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’s introduction of $\tau$ (tau) as the “true circle constant” has had a significant impact on geek culture, including changing the time of day MIT announces its admissions decisions, being featured in the official Google 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 been featured in The Wall Street Journal and is celebrated annually on 6/28 by thousands of math enthusiasts around the world.
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Michael Hartl is a physicist and entrepreneur. He is the author of over a dozen books, including Learn Enough Python to Be Dangerous and the Ruby on Rails Tutorial, and was cofounder and principal author at Learn Enough (acquired 2022). Previously, Michael taught theoretical and computational physics at the California Institute of Technology (Caltech), where he received a 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.