Reference to the Nautical Almanac shows that this is the sun’s declination on May 5 and August 9. We may therefore conclude that the Long-stone was erected to mark the May sunrise, as seen from the Tregeseal Circle.
Had we been dealing with a star, instead of the sun, the only modification necessary in the process of calculating the declination would have been to omit the semi-diameter correction of 14′.
Having obtained a declination, we must refer to the curves given on [pp. 115]-[6] in order to see if there is any star which fits it, and to find the date.
Take, for example, the case of the apex of Carn Kenidjack, as seen from the Tregeseal circle—
Az. = N. 12° 8′ E.; hill = 4° 0.′ lat. = 50° 8′.
This gives us a declination of 42° 33′ N., and a reference to the stellar-declination curves ([p. 115]-[6]) shows that Arcturus had that declination in 2330 B.C. From the table given on [p. 117], we see that at that epoch Arcturus acted as warning-star for the August sun.
In cases where the elevation of the horizon is 30′, or in preliminary examinations, where it may be assumed as 30′, the refraction exactly counterbalances the hill, and therefore the true zenith distance at the moment of star-rise is 90°. Hence the N.P.D. of the star may be found from the following simple equation—
(3) cos Δ = cos A cos λ
where Δ and A have the same significance as before and λ is the latitude of the place of observation.