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Megalithic Lunar AstronomyAlexander Thom amassed a great deal of hard evidence for the existence of a megalithic calendar of 16 'months' or intervals. This evidence was astronomically gained from careful surveys of many sites. During this experience his attention had been drawn to a number of major stone settings which appeared to indicate the rising or setting points of the Moon at it's furthest extremes. The Moon reaches it's monthly declination extremes,- furthest north and furthest south- two weeks apart, with one complete cycle every month. The Sun takes six months to travel between it's extremes, the solstices and twelve months to complete one cycle.
Lunar StandstillsThe Sun is regular in it's cycle. The position of the solstices changes very slowly. In 4000 years these have moved less than half a degree of declination,- 27 arc minutes.
Observing limitations of horizon astronomyWith the equipment available to Bronze Age astronomers it would be impossible to accurately measure the position of the Moon in it's orbit at any point other than the standstills. This is due partly to the high rate of change in declination over 24 hours but also to the problems raised by several minor discrepancies in the regularity of the orbit. None of these minor cycles can be sorted out and quantified at any other time than at a Lunar Standstill
Stake setting at a lunar extreme
A distant object- the Moon- will appear to move in the same direction as a mobile observer with relevence to an object in the foreground- the horizon. The relationship is strict but in proportion to the length of the alignment to the foresight on the horizon.
For an alignment length of 8.8 kms an oberver may 'move' the Moon across a horizon mark by it's own diameter-half a degree-with a step-aside progression of about 224 metres.
For an alignment of 29 kms the step-aside needed to make an angular displacement of the Moon of half a degree will be nearer 800 metres.
Observing limitations of horizon astronomy
"It was only natural that a people looking critically at the setting Sun would use the same observing method on the Moon. But here they found a much more complicated motion. They would see that it was only at the standstills that anything like repeatable results could be obtained. But the rapidity with which the Moon passes through the monthly declination maximum set them a serious problem, namely that of extrapolation. Did they solve this problem by first making it geometrical? If they tried the effect of stepping back each night as shown in Fig. 1.1 they would have been able to apply their knowledge of the geometry of the flat arc. We know how much time they must have spent on the problem at numerous observatories throughout the country, and it would be strange indeed if they had not somewhere tried out this method of avoiding confusion as the stake position moved first one way and then the other. This method would show the constancy of what we have called the sagitta (G), and would have led to the method of finding G at any site with sufficient clear ground."