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. He had his known point
from whence to start on a world-journey, and he would use it; he would
calculate the circumference of the earth--and he did it. By
observations made at Alexandria, he ascertained its latitude compared
with that of Syene; and so ascertained what proportion to the whole
circumference was borne by the 5000 stadia between Alexandria and Syene.
He fell into an error, by supposing Alexandria and Syene to be under the
same meridians of longitude: but that did not prevent his arriving at a
fair rough result of 252,000 stadia--31,500 Roman miles; considerably
too much; but still, before him, I suppose, none knew whether it was
10,000, or 10,000,000. The right method having once been found, nothing
remained but to employ it more accurately.
One other great merit of Eratosthenes is, that he first raised Geography
to the rank of a science.
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