Field Visit

Date 2013

Event ID 994371

Category Recording

Type Field Visit

Permalink http://canmore.org.uk/event/994371

NB 21295 32950 and NB 21300 33017 A natural phenomenon has been observed close to the Callanish I monument which could be significant for the interpretation of the monument as a whole. Approximately 20m to the S of the monument lies Cnoc an Tursa, a hillock with distinctive rounded outcrops of bedrock – a Roche Moutonee. One large rounded block sits atop two others on the N side of the Cnoc, nearest the monument. Stones flanking this group create a group of five stones. The aperture between the first three forms a small, much discussed cave. When GM Coles and T Rees excavated trenches immediately N of the cave in 1993–4, they discovered many interesting features including rows of posts, pits and ditches containing pottery and flint. A continuity error in the 1994 DES report creates uncertainty about the position of pits and a ‘wing wall’, which could be on the W, E, or possibly both. It also doesn’t tell us whether the stones left in the cave were replaced exactly or just randomly backfilled. Nevertheless, this area has clearly been the focus of special attention at some point in the life of the monument.

Many monuments start life as natural places perceived to be special in some way and it is possible that this cave was such a place. It might also be worth noting the similarity between the cave and its flanking rocks with the entrance and forecourt of a chambered tomb.

Around midday on a bright sunny day in February, 2013, a defined beam or shaft of light was noticed by Ian Mchardy to be emanating from the back of the cave, around 50 to 150mm wide and stretching 2 or 3m out to the N onto the grass ‘forecourt’. The ‘forecourt’ area around the shaft was in deep shade, intensifying its effectiveness. The potential significance of this beam was realised and the discovery discussed with Margaret Curtis, who has documented it on bright days throughout 2013.

Investigation revealed that the bright sun was shining through a small slit between the rocks, which was further restricted by small trapped chock stones. The shaft moved W to E as the sun moved E to W, not unlike the hand of a clock. It was clear that this could only happen for a limited duration because of the sides of the cave – at around midday, from around 10.50 until 12.30 according to Margaret’s observations to date. Closest to midday the beam shines down directly into the cave, onto a step or seat like stone, as well as out onto the forecourt. Subsequent recording of the beam by Margaret has proved that the length of the beam and the surrounding shadow vary from summer to winter. In summer, the high sun casts a small shadow and beam, and in winter both beam and shadow are long.

Therefore these stones and the forecourt create a natural sundial, which with the addition of simple markers or posts in the ground at the various beam lengths on a N–S axis could tell not only the midday but the time or even month of the year.

Could this have possible in the Neolithic or before? The sun has changed only very slightly in declination since the Neolithic, about the width of their discs over the period since 2000 BC and so slightly more than this since 2,900 BC or before. The difference would mean that the sun and moon were slightly further S in rising and setting, hence only potentially negating the sun dials operation in midwinter, if the sun was so low that the Cnoc an Tursa cast a shadow over the group of five. This seems unlikely given the topography and magnitude of change.

Is this what the lines of pits or postholes found by Coles and Rees were? Access to a plan of their excavation may allow us to test this hypothesis, by comparing the excavated features with observation and recording of the actual sun beam through the year. An eight fold yearly division would require five post markers on a N–S line. Twelve months would be easily represented by seven posts, but 12 lunar months would fall just short of the solar year. A division of the year into 13 months would be rather difficult, because 8 posts would represent 14 months.

There is also a possibility that on the southern extreme of the moon, a bright full moon’s light could create a ‘moon beam’ allowing the setting to work as a ‘moondial’. Could this have led the builders of Callanish to understand the long, 18.6 year cycle of the moon, as has often been argued to be the case, but yet to be widely accepted? It is hard not to suggest that understanding this natural sun dial would pre-suppose a certain amount of knowledge and lead to a lot more knowledge of the cycles of the heavens. This knowledge would have been extremely useful to a farming population.

Ian McHardy and Margaret Curtis, 2013

(Source: DES)