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#21 BALLYMEANACH Thom A2/12,Ruggles: AR15 (DUNCRACAIG) NR 8337 9641

(See Thom, Megalithic Lunar Observatories, pp52-53)

The main alignment is formed by 4 large stones, all slabs and up to 4m. in height. The east faces of the two central stones have many cupmarks.  To the NW the stones give a lunar alignment. (See note later).  Thom gives an alignment to the SW for the winter solstice, (MLO p52-53 ) but the horizon is now tree covered.  According to Thom the stone pair gives an alignment for the lunar southern standstill, but again the horizon is now tree covered.


(See Note2 at the end regarding visibility/ use of the moon setting at A2 ).

Observations were made of the 4 stone alignment towards the NW and the results were in close agreement with Thom, particularly for the top LH corner of the rock at A2 (3.2km.).  OP was 2m behind the stones and as close as possible to the line of the stones while still permitting views of A1 & A2 from each of LHS and RHS.  Two measurements past each of LHS and RHS on separate occasions were made.  ( The separation of the OPs was about 1.3m.)


The LH end of the rock outcrop is about 1.5 - 2 m. high.   At 3.2km. this corresponds to about 1'.5.   The limit of resolution of the eye is about 1' arc and so the small angular size would not be a problem.

It remains to consider the alignment A1 for the centre of the moon close to the slope at 321°.4.  It is not possible to measure accurately the centre of the moon directly and in any case what would be the purpose?  However what would be useful to know is the lunar diameter which typically varies between 29'.4 and 32'.4 during each lunation (non-circular orbit). I.e. a semi-diameter range of ±1'.5 which is significant.  The fact that the centre of the moon 'misses' the hill slope at A1 by about a minute of arc is unimportant. The fact that the slope is parallel to the path of the moon for some 10'  would give sufficient time for two observers - one left, one right - to find the places along the line at right angles to the OP where one limb or other of the moon just touched the slope. Their separation would give a measure of the moon's diameter (in any convenient unit of length) and hence allow a correction if necessary for the alignment at A2.  e.g. observing a short distance left or right of the OP.   (See Thom MLO, Nether Largie pp 47/8 )

(The above discussion is only intended to explain that the A1 'alignment' could be used as Thom suggested.  It is viable.  This does not mean that it was necessarily used. It could be chance, but see Note2 below)

Note 1:
Thom later had some doubts about the alignments when he wrote in SRSS (1990)   ......... "To the NW it may be lunar but there are too many foresights." It is suggested there is only one clear, unambiguous foresight, and it is the one used.    Namely the rock outcrop, and probably the top LH corner of it.

Note 2:
Visibility of Moon at Moonset.   (3rd quarter in Sept.would set in daylight.)  At 1st quarter the moons last light would be at the moment of moonset.  For the setting moon it would be possible to follow the northern limb of the moon down (e.g with a slanted stick).  A position for observing the lower limb on the rock outcrop could easily have been found.  Perhaps the stones were positioned as found because of a desire for the additional alignment at A1.(?)

( Note that a very similar alignment exists at both Nether Largie and Torbhlaran, (i.e. for the extreme northernmost moon), except that in those cases the lower limb is used thus avoiding the difficulties mentioned above.)