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CHAPTER VII

ASTRONOMICAL OBSERVATIONS AT STONEHENGE IN 1901

( footnote 62:1 )

AN investigation was undertaken by Mr. Penrose and myself in the spring of 1901, as a sequel to analogous work in Egypt and Greece, with a view to determine whether the orientation theory could throw any light upon the date of the foundation of Stonehenge, concerning which authorities vary in their estimates by some thousands of years. Ours was not the first attempt to obtain the date of Stonehenge by means of astronomical considerations. In Mr. Godfrey Higgins’ work ( footnote 62:2 )he refers to a method of attack connected with precession. This furnished him with the date 4000 B.C.

More recently, Prof. W. M. Flinders Petrie, ( footnote 62:3 ) whose plan of the stones is a valuable contribution to the study of Stonehenge, was led by his measures of the orientation to a date very greatly in the opposite direction, but, owing to an error in his application of the change of obliquity, clearly a mistaken one.

The chief astronomical evidence in favour of the solar temple theory lies in the fact that the "avenue," as it is called, formed by two ancient earthen banks, extends for a considerable distance from the structure, in the general direction of the sunrise at the summer solstice, precisely in the same way as in Egypt a long avenue of sphinxes indicates the principal outlook of a temple.

These earthen banks defining the avenue do not exist alone. As will be seen from the sketch plan (Fig. 15), there is a general common line of direction for the avenue and the principal axis of the structure; and the general design of the building, together with the position and shape of the naos, indicates a close connection of the whole temple structure with the direction of the avenue. There may have been other pylon and screen equivalents as in other ancient temples; which have disappeared, the object being to confine the illumination to a small part of the naos. There can be little doubt, also, that the temple was originally roofed in, and that the sun's first ray, suddenly shining into the darkness, formed a fundamental part of the cultus.

With regard to the question of the roof, however, the above suggestion, I now find, is not new, the view having been held by no less an authority than Dr. Thurnham, who apparently was led to it by the representations of the Scandinavian temples as covered and enclosed structures.

Since the actual observation of sunrise was doubtless made within the sanctuary itself, we seem justified in taking the orientation of the axis to be the same as that of the avenue, and, since in the present state of the S. W. trilithon the direction of the avenue can probably be determined with greater accuracy than that of the temple axis itself, the estimate of date must be based upon the orientation of the avenue. Further evidence will be given, however, to show that the direction of the axis of the temple, so far as it can now be determined, is sufficiently accordant with the direction of the avenue.

The orientation of this avenue, may be examined upon the same principles that have been found successful in the case of Greek and Egyptian temples—that is, on the assumption that Stonehenge was a solar temple, and that the greatest function took place at sunrise on the longest day of the year. This not only had a religious motive; it had also the economic value of marking officially and distinctly that time of the year and the beginning of an annual period.

It is, indeed, possible that the present structure may have had other capabilities, such as being connected with the May year, the equinoxes or the winter solstice; but it is with its uses at the summer solstice alone that we now deal.

There is a difference in treatment between the observations required for Stonehenge and those which are available for Greek or Egyptian solar temples. In the case of the latter, the effect of the precession of the equinoxes upon the stars, which as warning clock stars were almost invariably connected with those temples, offers the best measure of the dates of foundation; but in Britain, owing to the brightness of the dawn at the summer solstice, such a .star could not have been employed, so that we can rely only on the secular change of the obliquity as affecting the azimuth of the point of sunrise. This requires the measurements to be taken with very great precision, and as the azimuth of the place of sunrise varies with the latitude, and as a datum point on the horizon in a known position was also required, Colonel Johnston, R.E., the Director-General of the Ordnance Survey, was asked for and obligingly supplied the following particulars:

Centre of stone circle, Stonehenge

Lat. N.    51° 10´ 42"
Long. W.     1° 49´ 29"    



Centre of spire, Salisbury Cathedral

Lat. N.   51° 3´52"
Long. W.  1° 47´ 45"  

The real point was to determine the direction of the so-called avenue. Measurements taken from the line of the bottom of the ditch assisted materially those taken from the crown of the bank itself. With this help and by using the southern bank and ditch whenever it admitted of recognition a fair estimate of the central line could be arrived at. To verify this, two pegs were placed at points 140 feet apart along the line near the commencement of the avenue, and four others at distances averaging 100 feet apart nearer the further recognisable extremity, and their directions were measured with the theodolite, independently by two observers, the reference point being Salisbury Spire, of which the exact bearing had been communicated by Colonel Johnston.

This bearing was also measured locally by observations of the Sun and of Polaris, the mean of which differed by less than 20? from the Ordnance value. The resulting observations gave for the axis of the avenue nearest the commencement an azimuth of 49° 38´ 48", and for that of the more distant part 49° 32´ 54". The mean of these two lines drawn from the central interval of the great trilithon, already referred to, passes between two of the sarsens of the exterior circle, which have an opening of about 4 feet, within a few inches of their middle point, the deviation being northwards. This may be considered to prove the close coincidence of the original axis of the temple with the direction of the avenue.

This value of the azimuth, the mean of which is 49° 35´ 51", is confirmed by the information, also supplied from the Ordnance Survey, that from the centre of the temple, the bearing to the N.E. of the principal bench mark on a hill, about 8 miles distant, the bench mark being very near a well-known ancient fortified British encampment named Silbury or Sidbury, is 49° 34´ 18"; and that the same line continued through Stonehenge, to the south-west, strikes another ancient fortification, namely, Grovely Castle, about 6 miles distant, and at practically the same azimuth, viz., 49° 35´ 51". For the above reasons 49° 34´ 18" has been adopted for the azimuth of the avenue.

The summer solstice sunrise in 1901 was also watched for by Mr. Howard Payn on five successive mornings, viz., June 21 to 25, and was successfully observed on the last occasion. As soon as the Sun's limb was sufficiently above the horizon for its bisection to be well measured, it was found to be 8´ 40" northwards of the peak of the Friar's Heel, which was used as the reference point; the altitude of the horizon being 35´ 48". The azimuth of this peak from the point of observation had been previously ascertained to be 50° 39´ 5", giving for that of the Sun when measured, 50° 30´ 25"; by calculation that of the Sun, with the limb 2´ above the horizon, should be 50° 30´ 54". This observation was therefore completely in accordance with the results which had been obtained otherwise.

The time which would elapse between geometrical sunrise, that is, with the upper limb tangential with the horizon, and that which is here supposed, would be about 17 seconds, and the difference of azimuth would be 3´ 15".

The remaining point was to find what value should be given to the Sun's declination when it appeared showing itself 2´ above the horizon, the azimuth being 49° 34'´ 18".

The data obtained for the determination of the required epoch were as follows:—

(1.) The elevation of the local horizon at the sunrise point seen by a man standing between the uprights of the great trilithon (a distance of about 8000 feet) is about 35´ 30", and 2´ additional for Sun's upper limb makes 37´ 30".

(2.) - Refraction + parallax, 27´ 20".

(3.) Sun's semi-diameter, allowance being made for greater eccentricity than at present, 15´ 45".

(4.) Sun's azimuth, 49° 34´ 18", and N. latitude, 51° 10´ 42".

From the above data the Sun's declination works out 23° 54´ 30" N., and by Stockwell's tables of the obliquity, which are based upon modern determinations of the elements of the solar system, ( footnote 67:1 ) the date is found to be 1680 B.C.

It is to be understood that on account of the slight uncertainty as to the original line of observation and the very slow rate of change in the obliquity of the ecliptic, the date thus derived may possibly be in error by 200 years more or less; this gives us a date of construction lying between say 1900 and 1500 B.C.

In this investigation the so-called Friar's Heel was used only as a convenient point for reference and verification in measurement, and no theory was formed as to its purpose. It is placed at some distance, as before mentioned, to the south of the axis of the avenue, so that at the date arrived at for the erection of the temple the Sun must have completely risen before it was vertically over the summit of the stone. It may be remarked, further, that more than 500 years must yet elapse before such a coincidence can take place at the beginning of sunrise.

In an Appendix certain details of the observations are given.

In the next chapter I propose to show that an independent archæological inquiry carried out, in a most complete and admirable way, just after Mr. Penrose and myself had obtained our conclusion, entirely corroborates the date at which we had arrived.


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Footnotes
62:1 This chapter and the end of the previous one are mainly based the paper communicated by Mr. Penrose and myself to the Royal Society (see Proceedings, Royal Society, vol. 69, p. 137 et seq.

62:2 The Celtic Druids. 4to. London. 1827.

62:3 Stonehenge, &c. 1880.

67:1 Smithsonian Contributions to Knowledge, vol. xviii. No. 232, table 9. Washington. 1873. For curve, see page 130.
 

Next Chapter: Chapter VIII. Archeological Observations at Stonehenge, 1901