The Pole is an ancient unit of measure. Today it is virtually never used but formerly it used to be taught as part of the imperial system of measurement until the 1950s and 60s and probably even after that in some more traditional educational establishments. As the imperial system became formalised, so the Pole was fixed at 198 inches, or 5.5 yards.

It is probable that the Pole once provided the basis for a system of measures. If so, only one has come down to us. This is the Celtic League, which was still being used in the western parts of Britain and Ireland until a couple of hundred years ago. This measure was 1.5625 miles or 500 poles.

I am indebted to Henry Lincoln and his book ‘The Holy Place*’ for the suggestion that it would be worthwhile to measure in poles when exploring distances between ancient sites. Certainly in the Vale of Glamorgan at least we find that the distances between ancient sites appear as round numbers when measured in poles. The length of the lines locating Tinkinswood – two at 8,000 and one at 4,000 – is a case in point.

Although today the Pole is fixed at 198 inches we cannot know whether it was always this length precisely. It could have been slightly more than this, or it could have been slightly less. The evidence seems to lie in the latter direction, that is that the Pole was slightly shorter in ancient times.

For instance, in measuring such ‘round number’ distances it appears consistently that the number of poles is about half a per cent less than one might expect if the pole is taken as having a length of 198 inches. This disappears if the pole is given a value of around 197 inches.

There is another, perhaps more convincing piece of evidence. The earth’s equatorial circumference is 40,077 kilometres, slightly larger than the 40,000 kilometres it was judged to be when the metre was fixed at the end of the eighteenth century.

We know that it is possible to measure the circumference of the earth by observing the length of shadow cast by the sun at two different points on the earth’s surface. Who did this first we don’t know but we do know that the ancient Egyptians measured this circumference, quite accurately. They based the 0.729 inch unit used in the building of the Great Pyramid and known to us as the ‘digit’ upon it. 0.729 inches are exactly 1/100,000 of 1 minute of arc at the earth’s circumference where this is measured as 39,996 kilometres.

Now eight million poles also approximate to the earth’s circumference. If the pole is given a value of 198 inches the circumference would be 40,234 kilometres. But if the pole were slightly shorter – 197 inches – this would imply an earth circumference measure of 40,030 kilometres. The latter is clearly a more accurate measurement and provides confirmatory evidence that the ancient pole could well have been slightly shorter than the modern unit.

We do not know why the ancients should have wanted to divide the earth’s circumference into 8 million units (or 16,000 if the Celtic League is used) rather than, say, 10 million, but it seems no more implausible that the 40,000 kilometres used as the basis for the metric system. And it does have the advantage that anybody can divide a circle by 8 or 4. To divide a circle accurately by 10 is not easy.

Even taking the pole as 198 inches, we can say that there are 200 poles to the kilometre with an accuracy of almost 99.5%. That makes working with a 1 in 50,000 Ordnance Survey map very simple. 10 centimetres on the map equals 1,000 poles (actually 988.4 poles). And if we assume the pole to have been 197 inches rather than 198, the correspondence is almost exact.

*Henry Lincoln, The Holy Place, Jonathan Cape, London, 1991