# Happy Tau Day!

Have you ever wondered why it takes two $\pi$ to go only one turn around a pie?  Or why everywhere in math you see:

$e^{2\pi i} = 1$

Euler’s Equation

or

Cauchy's integral formula

or

The Gaussian Distribution (aka the Bell Curve)

or

Stirling's Formula for approximating the factorial function

Looking at the $\latex \pi$’s in math, you’ll start to notice that more often than not, there are two $\pi$‘s, not just one.

People being what they are, they have gotten all ideological about it.   They’ve defined

$\tau = 2\pi.$

Here $\tau$ stands for “t”, as in “turn”, as in one $\tau$ is one turn of the circle.  There is a movement afoot to use $\tau$ instead of $\pi$ in math.   To the believers, today is $\tau$ Day (which makes March 14th $(1/2) \tau$ Day).  They have a manifesto, a website, a guru, media coverage, a T-shirt:

Every revolution needs a good T-shirt

an anthem :

and a local devotee.