The landscape around Carnac on the south coast of Brittany, northwestern France, is so full of megalithic monuments that one almost grows blase about them. There are huge passage tombs (also referred to as passage graves) and other types of burial monument; great multiple alignments of standing stones several hundreds of meters (over a thousand feet) in length; stone circles and variants, known locally as cromlechs; and numerous individual standing stones, often themselves of considerable size (some are several meters [over 15 feet] high). Le Grand Menhir Brise (the Great Broken Menhir) is situated close to the modern town of Locmariaquer, adjacent to two burial mounds of considerable size: a passage tomb known as the Table des Marchand (Merchants’ Table), and a 160 meter- (close to five-hundred-foot-) long tumulus known by its Breton name Er Grah, much of which was destroyed in the mid-twentieth century to make room for a visitors’ parking lot, but which has now been carefully restored.
The Great Menhir stands out as by far the largest single standing stone in the area, indeed the largest standing stone in Europe; incredibly, it weighed well over three hundred tons and measured some 20.5 meters (67 feet) from its base to its tip. It does not stand now: at some stage it broke, the top part falling one way and the lower part in the opposite direction. The top part split into three on impact, so that the stone now lies in four huge pieces. The fact that the whole menhir did not topple over in the same direction implies that it could not have fallen while being erected, nor could it have been deliberately pushed over. The only viable conclusion seems to be that it was shaken from side to side by an earthquake: in other words, it fell naturally.
The Scottish engineer Alexander Thom, famous for his interpretation of the British megalithic sites as solar and lunar “observatories” of considerable precision, interpreted the menhir as a “universal” lunar foresight that was used to track the changing rising and setting positions of the moon. The rising point of the moon moves up and down the eastern horizon (and similarly the setting point on the western horizon) between limits that are reached once every month. These limits themselves vary over a cycle of 18.6 years, so that in every nineteenth year the range of rising (and setting) positions of the moon is at its widest, while nine and a half years later it is at its narrowest. The outer and inner “limits of the limits,” where the moon rises and sets at times that are (rather misleadingly) referred to as its major and minor “standstills,” form eight lunar horizon targets that, if known, would have helped people keep track of the 18.6-year cycle. All eight horizon targets were, according to Thom, accurately marked by the Great Menhir. This was achieved by carefully placing backsights some distance away in each of the eight opposite directions, to mark places where people could stand and observe the rising or setting of the moon in relation to the Great Menhir in the distance.
The largest two pieces of Le Grand Menhir Brise viewed from the west, with the dolmen known as La Table des Marchand visible in the distance.
Unfortunately, there are many problems with this interpretation. How could the positions of the backsights have been fixed without observing programs lasting many generations? How useful would the “inner” target markers have been in practice? (The moon passes these positions twice every month!) What happened when a critical observation was missed owing to bad weather? Given these and many other concerns, people were quick to ask whether Thom’s evidence could really support such an idea. One problem that soon emerged was that the alleged backsights formed a motley mix of prehistoric monuments, and in four out of eight cases were probably not genuine prehistoric monuments at all. Yet the landscape in this area is strewn with impressive monuments. Wouldn’t it be quite easy to find an equally convincing backsight in almost any direction from the Great Menhir? The answer, it is now generally agreed, is: very likely indeed.
The importance of this example is methodological. How could Thom have been so selective with the evidence without realizing it? As we know from the accounts of those who worked with him, he merely set out to identify suitable backsights in each of the eight directions he considered significant. Why didn’t he at least try some other, randomly chosen, directions as a control? The probable answer is that he was too convinced by his own the-ory—that Neolithic people observed the motions of the sun and moon to great precision—to feel it necessary. He was not alone in this. It is natural to want to find evidence that supports a favorite theory, especially if it is one that has taken a lifetime to develop. Yet it is precisely for this reason that correct field methodology, which ensures that one gives due consideration to all the evidence, is so critically important.
In fact, The Grand Menhir is now known to have stood at the end of an alignment of more than a dozen stones, varying in height. Most of the smaller stones in the row were subsequently felled and some at least were then reused in the construction of tombs. A split carving shows without any trace of doubt that one former menhir—possibly one of this row—was split asunder, with one end ending up as the capstone at the Table des Marchand and another as the capstone at the tomb of Gavrinis several kilometers away across a strait (it is now on an island). Not only does the very early date for menhir construction that this implies drive a final nail in the coffin of the lunar foresight hypothesis (since the lunar targets shift slightly over the centuries owing to the changing obliquity of the ecliptic). More importantly, it shows the potential complexity of the archaeological and chronological context, and the vital importance of taking this into account when formulating astronomical theories.
