The Achara Stone


I have over the last few years been carrying out an investigation into possible purpose(s) of standing stones in West Scotland.     It is generally accepted that these were erected sometime in the period 3000 – 1500 BC, but for unknown purpose.   However there is a possibility that Achara at least may have been erected by Bronze Age Man around 1800 BC as a ‘backsight marker’ (i.e. viewing point) of the moon on the hill slope rising inland above Kentallen.

Alexander Thom (1894 – 1985) first proposed in detail the idea that Ancient Man used the horizon as a giant protractor for observations of the Sun/ Moon at key positions in their orbits; the observing position, or ‘backsight’ once found being marked permanently by a stone.    The Achara stone is one of the finest and so clearly it was considered important.

The sun has a straightforward movement over the year with which we are all familiar.    However the moon’s orbit is inclined at some 5 ° to the plane of the earth/sun orbit, and so its movements in the sky are much more complicated.   The sun moves from about 23 1/2° N. to 23 1/2° S and back again.   The Moon’s extremes are up to 5° more and less.   I.e from nearly 29° N. to 29° S. but then gradually changing to being 5° less than the sun (about 18° N. to 18° S.) and then increasing slowly again to the maximum.   Slowly is the operative word the whole cycle taking over 18 years.   Now there is a further but important complication.    Because of the inclination of the moon’s orbit there is a small (about 1/3 moon diameter) ‘wobble’ in the moon’s maximum position of period about 5 ½ months (173 days).    The effect is that at the maximum north ( the ‘ Maximum Standstill’ ) the moon will sometimes reach a little further north than expected and sometimes a little less (in an entirely predictable way) during the 5 ½ month period.    (This ‘wobble’ or ‘perturbation’ of course continues on as the moon no longer reaches its maximum north (& south) but that is the only time when it can be conveniently observed.)   We cannot now go to a site and expect to observe what Bronze Age Man observed because the earth precesses slowly on its axis like a slow spinning top with the result that the alignments are now some 1/2° wrong. So we must use a theodolite, make measurements and work backwards.   (It is this slow, steady precession that enables us to say when the alignment was correct and so date the site).

It is important to understand that Ancient Man had no need to approach this whole matter as we must now do.   He simply needed the ability to observe, record and to erect stones.    He also would have required a working calendar and the ability to count.   There is strong evidence for the first and the second inevitably follows.
My measurements show that the foresight from the Achara stone was the hill side to the NE of the stone, rising above Kentallen.   This consists of a steep rocky bluff and further up the slope a small but distinct conical bump.   These were both for the lower limb of the moon (one with the ‘wobble’ and the other without) at its extreme North around 1800 BC.     Around this date I believe that the rising moon as described would have appeared to glide up the slope just touching one of the two points depending on the ‘wobble’.    Unfortunately intervening trees at present obscure much of the relevant hillside.   However it can be conveniently seen from the road when driving towards Kentallen and just before leaving the 40 mph limit.
The Danish astronomer Tycho Brahe (1546 – 1601) is credited with being the first to determine the moon’s ‘wobble’.   It is a fascinating thought that perhaps Bronze Age Man discovered it over 3000 years earlier. If he did then a likely reason would be prediction of (lunar) eclipses as the timing of these is closely linked to the time of the ‘wobble’.   However after a few centuries the Earth’s axis would have tilted enough to throw any alignment off.   Their original purpose would then have been gradually forgotten.

Note:-   This work is by no means proved yet but continues.
 By Thomas Gough

Back Settlement
Cuil Bay
Back Settlement
Back Settlement
Mouth of Duror
Achara Stone