Nearly 2000 years before Christopher Columbus set off on his journey to reach land by sailing west across the Atlantic, the ancient Greeks had already used their sophisticated grasp of geometry to correctly calculate the size and shape of the Earth. From the shadow cast by the Earth on the moon during a lunar eclipse, the Greeks figured out that our planet was a sphere. And by measuring the angles made by solar rays on summer’s first day in two parts of Egypt, Erastothenes calculated the size of the Earth.