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October 1977 IndexBulletins Index

Caithness Field Club

Caithness Field Club Bulletin
1977 - October


A Megalithic Summer Solstice Observatory

L. J. Myatt

As sunset was approaching on midsummer's day, some fifty members of the Field Club walked up the ridge of Cnoc Na Maranaich in Dunbeath Strath to observe the setting sun from the large standing stone, at grid reference ND 132332, near the summit.

The stone, pointed at the top, and now leaning slightly, stands to a height of 2.5m and is 1m wide at the base and 0.28m thick. Some 4.2km distant from the stone, in a north-westerly direction, is a well defined notch in the horizon formed between the two hills Cnocan Conachreag and Cnoc Fuarain.

It was Professor Thom who first suggested that this site could have been used to determine accurately the date of the summer solstice. By careful and repeated measurements of the azimuth (compass bearing) and altitude (angular elevation) of the notch he was able to calculate the declination of the sun such that it would set with its upper limb finally disappearing in the notch at sunset. This gave a value of 23deg. 53.1min.

The declination of an astronomical body, such as the sun, is the angle it makes above, or below, the plane of the earth's equator. At the present time the declination of the sun varies throughout the year between the limits of +23.5deg, at midsummer, when it is above the plane of the equator, to -23.5deg, at midwinter, when it is below. At any other time of the year the declination will lie between these two extreme values. For example at the two equinoxes the declination of the sun is 0deg. when it is directly above the equator. The annual maximum value of declination (now 23.5deg.) has been decreasing slowly at the rate of about 0.5deg. in the last 4000 years at a known rate, and from this knowledge Thom was able to estimate from the declination indicated by the notch that the sun would have set with its upper limb finally setting in it at the date of approximately 1590 BC.

Whatever the reason for erecting this standing stone in this particular position, there can be little doubt that the above phenomena would have been seen by an observer using the stone as a backsight and the notch as a foresight round about 1590 BC. The diagram Fig. 6(a), shows the profile of the distant horizon with the notch as seen from the stone plotted from the contours on the 1:25000 large scale Map. Also is shown the path of the midsummer setting sun calculated for 1590 BC. This indicates how the last flash of the upper limb of the sun would occur in the notch on setting.

Diagram Fig. 6(b) shows the same horizon profile with the path of the midsummer setting sun as at the present time. This is precisely what was observed by Field Club members this year. The reduction in the declination of the sun over the last 3567 years together with the effect of the sloping horizon causes it to set with the upper limb finally disappearing some 1.5deg. to the west of the notch.

This serves to emphasise one of the difficulties of identifying such possible sites because of the slight change in position of the setting sun over the last 4000 years. It can only be done by accurate survey work such as Thom and others have carried out, working to seconds of arc in measuring the necessary angles.

Is there other possible evidence of sites where standing stones could have been used to determine the date of the solstice by similar alignments towards the sun? Professor Thom has shown convincing evidence for such sites in Argyllshire. Each of these uses a large standing stone as a backsight and a distant feature on the horizon-as a foresight. Details of four possible sites are given below.



Distance (km)



Corra Bheinn, Jura


Setting midsummer


Cara Island


Setting midwinter


Beinn Shiantaidh, Jura


Setting midwinter

E. Loch Tarbert, Jura

Ben Tarsuinn, Arran


Setting midwinter

In each case these sites give an accurate alignment towards the top limb of the sun at the appropriate solstice in the second millennium BC. A further interesting feature of the Kintraw site is that the horizon foresight is not visible from the standing stone. However, across a deep gorge behind the stone is a rising hillside on which is a ledge with a large stone lying on its side. Excavation of the ledge by Dr. Euan Mackie revealed it to be man made, and Thom has shown that it is correctly positioned to have been used as an observing platform for the setting midwinter sun falling below the horizon foresight 31km away.

On each of the days before and after the solstice the change in setting or rising position of the sun amounts to only about 30 seconds of arc in azimuth, depending upon the latitude and the profile of the horizon. The daily movement then increases rapidly until at the equinoxes it is about 32 minutes of arc, which is roughly equal to the diameter of the sun. The argument is often made that sufficient accuracy of measurement could not be obtained by the method suggested and that it would be affected by which side of the stone was used as the backsight. This criticism is, however, unfounded for the above sites when the facts are properly considered for it will be noticed that in each of the alignments given, the distance between the backsight and the foresight is very great, as it needs to be for a solstitial site. On the Kintraw platform it would require a sideways movement of some 5.8m to give a change in azimuth of the distant horizon notch of 27 seconds of arc, equivalent to the daily change in setting position of the sun at solstice. At the Cnoc Na Maranaich site the alignment is much shorter, amounting to only 4.2km, and the daily change in azimuth at the solstice would be 33 seconds of arc. To produce this difference at the standing stone would nevertheless require a sideways movement of 0.6m, and the stone is only 0.28m thick. It would therefore introduce insufficient error whichever side of the stone was used for the alignment. For a site such as Stonehenge however, where midsummer sunrise is popularly believed to be indicated by an alignment from the centre of the circle to the Heel stone the distance is very short. It would only require a sideways movement of 9mm to give a change in azimuth equivalent to the daily movement of the setting sun at the solstice. Despite the present day ceremony which is annually performed at Stonehenge, the use of this particular alignment as an accurate indicator of the solstice would seen doubtful.

There can be no doubt however that the large standing stone near the summit of Cnoc Na Maranaich would have given an accurate indication of the summer solstice in about 1590 BC. That other similar sites are known elsewhere, which could also have been used for the same purpose in the second millennium BC can only add support to the suggested reason for its erection.


Thom Megalithic Lunar Observatories Oxford 1971

Thom & Thom

A Megalithic Lunar Observatory in Orkney: The Ring of Brogar J. Hist. Astron. 4 1973

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