A Greek Mystical Sect Deduces the Shape of the Earth

The Pythagoreans (they looked upon Pythagoras as their founder) inhabited the Greek colonies of southern Italy. Unlike the great Babylonian astronomers who collected huge datasets and looked for models to predict certain celestial events, the Pythagoreans searched for models that described the true nature of the "cosmos".

The Greeks debated both the accuracy of predictions and the reality of the models. They included philosophical (see Aristotle's writings) as well as mathematical (see Plato's writings) arguments. 

The logic the Pythagorians used to arrive at their conclusion that the Earth was a sphere did not survive to the present day. But Aristotle may have captured their reasoning when he wrote that the shadow of the Earth on the Moon (always round) decided the issue. He also argued that since the stars one sees in the sky change as one travels north and south – meaning that the Earth was a sphere of no great size - confirmed the model and eliminated other possible solutions, like a cylndrical shape proposed by Anaximander two hundered years earlier.

Today, the histories we tell our children often imply that common knowledge of the shape of the Earth changed only with the partial and total circumnavigation of the globe by the likes of Columbus and Magellan. But it’s closer to the truth to say that all educated people after the Greek astronomers (including those in post-Roman Europe) held that the Earth was round.