Caithness Sites and a Potential Observing Method

At Mid-Clyth in Caithness in northeastern Scotland, there is a grid that perhaps once held as many as ~250 stones on a gently sloping hillside: "the Hill o' Many Stanes." Thom (1971/1973/1978, pp. 23-25; 86-90) saw the establishment of the extreme azimuths of the rising and setting Moon as a principal goal of megalithic astronomy, perhaps for eclipse purposes, or because of interest in the Moon for its sacre-dotal, luminary, or tidal effects. In ยง3.2.1, we mentioned Thom's hypothesis that three components of the Moon's motion could be measured from at least some selected megalithic sites. Thom believed he had found evidence that the requisite information was obtained by virtue of the alignments; he also believed that the information was stored in the geometric grids at Mid-Clyth and perhaps elsewhere and, moreover, that the grids provided computational assistance to determine those components.

The method of encoding the observational information allegedly began with a distance stepped off nightly on either side of the date of extreme azimuth of moonrise/set. The careful marking of where the observer stood to observe a given phenomenon would provide data to determine the moment of extreme azimuth (and therefore declination). We now discuss how this could be accomplished for particular sites (see Chapters 2 and 3 and references cited therein for the geometry and spherical trigonometry background).

For any particular alignment, the azimuth of a rising (or setting) object depends on the declination of the object, 8, the zenith distance, z, and on the observer's latitude, f. In general, the relationship (2.1) can be expressed as sin 8 = cos z x sin j + sin z x cos j x cos A. (6.7)

The basic idea is this: The declination of the moon rises to a maximum sometime around its extreme rise (or set) azimuth during the month. The variation of the declination over a couple of days on either side of the maximum is assumed to be representable by a parabola of the form

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