Most people have heard of Pi Day, which celebrates the mathematical constant π or 3.14159265… The digits go on for more than one trillion decimal places, but we’ll stop there! π is the ratio of the circumference of a circle to its diameter, and prompts celebration each March 14th. That celebration often involves delicious pie and pi recitation contests. But some mathematicians think a different circle constant is more deserving of fanfare and celebration.

Tau or τ, is equivalent to 2π, or 6.28318530… That makes today, June 28th, the perfect day to sing its praises. Michael Hartle, author of “The Tau Manifesto,” believes tau is more important than pi because circles are more naturally defined by their radius than their diameter. In addition to being found naturally in equations used in a wide variety of engineering and mathematics applications, tau is a more perfect constant to represent the perfect circle.

**How can my class celebrate? **

Involve your class in the tau vs. pi debate with a classroom discussion. Read “The Tau Manifesto” together and watch Michael Hartle discussing the importance of tau and the difficulty of getting people to break out of their comfort zones after decades of teaching pi as the preferred constant to measure a circle across the planet.

Follow that up with Vi Hart’s “Pi is Still Wrong” animated song.

Check out this “Pi vs. Tau Smackdown” from Numberphile.

Once everyone is well informed, discuss. Have your students articulate why they would choose pi or tau. Then explore the relationship between music and math with Michael Blake‘s “What Tau Sounds Like” composition.

If you’re worried about missing out on all the delicious pie during Pi Day, don’t forget, you can still celebrate Tau Day with some delicious baked treats. Tau is 2 x pi, so this holiday gets twice the pie!

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