Tri Radial Cairns

Tri-radial cairns are a distinctive type of Bronze Age monument found on the moorlands of northern England, comprising three radial "arms" of stones, typically no more than two meters (six feet) long and half a meter (two feet) high, extending out from a common center. Modest in size and easily mistaken for sheep shelters or other more modern constructions, their status as prehistoric monuments was only recognized in the 1990s. Over twenty examples are now known, with a major concentration of eight at Lordenshaws in Northumberland.

The state of preservation of the known tri-radial cairns varies. It is often difficult to determine with any great precision the original direction of each of the arms, even assuming that they were straight in the first place. Nonetheless, there is a clear general consistency in the orientations, and this invites an astronomical explanation. In each case one of the arms points approximately due north, always to within about thirty degrees and in the majority of cases to within ten degrees. The remaining two arms point southeastward and southwestward respectively, with a similar scatter in the "best-fit" directions.

A group from the Border Archaeological Society who discovered the cairns has suggested that they might have been constructed by making observations of the sun around the time of the summer solstice. The technique would have been to erect a vertical post and to build the three arms in the direction of the shadow at sunrise, noon (when the shadow is shortest), and sunset. This might not have been done to any great precision, so we might expect a considerable scatter in the resulting orientations even at the time of construction. (It would also require clear skies, but given the tiny change in the sun's motions on days near the solstice, there would be many opportunities to take advantage of suitable weather conditions.) The result would be radial orientations concentrated around azimuths of 0 degrees, 135 degrees, and 225 degrees respectively.

But there is a simpler explanation that fits the data just as well—in fact, better. Suppose that one arm was oriented to the north, either by using the sun's shadow at noon (on any day of the year), or by reference to the diurnal motions of the stars at night, the other two arms being simply placed at regular intervals round the cairn. This would result in radial orientations concentrated around azimuths of 0 degrees, 120 degrees, and 240 degrees. Of course this begs the question of why the chosen number of arms was three, but the explanation could well have nothing to do with astronomy.

There is no conclusive evidence to support either of these explanations, but the fact that they are both viable shows the dangers of fitting particular astronomical orientations too readily, and not considering equally plausible alternatives. It also demonstrates the principle known as Occam's razor, which advises us that there is no need to resort to a complex hypothesis to explain some data if a simpler one will explain the data just as well. Here, the "regular spacing" explanation is arguably the simpler one and more consistent with the data.

See also:

Azimuth; Diurnal Motion.

References and further reading

Ford, Bill, Philip Deakin, and Manuella Walker. "The Tri-radial Cairns of Northumberland." Current Archaeology 182, 16(2), Nov. 2002, 82-85.

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