Anti-π activists celebrated Tau Day. ‘People find themselves almost violently angry at π,’ explained theoretical physicist Michael Hartl. ‘They feel like they’ve been lied to their whole lives.’

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The Tau Manifesto is dedicated to one of the most important numbers in mathematics, perhaps the most important: the circle constant relating the circumference of a circle to its linear dimension. For millennia, the circle has been considered the most perfect of shapes, and the circle constant captures the geometry of the circle in a single number. Of course, the traditional choice for the circle constant is π—but, as mathematician Bob Palais notes in his delightful article “π Is Wrong!”1, π is wrong. It’s time to set things right.