(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,[14] 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.
[11] This chapter and the end of the previous one are mainly based on the paper communicated by Mr. Penrose and myself to the Royal Society (see Proceedings, Royal Society, vol. 69, p. 137 et seq.).
[12] The Celtic Druids. 4to. London. 1827.
[13] Stonehenge, &c. 1880.