Maps and charts (II): In 240 BC, it was clear in Egypt that the Earth was round and that it was 25,000 miles around

Best Defense is in summer re-runs. This item originally ran on November 25, 2013.

Another thing I learned in Lloyd Brown’s The Story of Maps was that in 240 BC, Eratosthenes calculated not only that the earth was round, but also that it was roughly 24,000 miles in circumference.

He did this with a brilliant insight followed by a simple bit of arithmetic: He began with the difference between the angular height of the sun from Alexandria (where he was the librarian) and from Syene (AKA Aswan), a city about 500 miles south of it. On the day of the solstice, the difference was just under 7.5 degrees, or one-fiftieth of a circle, which of course is 360 degrees. So, doing the math, he figured that 7.5 was to 360 as 500 was to X, which made X, the circumference of the world, roughly 25,000 miles. Pretty cool — and very close to the actual measurement of 24,901 miles at the equator.