We all know about Pi Day, that tasty holiday on March 14 where celebrations include reciting digits of Pi and eating pie.

we’re seeing headlines about “pi being under attack” and the movement to replace pi with another constant... tau?

Why tau instead of pi? We all know that pi (p) is the circumference (*C*) of a circle divided by its diameter (*D*):

Pi (p) is used constantly (no pun intended) in math, and shows up everywhere in nature. It’s irrational, meaning you can’t get it from a fraction of whole numbers, and it’s also transcendental, meaning that it’s not the root of a non-constant polynomial equation. Pi, however, isn’t the most fundamental constant that could be used in geometric and mathematical expressions: it’s actually “a confusing and unnatural choice” to be used as a circle constant, according to Dr. Michael Hartl of TauDay.com and author of the *Tau Manifesto*.

A more fundamental ratio for the circle constant is that of a circle’s circumference (*C*) to its radius (*r*):

This number is numerically equal to 2p (since the diameter is equal to 2*r*), and is called tau, or t, and like p, is also irrational and transcendental. Tau makes much more sense in mathematical formulas, especially in trigonometry: its value relates the full turn of a circle, not just half as with p, to a constant. Tau describes 360°, not just 180°. In this context, t is the more natural choice to use not just when talking about angles on a circle, but in general mathematics and beyond.

Therefore,

The date 6/28(June 28th), which is Tau Day. The number Tau is 2pi, or 6.28 (followed by many more decimals).

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