He had before suggested, as is supposed, to Ptolemy Euergetes, to make him the two great copper armillae, or circles for determining the equinox, which stood for centuries in "that which is called the Square Porch"--probably somewhere in the Museum. By these he had calculated the obliquity of the ecliptic, closely enough to serve for a thousand years after. That was one work done. But what had the Syene shadows to do with that? Syene must be under that ecliptic. On the edge of it. In short, just under the tropic. Now he had ascertained exactly the latitude of one place on the earth's surface.英文原著 · 《Alexandria and her Schools》