Ballochroy is one of many hundreds of small megalithic monuments found in western Britain. The casual visitor is unlikely to be greatly impressed at the sight of it: a row of three standing stones, one broken off, occupying an unassuming location in a field behind a barn at Ballochroy farmhouse on the west coast of the Kintyre peninsula, Argyll, Scotland. There is, however, a good view over the coast to the west, and this view is key to understanding its significance, for this modest monument is one of the earliest and most famous examples of a megalithic “observatory” put forward by Alexander Thom, during the 1950s. It assumed a central place in the controversies that raged for more than two decades over Thom’s theories.

Ballochroy encapsulates Thom’s idea that prehistoric Britons used features on distant horizons as astronomical foresights in order to observe and record the motions of the sun and moon to remarkable precision. The central stone at Ballochroy has a broad, flat face oriented across the alignment that points northwest, directly at the slopes of Corra Bheinn, a mountain on the Island of Jura some 31 kilometers (19 miles) away. On the summer solstice, the tip of the sun’s disc twinkled down the indicated slope; a couple days before or after, when the sun’s path was just slightly lower, it would not have been visible. The row of three stones itself points southwestward toward a small island called Cara Island about 12 kilometers (7 miles) away. Close to the winter solstice, the tip of the sun’s disc gleamed to the right of the island as it set; on the solstice itself this would not have been the case.

The best evidence supporting the theory that Ballochroy was a “solar observatory” is that there are not one but two foresights at the same site, marking the setting sun at both of the solstices. Surely such a coincidence could not have arisen by chance? And yet many critics raised doubts. One of the other standing stones in the row also has a broad, flat face pointing northwestwards, but this one points at a different mountain. And the alignment along the row is very broad, encompassing not only the right-hand end of Cara Island but also its left-hand end and central peak as well. If we are fair with the data, then we should admit the existence of at least a few other candidates for foresights that are equally plausible but have no ready astronomical explanation.

On the other hand, the fact that the claimed alignments are so precise means that we can use the slow change in the setting path of the solstitial sun century by century (due to the gradual change in the obliquity of the ecliptic) to calculate the best-fit dates for the two foresights and see if they coincide. The result is stunning: the best-fit date is pretty much the same for both foresights, around 1600 b.c.e., and this is a date that is certainly plausible archaeologically.

Yet archaeological and historical evidence has all but destroyed the idea that Ballochroy is a precise solstitial observatory. There is, and was, more to this monument than three stones in a row. A drawing by the antiquarian Edward Lhuyd in 1699 clearly shows three cairns in line with the three stones, one of them so large that it would have blocked the view to Cara Island, together with a fourth stone beyond. The burial cist (a box-shaped tomb with four side slabs) that was originally covered by this mound is still visible at the site, although the mound and the other features sketched by Lhuyd have been destroyed. The remains of this cairn were excavated in the 1960s, and the archaeological evidence indicates that it is very unlikely to have been constructed as late as the mid-second millennium b.c.e. If the stones were erected at the date indicated by the two alignments, then one of them was always blocked by the cairn. Though a few have argued that people might have stood atop the cairn to make the observations, this is special pleading. Being realistic, we are forced to conclude that the solstitial alignment of the stone row at Ballochroy, if intentional at all, was only of a low precision: it was not an observing instrument.

The example of Ballochroy remains important to archaeoastronomers because it demonstrates the dangers of enthusiastically endorsing alignments that seem to fit an astronomical theory while ignoring other possibilities because they don’t. This isn’t being fair with the evidence. It also shows the importance of the broader context of archaeological and, where we have it, historical evidence.

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