Friday, 3 February 2012

229 degrees

There are no definitive answers in archaeology, even though it is a discipline based on the material remains of the past. A pot is a pot. But what was it used for...

Look at Stonehenge. It is one of the largest examples of material remains surviving to the present day. It is in the landscape. But what was it built for? We know that when you look through the uprights at the Heelstone it marks the position of the midsummer sunrise, but does that mean that Bronze Age people worshipped the sun?

I have a couple who are students of mine and had a look at the material remains of Stonehenge and the Martin's Down Bank Barrow here in Dorset.

Looking at Stonehenge they read that the height of the horizon is 0.6 degrees behind the Heelstone. From the centre of the horseshoe-shaped trilithons an observer can see the sun rise behind the stone. So far so well documented. Of course, in the opposite direction one would be able to see the midwinter sunset too. The direction opposite midsummer sunrise is 229.3 degrees and midwinter sunset is 229.8 degrees. The situation of the monument has to be less than 1 degree in horizontal relation to the horizon or the observation of the sunrise/sunset will not work. 

In Dorset the Bank Barrow is dated to the second quarter of the of the 3rd millennium (c.2800-2500BC according to C14 from ditch fill samples - as near as damn it to this phase of SH). It has a bearing of SW-NE, similar to Stonehenge and is c.180m long. By finding out the grid reference of the ends of the BB it is possible to find out the exact bearing. When my students did the calculations the bearing was 229 degrees. Looking along in the SW direction from the BB a hill is seen, the Knoll near Puncknowle. The height of the BB is 193m OD and the height of the Knoll is 179m OD, or just 1 degree in elevation between the two. 

My students suggested that the builders sought out a flattish hill to build the BB with a feature on the horizon they could line up at 229 degrees and near horizontal to the proposed monument. The BB has to be 100m+ to accurately frame the sun (which is 0.5 degrees across - 1m in 100m length is c.0.5 degrees). The position of the sunset varies across the year but changes very little in the few days either side of midwinter. To be precise to a single day one must be able to observe a change equal to about 1/60th of the diameter of the sun.  Observing that it occurred within a two day period is easier, requiring a precision of about 1/16th of the diameter of the sun. A marker (natural or not) lying close to the midwinter sunset direction might help in deciding the day, eg if a quarter of the sun was still visible.  

I would like to do this simple survey on other monuments and check their bearings too. But what does this tell us about why such monuments were built? The only concrete evidence is that they were concerned with the setting and sunrise of the sun at certain times of the year. Why is another matter and this crosses the line into interpretation and theory. 

Bank Barrows have puzzled archaeologists since they were recognised as distinct from burial mounds in the last century. They are very much like burial mounds but with no burials. Apart from pointing in the same direction as Stonehenge why is the Martin's Down example built like a burial mound, with flanking ditches and probably has the same internal palisade structure? What is the notch at one end for (if it is contemporary)? This shape and construction technique must mean something too, just like our churches are in the shape of the cross. This question goes beyond the purely material remains that still dominate our Dorset landscape. 

Keep looking in as more surveys are done on other monuments.

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