The Tau Manifesto
Happy Tau Day (6.28)!  
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!", pi is wrong. It’s time to set things right.
By Michael Hartl.