The hill in the background is Tor y Foel 551m. The alignment to the stone is 4deg south of West. Making allowance for the angle of altitude of the hill, and the precession of the equinoxes, viewed from the stone, the sun would set on the peak on the evening of the spring or autumn equinox around 1800BC. The angular error is a fraction of a degree. This is therefore a convincing calendrical alignment. Despite the comment below, I do not doubt that the stone is in fact an authentic bronze age megalith. The Coflein page referenced below merely states that the stone may have been used as be a mediaeval boundary marker, which is not to say that it was erected during the mediaeval period. Curious how comments become changed in the recounting ("Coflein describes it as mediaeval.") There are many examples of mediaeval documents which describe boundaries in terms of pre-existing or natural features - rivers, trees, roads, etc. There is no reason why the stone could not be a genuine megalith which was later used as a boundary mark, so I would presume it to be genuine until proven otherwise by documentary evidence that it was erected during the historical period.
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