Ptolemy's Almagest

c. 150 CE — Egypt, Roman Egypt

Today: Alexandria, Egypt

Working in Alexandria, Ptolemy compiled a mathematical model of the heavens that put Earth at the centre and explained the planets' looping motions with circles turning on circles. It was wrong, and it worked — accurate enough to predict positions for centuries, which is why it went unchallenged for 1,400 years, translated into Arabic as the Almagest and taught from Baghdad to Oxford. His geography, with its coordinates and its badly undersized Earth, was still being consulted by navigators in the 1490s.

Worth knowing: Ptolemy's map underestimated the planet's size, and Columbus used numbers descended from it to argue Asia lay a short sail west. He was wrong, sailed anyway, and hit a continent Ptolemy never knew existed.

Pattern: Information-medium revolution — A new way to record or transmit information lowers the cost of copying ideas and reorders who holds knowledge and power.

Entry 76 of 240 in Precedent, a walk through the whole human story in order.