Tau Day is a celebration of the circle constant $\tau = C/r =$ $ 6.283185\ldots$ Founded in 2010 with the publication ofThe Tau Manifesto byMichael Hartl, Tau Day takes place annually on June 28 (6/28 in the American calendar system).

The Tau Manifesto’s introduction of $\tau$ (tau) as the “true circle constant” has had a significant impact on geek culture, including changing thetime of day MIT announces its admissions decisions, being featured in the officialGoogle calculator, and being supported by a large number of computer programming languages (including Microsoft .NET, Java, Rust, and Python).

Including the English-language original,The Tau Manifesto is currently available in eight different languages. Its associated holiday,Tau Day, has beenfeatured inThe Wall Street Journal and is celebrated annually on6/28 by thousands of math enthusiasts around the world.

To get all the latest updates on tau, read theState of the Tau update, andjoin the official Tau Email List.

About the author

Michael Hartl is a physicist and entrepreneur. He is the author of over a dozen books, includingLearn Enough Python to Be Dangerous and theRuby on Rails Tutorial, and was cofounder and principal author atLearn Enough (acquired 2022). Previously, Michael taught theoretical and computational physics at theCalifornia Institute of Technology (Caltech), where he received aLifetime Achievement Award for Excellence in Teaching and served as Caltech’s editor forThe Feynman Lectures on Physics. He is a graduate ofHarvard College, has a Ph.D. in Physics fromCaltech, and is an alumnus of theY 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 memorized52 decimal places of \( \tau \).