No, really, pi is wrong: The Tau Manifesto by Michael Hartl
Welcome to the Tau Manifesto. This 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 of circle constant is pi—but, as mathematician Bob Palais notes in his delightful article “pi Is Wrong!”,1 pi is wrong. It’s time to set things right.