Operation Megalithic Lift - The Stones and the Stars: Building Scotland's Newest Megalith - page 187

F IG . 7.3 Midsummer sunset as an example of the graphical calculation
ment to the skyline event was marked (this time, with parallax
taken into account), and likewise the true horizon. Then the bear-
ings one degree east and west were similarly marked.
So in each 'working photograph' I now had a small central
grid (to minimize distortion) 2° on a side, centered on the the-
oretical alignment. On each grid I then plotted the theoretical
rising or setting of the heavenly body concerned, and where it
crossed the 0° and +1° lines I corrected upwards for the mean
values of atmospheric refraction given by Thom in Megalithic
Sites in Britain .
The last step was then to draw the apparent rising or setting
path and where that line cut the skyline in the photograph, this
was the azimuth on which the rise or set would occur, and there-
fore the bearing on which the marker stone should be placed. Of
course running the calculations backwards in this way produced
far less accurate results than the Thoms' surveys of prehistoric
sites, but since our circle would be only 40 ft across and we were
not working to distant foresights, I hoped the shortcuts were per-
missible. As at Kintraw, where the stone did not break the skyline
as seen from the hillside view station, each of our stones would be
only a pointer to the celestial event.
The simplified diagram of Fig. 7.3 illustrates the principle. Fas-
cinatingly, Gavin Roberts recognized a similar pattern of scratches