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KRONOS Vol VIII, No. 3
Observations At Kintraw
T. MCCREERY, A. J. HASTIE, AND T. MOULDS
Copyright (C) 1982 by Cambridge University Press.
We have made six visits to the Kintraw site which has been claimed by Professor Alexander Thom as a prehistoric astronomical observatory used for the detection of the midwinter solstice. This claim has been disputed on several grounds, notably that the foresight is not visible from the backsight on the ledge overlooking the menhir. From the results of our observations at the backsight we contend that this particular dispute is easily resolved. Other work done by us on the ledge indicates that it could not have been used as an observation platform to determine the exact day of the midwinter solstice.
Of the many megalithic sites claimed by Professor Thom as remains of prehistoric observatories none has proved more contentious than the proposed midwinter solstitial site at Kintraw (A. Thom 1971, pp. 37-40). This consists of cairns and a menhir which Thom claims indicates a horizon foresight, the col between Beinn Shiantaidh and Beinn a' Chaolais on the island of Jura, 45 km away. Since the view of this col from the area around the menhir is partially blocked by the intervening ridge of Dun Arnal, Thom suggests that preliminary observations were made from a ledge on a hillside to the north east of the site, and overlooking it. It is the question of the validity of this claim which has produced such a vigorous debate (MacKie 1976, 1981; Patrick 1981).
Evidence apparently supporting Thom's suggestion was furnished by the discovery on the ledge, and at exactly the position required for a backsight, of two boulders inclined towards each other so that their inner faces form a V-shaped notch pointing over the menhir and towards the foresight. Subsequent excavation revealed a compact layer of stones behind this "boulder notch" (MacKie 1974) and petrofabric analysis of the layer seemed to indicate that it was man-made (Bibby 1974,1983). Other investigators disputed both the reliability of petrofabric analysis, the use of which in this context is highly original (McCreery 1980,1983), and the visibility of the col on Jura from the boulder notch (Patrick 1981). We attempt here, on the basis of our own observations from the ledge, to resolve this latter dispute and to examine more widely the claim that the exact day of the solstice could have been determined using the ledge as an observing platform. We offer no comment on the petrofabric analysis since the viability of the site as a solstitial observatory depends on other, more important factors. However, the presence of a layer of stones in this area alone may perhaps be explained by the configuration of the slope above it. Here, there is a shallow couloir which would tend to act as a funnel channelling all stones moving down this part of the hillside into the area around the boulder notch and to its left.
Between September 1980 and October 1981 we visited Kintraw six times. On each occasion visibility was good enough for the col to be seen clearly from the hillside above the ledge, and observations were made with a telescope (x45) from significant positions on the ledge. The results are presented in Table I.
To an observer standing on the stone layer behind the boulder notch, the col was sometimes visible and at other times obscured by the vegetation on Dun Arnal (Fig. 1). We think that three factors are responsible for this:
(A) The observer's height. Changes in eye level of a few inches are critical.
(B) Unpredictable, but substantial variations in terrestrial refraction, which changes from hour to hour, along the length of the alignment from Jura to Kintraw.
(C) The foliage on the clump of trees indicated in Figure 1(1). In summer this is thick enough to be virtually opaque, but in winter and early spring, it is sparse enough for it to be just possible to see through to the skyline beyond. We believe that this factor is less important than the two mentioned above, for only the tops of the trees forming the clump rise above the outline of the ridge, as viewed from the ledge.
[Fig 1: Viwe from the boulder notch. The upper line shows the maximum observed elevation of the col (optimum conditions), the lower line shows the minimum elevation. Observer's eye height 5 ft 7 ins. BEINN SHIANTAIDH, BEINN A'CHAOLAIS, TREE CLUMP, DUN ARNAL]
Standing directly behind the boulder notch an observer whose eye level was 5'0" above the stone layer was never able to see the col. To an observer whose eye level was 5'7" above the layer the col was sometimes just visible, rising marginally above the trees on Dun Arnal, but conditions changed rapidly, presumably because of variations in terrestrial refraction, on two of our visits. On one of these occasions, 13th October 1981, at 2:45 p.m. the col appeared clearer of the ridge than on any previous visit. At 3:30 p.m., after a squall had passed over Jura, it was obscured by the trees on Dun Arnal, but ten minutes later was again visible, though barely so. Prior to our last visit we attempted to correlate our observations with values for terrestrial refraction calculated using mean values for the refraction constant (A. Thom 1971, p. 31), and meteorological data from the nearest weather station at Rhuvaal on Islay. These values (Table I) show no relationship to what was actually observed. This tends to confirm Moir's suspicion that although Thom used only average values for terrestrial refraction in his published work (A. Thom 1958), "his actual raw data ... was very scattered" (Moir 1981).
These observations enable us to resolve the long standing dispute between Dr. Patrick and Dr. MacKie. Patrick has asserted that he "could not satisfy himself that it was possible to see the base of the notch using a theodolite with a thirty power magnification", and has published a photograph to illustrate this (Patrick 1981). MacKie claims that Patrick's statement is "most incomprehensible" and "nonsense which Patrick has never explained" (MacKie 1981), and has also published a photograph (MacKie 1977, p. 106). Patrick's observations, made from an eye level position 5'9" above the stone layer, are remarkably consistent with our own, from a similar height, and his published statements require no further explanation. MacKie's photograph shows the col much higher above the ridge than we ever noted in any observation at the boulder notch. We believe that this may be explained at least partly by MacKie's personal height. Perhaps also, the photograph was taken before the ledge was excavated down to ' the stone layer. The gain in height resulting from such factors, together with different refraction conditions, can explain the discrepancy between the two published photographs.
This controversy is, however, made meaningless by a further difficulty. According to Thom, several days before the solstice the observer on the ledge would have so positioned himself that he saw the Sun flash in the col at the moment of sunset, and would have marked the position. As the setting position moved southwards, the position from which the observer saw the Sun setting in the col moved to the right (north west) along the ledge till the day of the solstice. On the following day, as the sunset once more advanced to the north, the observing position retreated to the left, leaving the extreme right hand position, subsequently marked by the boulder notch, to identify the exact day of the solstice. It is evident that this position could not have been obtained if the positions appropriate to the days immediately before (and after) the solstice had not also been established.
At Kintraw the observing position for the day before (and after) the solstice is 19 ft. to the left of the boulder notch. Fig. 2 shows the view from that position.
[Fig 2: View from position 19ft south east along ledge from boulder notch showing maximum observed elevation of Jura Hills. Observer's eye height 5ft 7ins.]
Regardless of conditions, the col was always obscured by Dun Arnal itself and not only by the vegetation on it. This was also true for the observing positions two and three days before midwinter, at 75 ft. and 170 ft. to the left respectively. MacKie wrongly states that the observing position for the day before the solstice is 8.62' to the left of the boulder notch(2) (MacKie 1976, p. 174), but even from this position the col was invariably obscured, sometimes apparently only by the vegetation on Dun Arnal, but sometimes by the hill itself (Fig. 3). It is quite clear, therefore, that the series of observations which Thom's theory requires could no more have been made from the ledge than from the field beside the menhir itself.
[Fig 3: View from position 8ft 6ins south east along ledge from boulder notch. The upper line shows the col obscured by vegetation only (optimum conditions). The lower line shows the col obscured by Dun Arnal. Observer's eye height 5ft 7ins].
Patrick has suggested that a broad shoulder of the hillside which he calls a "crest", about 30m above the ledge, might have been a suitable observing platform, and that one recumbent stone there is aligned on the Paps of Jura (Patrick 1981; Heggie 1981, p. 190). However, if observations of the Sun setting in the col were made from the crest instead of the ledge they would have had to be made from positions further to the south east, as the altitude of the col is less from the crest than from the ledge. As the hillside drops away sharply in this direction there is simply no room for such a series of positions to be established. There are many large recumbent stones on this part of the hillside. All lie with their long axes pointing down the fall line of the slope, which is exactly what is to be expected if their position is due to natural causes. That they also point roughly towards the Paps of Jura is coincidental.
Several further objections must be advanced against Thom's interpretation of the site and to a lesser extent against Patrick's suggestions. The suggested use of a long alignment means that the altitude of the foresight is small (<1-degree) and this leads, as we saw on our visits, to very pronounced changes in terrestrial refraction. Further changes in the Sun's apparent position will be caused by variations in astronomical refraction and the effect of "graze", i.e. the rays of light from Jura being bent by Dun Arnal in their passage to the ledge (A. S. Thom 1981). Finally, the Sun will not generally attain its declination maximum at the instant of setting. Cumulatively these factors so affect the Sun's apparent position that, even if the visibility of the foresight from the ledge were not in dispute, it is most improbable that the boulder notch could have been used as "the permanent observing point from which the day of the midwinter solstice was regularly checked" (MacKie 1974). Its use in this context would lead to quite inconsistent measurements of the length of the year.
We believe that the evidence presented here is not compatible either with Thom's hypothesis that the ledge could have been used as an observing platform for work leading up to the establishment of a permanent backsight at the menhir, or with Patrick's suggestion that the crest of the hill might have been used in a similar fashion.
1. Due to circumstances beyond our control none of the photographs we took from the ledge was suitable for reproduction. The text figures are based both on these photographs and on notes and sketches made at the time.
2. MacKie (1974, p. 174) has confused the Sun's azimuthal change, which determines the sideways distance to be moved at each daily observation, with its declination change. At Kintraw, the azimuthal change is about 28" for the declination change of 12" in the twenty-four hour period before and after the solstice.
Acknowledgements: The authors wish to thank Mr. J. Mooney and Mrs. M. Hastie for their help in preparing the final copy.