named by Archaeology. These stone rows are a major complex monument
but here we consider only the section beside the road to the east. Unlike the
Le Manec Kermario and Kerlestan alignments which start north of Carnac,
Erdevan’s alignments are, like the Crucuno rectangle accurately placed east and
Around Carnac in Brittany the land is peppered with uniquely-formed megalithic designs. In contrast, Great Britain’s surviving monuments are largely standing stones and stone circles. One might explain this as early experimentation at Carnac followed by a well-organised set of methods and means in Britain. What these experiments near Carnac were concerned with is contentious, there being no appetite, in many parts of society, for a prehistory of high-achieving geometers and exact scientists. Part of the problem is that pioneers interpreting monuments are themselves hampered by their own preferences. Once Alexander Thom had found the megalithic yard as a likely building unit, he tended to use that measure to the exclusion of other known metrological systems (see A.E. Berriman’s Historical Metrology. Similarly, John Neal’s breakthrough in All Done With Mirrors, having found the foot we still use to be the cornerstone of ancient metrology, led to his ambivalent relationship to the megalithic yard. Neal’s interpretation of the Crucuno rectangle employs a highly variable set of megalithic yards, perhaps missing the simpler point, that his foot-based metrology is supported as present within the dimensions of the Crucuno rectangle; said by Thom to be a “symbolic observatory” of the sun: this monument was an educational device, in which Neal finds the geometry of “squaring the circle” which, as we see later, was probably the Rectangle’s main metrological meaning.