The alignments at Carnac have attracted precise astronomical claims for over a century, and there is a specific reason to distrust measurements taken from them that has nothing to do with whether Neolithic people watched the sky.
A large proportion of the stones now standing were put back up around 1900.
When Felix Gaillard surveyed the site in the later nineteenth century, roughly two thirds of the seventeen hundred odd menhirs he counted at the main groups were lying on the ground. Zacharie Le Rouzic subsequently re-erected a great many of them. The work was done with the intentions of its period and without any record precise enough to reconstruct where each stone had originally stood, at what angle, or in some cases whether the socket found was the original one. A visitor today is looking at a monument substantially restored, and any measurement of an alignment to a fraction of a degree is measuring, in part, the judgement of restorers.
This bears directly on the best-known claim about the site. Alexander Thom surveyed Carnac in the early 1970s and argued that the rows functioned as precise lunar observatories and that they were laid out in a standard unit, the megalithic yard, common to Britain and Brittany. The argument had genuine appeal and it did not survive statistical scrutiny. Clive Ruggles re-examined the evidence and found the alignments consistent with chance once the number of possible sightlines was accounted for; Aubrey Burl called the megalithic yard a grotesque statistical misconception. The restoration problem sits underneath both critiques and makes the original claim close to untestable.
What can be said is duller and better founded. The rows were built in stages by farming communities over centuries, roughly between 4500 and 3300 BC, alongside tombs and enclosures. They cannot be dated directly, because stone carries no radiocarbon: the dates come from organic material in the sockets and from associated monuments, which date the activity around the stones rather than the stones. The scale implies sustained coordinated labour across generations. The purpose is not known, and the honest position is that it is not currently recoverable.
The megaliths of Carnac and the shores of Morbihan were inscribed on the UNESCO World Heritage list in July 2025.