Megalithic Observatories

In the mid-twentieth century, the engineer Alexander Thom proposed that many British megalithic monuments functioned in prehistory as devices for observing the sun, moon, or stars. Actually, the horizon was the real observing instrument. Given a sufficiently distant horizon, and provided that the observing position is closely enough defined, then any clear, unmistakable horizon feature such as a jagged hilltop or a sharp notch between two steep slopes can be used to pinpoint a particular direction. From the right observing position, the notch or other feature can be used as a foresight marking the rising or setting point of a celestial body to remarkable precision, perhaps to as little as a few arc minutes. For instance, the difference in the rising or setting position of the sun on the solstice and a couple of days earlier or later is normally imperceptible. However, by choosing a suitable observing position and horizon notch, an oberserver could distinguish the solstice, Thom observed, on the basis of whether or not a brief flash of sunlight from the very edge of the sun's disc appeared in the notch. Many megalithic monuments, he claimed, marked both the place to stand and the direction in which to look. They could, in this sense, appropriately be labeled "observatories." Classic examples include the stone row at Bal-

lochroy and the standing stone at Kintraw, both near the coast of Argyll, western Scotland.

Thom's ideas of high-precision alignments have not withstood the test of time. During the 1970s and 1980s many problems came to light, including day-to-day variations in atmospheric refraction, and extinction in the case of stars; and many difficulties were found with Thom's selection of data (see, for example, Brodgar, Ring of). Many archaeologists were deeply suspicious from the outset of the idea of "observatories" in prehistory. They did not necessarily dismiss out of hand the possibility of deliberate astronomical alignments; what disturbed them was the idea that monumental alignments were used primarily as observing devices. Surely, if the aim was merely to mark a rising or setting position, one could simply use a temporary marker such as a wooden stake, as is done by sun-watchers among the Mursi in Ethiopia. There would be no need to go to all the bother of erecting big stones. Given that big stones were used, it would seem more likely that they were encapsulating an alignment that was already well known, perhaps using it for symbolic or ceremonial purposes. This would be entirely consistent with the human tendency, identified by anthropologists, to encapsulate relationships of cosmological significance in monumental architecture. This sort of explanation also fits well with the lower-precision alignments that are more commonly indicated by research in archaeoastronomy today.

See also:

Cosmology; Palaeoscience; Science or Symbolism?

Ballochroy; Brodgar, Ring of; Kintraw; Megalithic Monuments of Britain and Ireland; Mursi Calendar.

Extinction; Refraction.

References and further reading

Ruggles, Clive. Astronomy in Prehistoric Britain and Ireland, 1-81. New Haven: Yale University Press, 1999.

Thom, Alexander. Megalithic Lunar Observatories. Oxford: Oxford University Press, 1971.

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