I now come to Britain. So far as my inquiries have gone, these clock-star observations were introduced into these islands about 2300 B.C.
In my statement concerning them I will deal with the astronomical conditions for lat. 50° N., as it is in Cornwall that the evidence is most plentiful and conclusive.
In that latitude and at that time Arcturus, decl. N. 41°, was just circumpolar with a sea horizon, and therefore neither rose nor set. Capella, decl. N. 31°, when northing was 9° below the horizon, so that it rose and set in azimuths N. 37° E. and N. 37° W. respectively; it was therefore invisible for a long time and was an awkward clock-star in consequence.
Fig. 61.—Arcturus and Capella as clock-stars in Britain.
AB = sea horizon.
A′B′ = horizon 3° high.
[Fig. 61] represents diagrammatically the conditions named, the circumpolar paths of Arcturus and Capella being shown by the smaller and larger circle respectively. A B represents the actual sea horizon and A′ B′ a locally raised horizon 3° high, whilst the dotted portion of the larger circle represents the non-visible part of Capella’s apparent path.
What the British astronomer-priests did, therefore, in the majority of cases was to set up their temples in a locality where the N.E. horizon was high, so that Arcturus rose and set over it and was invisible for only a short time, as shown in the diagram by the raised horizon A′ B′.
The two lists following contain the names of the monuments where I suggest Arcturus was used as a clock-star. In the first, the angular elevation of the sky-line as seen from the circle in each case has been actually measured, and the date of the alignment is, therefore, fairly trustworthy; but in the second list the elevations have been estimated from the differences of contour shown on the one-inch Ordnance map, and the dates must be accepted as open to future revision.
ARCTURUS AS A CLOCK-STAR.