Eratosthenes and the Circumference of the Earth

About two and a half thousand years ago, Eratosthenes (try saying that with half a mince pie in your mouth) managed to estimate the circumference of the Earth with only a 2% margin of error.  That’s pretty good and although he didn’t do it sat at home with a pipe (or whatever they might have smoked then) it didn’t take an epic voyage to do it. In fact it was done with maths.  Rogue Robot tells the tale.