Question: How did you descover that the world wasn't flat?

James Verdon answered on 1 Jul 2012:

Hi bethyboo,

We have lots of evidence that the world isn’t flat. Obviously, some of the most important evidence was collected when medieval sailors (like Magellan) sailed round the earth without falling off the edge. However, it was the ancient Greeks who first worked out that the earth was a sphere. They looked at how you could see different stars depending on where you were on the earth, and how the sun’s shadow had a different length and orientation depending on where you were. For instance, at the equator, at midday a vertical stick will cast no shadow, because the sun will be exactly overhead, while here the sun will be to the south of us, so a vertical stick will have a shadow. They realised that the only way to match these observations would be if the earth was a sphere, so the earth cannot be flat, it must be a sphere.

A man called Eratosthenes in about 240BC measured the lengths of the shadow of a vertical stick in Egypt and in Greece. He was then able to use these measurements to work out the circumference of the earth. His calculation, using only a stick and a ruler, and measuring shadow lengths, was correct to within 10% of the exact measurement that we now know from satellite measurements!

Obviously, I first learned that the earth was round when I read it in a book, or because my teachers told me. But the beauty of science is that, if you didn’t believe this, you can repeat the measurements yourself. You could go and measure shadow lengths at difference places around the earth and work out the earth’s circumference for yourself.

With all science, you might be learning things from books, but in theory at least you could go an redo the measurements and find out for yourself, which is how we can have faith in the results.