Today is “National Half-Tau Day” or, as others call it, it’s “National Pi Day”. Not apple or Cherry Pie, but what you multiply the diameter of a circle by to get the length of the circumference: Circumference = Diameter times Pi.

I’ve had a strained relationship with Pi. In 1997 I read a book called A Tour of the Calculus. That book opened my mind and sent me down the path that included creating TripInsuranceStore.com.

Here’s something I created in 1997 and I keep it next to my desk to remind me of the endless opportunities that exist:

The length of the curve on that half circle is Pi. Unfortunately, Pi is only one-half the circle’s diameter. For nearly 15 years, I’ve thought, why is Pi stuck with half a circle? Circles are commonly defined by their radius. It seemed odd to me that many of the Pi formulas contain 2pi. In my opinion, 2Pi is an odd way of expressing the Diameter of a circle. Why isn’t there a mathematical constant that would be the ratio of the circumference to radius of the circle?

In 2001, Bob Palais wrote essay called “Pi is wrong!“.

Enter Michael Hartl, a physicist by training who’s now an educational entrepreneur, wrote The Tau Manifesto (http://tauday.com). Tau is the ratio of the circumference to radius of the circle, while pi is circumference to diameter.

Tau, technically, is just pi multiplied by 2, so about 6.28. But Michael Hartl, considers this number a more elegant and appropriate circle constant than pi and thinks pi should be replaced by tau across the field of mathematics (with the proper factors of 2, of course).

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