III. On the orientation of certain Greek temples and the dates of their foundation derived from astronomical considerations, being a supplement to a paper on the same subject published in the Transactions of the Royal Society in 1893

The paper now presented to the Royal Society is a sequel to one on the same subject read here on April 27, 1893, and published in the Transactions for that year. In that paper the subject was explained at some length; it will, therefore, be unnecessary in this to repeat more than a very few explanatory observations. The aim of this inquiry is to deduce the date of the foundation of a Greek (or Egyptian) temple from its orientation, but I confine myself entirely to Greek temples, in which, however, the same practice was followed which had previously been reduced to a system in Egypt (vide ‘ Dawn of Astronomy,’ by Sir J. N. Lockyer). Almost all the temples in Greece and its Colonies had an Easterly frontage, and the principal religious function in each temple took place on the morning of the day when the sun, as it rose above the visible horizon, shone through the open Eastern door directly upon the sanctuary, where there was usually a statue of the deity in the centre. As some time was requisite for the priests to prepare for the ceremony, the orientation of the temple was so directed as to combine with the sunrise the previous heliacal rising or setting of some conspicuous star which could also be observed from the sanctuary. In the absence of clocks the heliacal rising or setting of stars was very greatly observed by the ancients—the meaning of the term being that the star, when very slightly above the horizon, should just be visible in the twilight, before being extinguished by the dawn. The angle of the orientation depended primarily on the time of year chosen for the principal festival, but it would be liable to a slight modification for the sake of combining an heliacal star with the sunrise, and it is the latter consideration which offers the means of determining the date of foundation, because the stars, owing to the precession of the equinoxes, are affected by a slow, but steady movement, which alters the amplitude, as it is called, of their rising or setting—viz., the angular distance from the true East or West as the case may be, and which is reckoned positive if towards the North, and negative if towards the South.