Next

CHAPTER XII

ASTRONOMICAL HINTS FOR ARCHÆOLOGISTS—Continued.

I NEXT come to the Sun observations.

First we must consider the astronomical differences between the rising of a star and of the sun, by which we generally mean that small part of the sun's limb first visible.

It is frequently imagined that for determining the exact place of sunrise or sunset in connection with these ancient monuments we have to deal with the sun's centre, as we should do with the sun half risen. As a matter of fact, we must consider that part of the sun's limb which first makes its appearance above the horizon; the first glimpse of the upper limb of the sun is in question, say, when the visible limb is 2´ high; and we must carefully take the height of the hills over which it rises into account.

The accompanying diagram will at once show the difference between the rising conditions we have now to consider. It deals with the summer solstice, as being the most precise case, in Lat. 59° N.

At this time the position of the sun, that is of the sun's centre, as given in the "Nautical Almanac," is represented by the double circle on the sea horizon.



Fig 36 The Conditions of “Sunrise” at the Summer Solstice in Lat. 50° N.

The azimuth of this position is N. 39° 16´ E. This is the equivalent of the declination of a star, but it will be seen that the real azimuths we want are very different. The dotted circles represent the actual position of the sun with regard to the horizon, the continuous circles the apparent positions caused by the lifting-up effect of refraction. We have the positions in azimuth of the apparent sun as it appears on a sea horizon, and when the horizon is formed by hills up to 1½° in vertical height.

To make this quite clear I give a table which has been computed by Mr. Rolston, of the Solar Physics Observatory, showing azimuths with hills up to 1½° high for lat. 59° N., and 51° N. nearly the latitude of Stonehenge, of the sun's upper limb for the summer solstice:—



The first important thing we learn from the table is that although at both solstices the azimuths of the rising and setting of the sun's centre are the same, these azimuths of the upper limb at the summer and winter solstices differ in a high northern latitude by some 5°. The difference arises, of course, from the fact that the limb is some 16´ from the sun's centre, so that considering the sun's centre as a star with fixed declination, at rising the limb appears before the centre, and at setting it lags behind it.


It will also be seen that at sunrise hills increase the azimuth from N., and refraction reduces it; while at setting, hills reduce the azimuth from S. and refraction increases it.

This diagram and table should fully explain the variation of azimuth at sunrise caused by the fact that from our present point of view we do not deal with the sun as a star.

To make the foregoing applicable for monuments in all latitudes between Brittany and the Orkneys, I give still another diagram, Fig. 37, also prepared for me by Mr. Rolston which will enable any archæologist to determine approximately, for the present time, the azimuth of sunrise at the summer solstice, without waiting for the 21st of June in any year actually to observe it.

FIG. 37.—The Azimuths of the Sunrise (upper limb) at the Summer Solstice.
The values given in the table have been plotted, and the effect of the height of the hills on the azimuth is shown. The range of latitude given enables the diagram to be used in connection with the solstitial alignments at Carnak, Le Ménac, and other monuments in Brittany.


As before stated, I have dealt with the solstice in this chapter because it affords us the most precise case. I hope to be able to deal with the May year sun in the same way later on.

 

Next Chapter: Chapter XIII. Stenness