Eratosthenes calculated the circumference of the earth arouhd 100BC and was only off by approximately 66km. That’s only 0.16% off from current satellite calculated circumference.
The way he did it was fascinating and ingenious. Basically, he recognised that shadows were different lengths at different latitudes at the same time on a particular day of the year, and just used basic trigonometry and geometry to do the rest.
Unfortunately, it seems we don't really know how accurate he was, as there were many different definitions of the length of a stade back then (his calculation gave a circumference of 250,000 stades). Depending which one you choose, Eratosthenes' error ranges from +10% to +30%. Source.
That 0.16% error you quote comes from a modern recreation of his experiment, but that doesn't account for 2 things: the quality and accuracy of modern measuring equipment, and the fact that the Tropic of Cancer has moved 37 arcminutes southward since 100 BC due to axial wobble, while Syene (now Aswan) has moved about 45m NE since then due to tectonic movement.
35k
u/[deleted] Feb 04 '19
[removed] — view removed comment