As June 28, 2015, approaches, the Internet is once again anticipating controversy as the mathematical constant pi comes under threat from a group of detractors who will be marking “Tau Day.” Tau Day’s revelers are campaigning for a constant twice as large as pi (about 6.28) to take its place, hence the June 28 celebration.

Tau proponents say that, for many mathematical problems, tau makes more sense and makes calculations easier.

As we learned in school, pi is the ratio of circumference to a diameter of a circle. Tau, on the other hand, is the circumference divided by radius. Tau-ists argue that pi is the wrong figure to be associated with the properties of a circle, which is more naturally defined by radius and not diameter. Therefore, pi, an essential in equations in math, science and engineering, should be replaced by tau. Not all fans of mathematics agree, however, and pi’s rich history means it will be a difficult number to unseat.

“I like to describe myself as the world’s leading anti-pi propagandist,” Michael Hartl, an educator and former theoretical physicist, told *BBC News*. “When I say pi is wrong, it doesn’t have any flaws in its definition — it is what you think it is, a ratio of circumference to diameter. But circles are not about diameters, they’re about radii; circles are the set of all the points a given distance — a radius — from the center,” Hartl explained.

Hartl credits Bob Palais of the University of Utah with first pointing out in 2001 that “pi is wrong” in an article that appeared in the *Mathematical Intelligencer*. However, it is Hartl who is responsible for the *Tau Manifesto* — calling tau the more convenient formulation and also instituting Tau Day to celebrate it.

Are you a Tau-ist? If so, there are numerous ways you can join with like-minded revelers from around the world in Tau Day celebrations:

- International Tau Day Meetup: This free virtual event will feature a short Tau Talk by Tau Day founder Michael Hartl, a Q&A session, and a chance to (virtually) meet other participants with a love of all things mathematical and true. Accounts of any real-life Tau Day parties will be most welcome, especially those that involve twice as much pi(e). The event will be held on June 28, 2015, 11:00 a.m. PDT/2 p.m. EDT/6:00 p.m. GMT.
- What Tau Sounds Like: Listen as a musician interprets the mathematical constant Tau to 126 decimal places.
- Join in on social media (hashtag: #tauday).
- Wear a Tau Day T-shirt.
- If you would like to try your hand at memorizing the digits of tau, here are the first 100,000 to get you started.
- Eat
*twice*as many pies as you normally would on Pi Day!