# How do we know $\pi$ is a constant? [duplicate]

How did the ancient Greeks discover that the ratio of a circle's circumference to its diameter is constant? It does not seem so intuitive. Thanks!

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## marked as duplicate by anorton, Dominic Michaelis, Henning Makholm, rschwieb, PaulApr 16 '13 at 20:50

It seems perfectly obvious to me. Like so many important discoveries $-$ Darwin's theory of evolution, for instance $-$ it only needed to be stated. –  TonyK Apr 16 '13 at 20:31