Pi is perhaps the most beloved number in the world. People devote an entire day to celebrating it and enthusiasts memorize it to hundreds of thousands of places. But Michael Hartl says it's time to lose pi in favor of a new symbol: tau. Hartl is the author of The Tau Manifesto, which argues that, quite simply, pi is wrong. He's also a physicist who has previously both studied and taught at Harvard and Caltech. So what is tau? Well, simply enough, it's 2*pi, or 2π . Whereas pi is defined as the circumference of a circle divided by its diameter (C/D=π), tau is defined as the circumference divided by the radius (C/r=τ).