Book 17
1.48
At Syene there is a well which indicates the summer solstice, because these places lie under the tropical circle,This was the general opinion of antiquity, and was reproduced by Eratosthenes, Hipparchus, Ptolemy, and others; in short, by all the Alexandrine school. At the time of Eratosthenes, the obliquity of the ecliptic was 23° 45′ 17″. Syene was therefore 20′ 6″ from being exactly under the tropic; for 24° 5′ 23″ (latitude of Syene)—23° 45′ 17″ = 20′ 6″. This would be the distance of the centre of the sun from the zenith of Syene; whence it follows that the northern limb of the sun was about 5′ from it. In the time of Strabo, the obliquity was only 23° 42′ 22″; the difference between the zenith of Syene and the northern limb of the sun was about 8′. Lastly, about 140 of the vulgar era, the obliquity was reduced to 23° 41′ 7″. Syene was then 24′ 16″ from the tropic, and its zenith was about 10′ from the northern limb of the sun; when the shadows of gnomons of any tolerable size must have been perceptible, and Syene could not have been any longer considered as lying under the tropic. As regards the well which served to ascertain the instant of the solstice, Pliny and Arrian both mention it. The formation of it no doubt belonged to a very remote period. In the time of Strabo, the rays of the sun could not have reached entirely to the bottom, but the shadow was so small that it was not sufficient to shake the ancient opinion. In fact, the angle being about 8′, and supposing the depth to have been 50 feet, the northern side would have projected a shadow of about 18 lines; the rest would have remained in fill light, and the reflexion would have caused the whole circumference of the well to appear illuminated. Letronne. [and occasions the gnomons to cast no shadows at midday].Kramer considers the passage between brackets to be an interpolation, as the same sense is conveyed in the passage which immediately follows. For on proceeding from the places in our country, in Greece I mean, towards the south, the sun is there first over our head, and occasions the gnomons to be without shadows at noon. When the sun is vertical to us, it must necessarily cast its rays down wells, however deep they may be, to the water. For we ourselves stand in a perpendicular position, and wells are dug perpendicular to the surface.
