No, really, pi is wrong…
The Tau Manifesto
by Michael Hartl
Tau Day is a celebration of the circle constant $\tau = C/r =$ $ 6.283185\ldots$ Founded in 2010 with the publication of The Tau Manifesto by Michael Hartl, Tau Day takes place annually on June 28 (6/28 in the American calendar system).
The Tau Manifesto proposed using the Greek letter $\tau$ (tau) to represent the true circle constant and ignited the Tauist movement that continues to gain strength with each passing year. To learn more, 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.

Buy Print Edition
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Get your own Tau Tee
Celebrate Tau Day in style with a Tau Tshirt from Teespring!
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 $\tau$.