Crucuno is one of a number of megalithic enclosures—settings of standing stones or boulders arranged to enclose a space—found in two clusters in Brittany, northwest France. One group is found toward the far northwest coast, around the modern city of Brest, and the other is found on the south coast of the Département of Morbihan, around Carnac. Often grouped together under the term cromlechs, some of these enclosures are very roughly circular or oval, while others are shaped like a horseshoe, a barrel, or a letter "D"; a few are closer to a rectangle or square. Crucuno, situated some five kilometers (three miles) northwest of Carnac, is certainly the most impressive of these. The twenty-two stones form the remains of what appears to have been an almost perfect rectangle, 33.2 meters by 24.9 meters (108.8 by 81.6 feet), oriented in the cardinal directions, with the longer sides east-west. Although the site has been largely reconstructed, the reconstruction appears to have been done faithfully, so that the layout can be trusted.

Amazingly, this simple structure combines geometrical perfection with cardinal orientation and astronomical alignment. In addition to being a near-perfect rectangle, the lengths of its sides were in the ratio 3:4. And in addition to the sides being oriented in the cardinal directions, the diagonals are oriented toward the rising and setting positions of the sun at the two solstices.

Yet we should guard against overinterpretation. The ratio of the sides does not necessarily imply that the builders were aware of the Pythagorean rule whereby the length of the diagonal is also a whole number of units (5).

Part of the megalithic rectangle at Crucuno, Brittany, France. (Courtesy of Clive Ruggles)

It could be just that, for aesthetic or other reasons, they chose to have the side lengths in the proportion 3:4 instead of, say, 2:3 or 4:5. Then there is the question of whether the solstitial orientation of the diagonals is fortuitous. Crucuno is located at a latitude (47.6°N) where, given a reasonably flat horizon, constructing a cardinally oriented 3:4 rectangle with its longer sides east-west will ensure the (approximate) solstitial orientation of the diagonals, whether or not this was intentional. On the other hand, perhaps the solstitial orientation of the diagonals was deliberate and the ratio of the lengths of the sides followed, unintentionally, as a result. Either way, it seems inconceivable that the latitude was deliberately chosen in order to achieve both solstitial orientations and Pythagorean proportions at once: the location of the site was surely determined in relation to the territory occupied by the people who built it, as is evident from its position within a tightly confined cluster of various types of megalithic enclosure in the Carnac area. Lastly, it is tempting to identify the east-west orientation as aligning upon the sunrise and sunset at the equinoxes, but this also makes several questionable assumptions, not least that the equinox as a concept meant something to people in prehistory.

The Scottish engineer Alexander Thom believed fervently that, in addition to a sophisticated knowledge of astronomy, "megalithic man" knew a good deal of geometry, including Pythagorean triangles, and used a precise unit of measurement, the "megalithic yard" of 0.829 meters (2.72 feet), in constructing monuments throughout the length and breadth of Britain, as well as in Brittany. The evidence he used to support this idea has now been largely refuted, but it is curious that the 3:4 ratio of side lengths is marked so clearly at Crucuno and that this implies a unit of 8.3 meters (27.2 feet), which is almost exactly ten of Thom's megalithic yards. The most probable explanation is that a unit of around this length tends to arise through local practices of pacing.

Most of the other recorded stone rectangles in Brittany are in a dilapidated state or have been removed entirely, but similarities have been noted with the station stones at Stonehenge. These four stones marked the corners of a rectangle whose side lengths were in the ratio 5:12, another Pythagorean combination yielding a diagonal of length thirteen units. The station stone rectangle is not cardinally oriented, but its shorter sides are themselves oriented upon midsummer sunrise and midwinter sunset, while the longer sides align roughly with an extreme rising position of the moon. This is similar enough to be intriguing, but different enough that it does not provide compelling corroborative evidence.

Nonetheless, despite the many problems and unresolved issues, the fact that people in the Neolithic or Bronze Age could have encapsulated four distinct characteristics—geometrical and numerological perfection, cardinal orientation, and astronomical alignments—in such a simple setting is truly remarkable. We do not have to accept Thom's megalithic yard at face value or postulate that the builders conceptualized Pythagorean geometry to appreciate this fact.

Nor do we have to assume that the builders chose their latitude in order to believe that the solstitial alignments of the diagonals were fully intentional. True, Crucuno would not have "worked" if it had been located much farther north or south. It is also true that such ideal latitudes are few and far between. For example, a square enclosure with similar properties would need to be placed much farther north—around Carlisle in northern England (latitude 55.0°)—and a 3:4 rectangle with its longer sides north-south would have to be located around latitude 59.6°, somewhat to the north of the Orkney Islands in northern Scotland. But what seems to us a case of remarkably good luck could, for people living in the Carnac area and unaware of differences farther afield, simply have seemed an intrinsic property of nature. In encapsulating the interconnectedness of geometry and astronomy in this modest but powerful way, the Crucuno rectangle may have provided a clear confirmation of the unity and integrity of the cosmos.

See also:

Cardinal Directions; Equinoxes; Solstitial Directions; Thom, Alexander (1894-1985).


References and further reading

Burl, Aubrey. Megalithic Brittany, 133. London: Thames and Hudson, 1985.

-. A Guide to the Stone Circles of Britain, Ireland and Brittany,

254-255. New Haven: Yale University Press, 1995.

-. The Stone Circles of Britain, Ireland and Brittany, 331-348. New

Haven: Yale University Press, 2000.

Giot, Pierre-Roland. La Bretagne des Mégalithes, 83. Rennes: Éditions Ouest-France, 1997. [In French.]

Le Cam, Gabriel. Le Guide des Mégalithes du Morbihan, 67. Spézet, France: Coop Breizh, 1999. [In French.]

Ruggles, Clive. Astronomy in Prehistoric Britain and Ireland, 82-83. New Haven: Yale University Press, 1999.

Thom, Alexander, and Archibald S. Thom. Megalithic Remains in Britain and Brittany, 19-20, 175-176. Oxford: Oxford University Press, 1978.

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