The Librarian who Measured the Earth

When did people first realize that the earth might be round rather than flat? Apparently Aristotle in the fourth century BC was convinced of the sphericity of the earth, because he observed the earth's shadow on the moon during a lunar eclipse. Less scholarly people like sailors also thought the world may be round because they observed ships as they approach the horizon. Their sails appear to dip into the ocean. But how round was the earth? No one had any idea. At least not until a curious and ingenious librarian named Eratosthenes (275-194 BC) discovered a remarkably simple method for measuring the circumference of the earth. This is the story of how he did that.
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