No, really, pi is wrong…
The Tau Manifesto
by Michael Hartl
Tau Day is an annual celebration of the circle constant $\tau = 6.283185\ldots$, which takes place every June 28 (6/28 in the American calendar system). To learn more about what $\tau$ (tau) is and why it’s important, read The Tau Manifesto, or watch one of the tau videos below.
To get all the latest updates on tau, read the State of the Tau update, and join the official tau email list.

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Tau Videos
About the author
Tau Day founder Michael Hartl is an educator, author, and entrepreneur. He is cofounder and principal author at Learn Enough and the Ruby on Rails Tutorial. Previously, he taught theoretical and computational physics at the California Institute of Technology (Caltech), where he received the Lifetime Achievement Award for Excellence in Teaching and served as Caltech’s editor for The Feynman Lectures on Physics. He is a graduate of Harvard College, has a Ph.D. in Physics from Caltech, and is an alumnus of the Y Combinator entrepreneur program.
Michael is ashamed to admit that he knows $\pi$ to 50 decimal places—approximately 48 more than Matt Groening. To atone for this, he has memorized 52 decimal places of $\tau$.