Having then true azimuths, the next question concerns the declinations of the stars which may have been observed.

The work of Stockwell in America, Danckworth in Germany,[19] and Dr. W. J. S. Lockyer in England, has provided us with tables of the changing declinations of stars throughout past time, or enough of it for our purpose.

An accurate determination on the 25-inch map of either the azimuth (angular distance from the N. or S. points) or amplitude (angular distance from the E. or W. points) of the stone or barrow as seen from the centre of the stone circle will enable us to determine the declination of the star at the time when it was observed.

I give a [diagram] which enables this determination to be made with the greatest ease for any monuments between Land’s End and John o’ Groats, whether the direction is recorded by amplitude or azimuth; the declination is read at the side from the value of either indicated, say, by a dot, at the proper latitude.

This, of course, only gives us a first approximation. The angular height of the point on the horizon to which the alignment or sight-line is directed by the stone or barrow from the centre of the circle must be most accurately determined, otherwise the declinations may be one or two degrees out.

In the absence of measurements it is convenient to assume, in the first instance, that the horizon is half a degree high, as with this elevation refraction is compensated, as the following table will show:

Elevation
of actual
horizon.		Bessel’s
refraction.		Combined
effect.
0° 0′0″34′54″ -34′54″
0°10′ 32′49″ -22′49″
20′ 30′52″ -10′52″
30′ 29′ 3·5″+0′56·5″
40′ 27′22·7″+12′37·3″
50′ 25′49·8″+24′10·2″
1° 0′ 24′24·6″+35′35·4″

In the absence of theodolite observations the actual elevation of the horizon can be roughly found by a study of the contour lines on the 1-inch map. The following heights will agree with the previous assumption of hills 1⁄2° high:

Distance1mileHeight=46feet
„2miles„=92„
„4„„=184„
„8„„=368„
„10„„=460„