A few years ago, a brilliant young woman named Vi Hart, describing herself as a “recreational mathemusician,” began posting a series of absolutely jaw-dropping videos on YouTube titled “Doodling in Math Class.” In them, using notebook and marker art, laser-sharp wit, humor, and a break-neck narrative style, Vi (rhymes with Pi) tackled interesting and entertaining aspect of mathematics. If you haven’t had the pleasure of seeing Vi’s videos, check out her channel. Sadly, Vi is not making very many videos on her YT channel since she went on to teach at Khan Academy, and most recently, eleVR where she’s doing VR development.

While Vi was doing her channel, one of her most popular and controversial videos was one in which she declared that Pi, the mathematical constant so beloved that it has its own day of the year, was wrong. Shocking, I know. But watch the video. In it, while baking pies, Vi outlines her argument for why Pi is wrong. And no, she is not suggesting that the concept, the constant itself, is incorrect, but that it is rather inelegant in practice and confusing to learn as a child. She joins mathematician Bob Palais in proposing a new constant, Tau, as a better way of calculating the ratio of a circle’s circumference to its diameter.

Instead of a circle being 2 Pi, 1 Tau is set as the radius of one turn, or as the ratio of a circle’s circumference to its radius rather than its diameter. She and other advocates of Tau see it as more natural than Pi, easier to teach, and it simplifies many formulas.

There is even a website for Tau Day that has been created to help promote the alternative constant and there is even a Tau Manifesto. You can read the manifesto and watch some more in-depth videos explaining the concept of Tau and how using it over Pi would impact various areas of science and engineering on the site. Tau Day is celebrated on June 28 (because the circle constant for Tau is 6.283185…). Mark your calendars!

While most of the math and science community seem either mildly entertained or annoyed by the whole “Tau movement,” there is a lot of merit to the idea of teaching/using Tau. And also to the idea of forgetting the whole silly business and sticking with the established Pi. On this episode of Numberphile, hosts Steve Mould and Matt Parker have a friendly debate over the pros and cons of Tau vs. Pi.

Vi and the Tauists have made some converts. In this video with University of Nottingham physics professor Phil Moriarty, he explains how he initially brushed off the concept but has since become a believer. He then does a really wonderful job of explaining how Tau works and why he is now a champion of the concept.

Related to Pi, not Tau, I couldn’t resist sharing this bonus Vi Hart video in which she rhapsodizes over the “proof of Pi = 4” and introduces the most awesome word ever in the process, “zigfinity.” I’ll let Vi explain in the way that only Vi can. Happy Pi Day (however you choose to slice it)!

Update:
Vi posted a new Pi Day video this morning: