This results from the fact that there is a slow change in what is called the obliquity of the ecliptic—that is, the angle between the plane of the earth's equator and the plane of the ecliptic; this change is brought about by the attraction of the other planetary bodies affecting the plane of the ecliptic. If these planes approach each other, the obliquity will be reduced; the present obliquity is something like 23° 27′; we know that 5,000 B.C. it was 24° 22′, nearly a degree more. A difference of 1° means, then, a difference of time of about seven thousand years. It may go down to something below 21°. Since the obliquity has been decreasing for many thousand years, a temple directed to the rising or setting sun at the solstice some thousands of years ago had a greater amplitude than it requires now.
It will be readily understood that if the orientation of the temple and the height of the hills towards which it points be accurately known, knowing also the precise obliquity of the ecliptic at different epochs, we have an astronomical means of determining the date of the original foundation of the temple, supposing, of course, that it was founded to observe the solstice.
But before I go into these matters it is essential that the evidence of Egyptologists should be considered. Very fortunately for us in these inquiries the temple of Amen-Rā is one of those most carefully studied by Mariette, so that the dernier mot of the archæologist is at our disposal.
Mariette, in his magnificent memoir on Karnak,[34] surpassed himself in the care and sagacity which he displayed in endeavouring to fix dates for the various structures in that wonderful temple-field, and among them the various parts of the temple of Amen-Rā.
In his maps, to which I now refer, each part of the temple is coloured according to the supposed date of its building. He points out first of all that the inscriptions on the walls must be disregarded, as they could have been put there at any date after the temples were built. On this point I quote Mariette's own words:—[35]
"Les couleurs marquées sur le plan servent à indiquer, au moyen de la légende explicative placée en marge, les époques diverses de la construction des temples et de leurs parties. Quelques mots d'explication sont ici nécessaires. Un mur porte les cartouches de Menephtah; mais il peut avoir été construit deux-cent-cinquante ans plus tôt par Thoutmès III. Les époques de la décoration ne sont ainsi pas toujours les époques de la construction. Pour avoir les époques de la décoration, il ne s'agit que de regarder les murs et les inscriptions dont ils sont couverts. Pour avoir les époques de la construction, tout un travail de confrontation, de comparaison, est nécessaire. Il faut s'assurer si les mêmes mains qui ont construit le mur l'ont décoré; dans le cas contraire, il faut faire intervenir l'archéologie dans toutes les branches de cette science qui touchent à l'observation des lieux, au mode de construction, à l'agencement des pierres, au choix et à l'appareillage des matériaux."
Taking the temple in its generality, he finds that, so far as his inquiries had carried him, parts were certainly built at a time an ancient as the twelfth dynasty—say 2400 or 3000 B.C., according to the authority in these matters that we may prefer.
Then again we have dates given and indications of kings through the eighteenth and nineteenth dynasties, and then again on to the times of the Ptolemies.
In such an inquiry we must have archæological dates on which we can rely. In the date assigned to the time of Mena by various Egyptologists we find a difference of nearly—in fact, rather more than—a thousand years in our authorities. In the twelfth dynasty we find a difference of five hundred years; but in the later dynasties, such as the eighteenth, the difference is reduced in some cases to ten years or so. So that in the later dynasties we know pretty well what time is in question. We are therefore on firm ground.
The first point to which I wish to call attention is that according to Mariette the building dates change along the open axis of the temple. From photographs I took when in Egypt I found reason to believe that the direction of the axis has been slightly changed at the west end.