Page:Proceedings of the Royal Society of London Vol 69.djvu/151

Rh 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 occupy about 17 seconds, and the difference of azimuth would be 3' 15".

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

The data thus obtained for the derivation of the required epoch are these :

(1.) The elevation of the local horizon at the sunrise point seen by a man standing between the iiprights 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 eccen- tricity 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 C 54' 30" N., and by Stockwell's tables of the obliquity, which are based upon modern determinations of the elements of the solar system,* the date becomes 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.

In this investigation the so-called Friar's Heel has been used only as a convenient point for reference and verification in measurement, and no theory has been 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.

We give in an appendix certain details of the observations.

We have to express our thanks to Sir Edmund Antrobus, Bart., for much kind assistance during our survey ; and to A. Fowler and Howard Payn, Esqs., for skilful and zealous co-operation in the mea- surements and calculations. As already stated, Colonel Duncan A. Johnston, R.E., Director-General of the Ordnance Survey, has also

ington, 1873.
 * ' Smithsonian Contributions to Knowledge,' vol. 18, No. 232, table 9. Wash-