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″.