As the world celebrates Pi Day on Friday, a movement is growing that claims pi — or 3.14, the ratio of a circle's diameter to its circumference — is wrong.

MORE: World celebrates 3.14

The Tau Day movement supporters — also known as Tauists — say the circle's constant should use the radius, not the diameter.

Tau (τ) comes out to 2π, or 6.28. Tau Day is celebrated June 28, and Tauists refer to Pi Day as Half Tau Day.

Circle radians tau

The Tauist argument is laid out in the Tau Manifesto, written in 2010 by theorist Michael Hartl: The definition of a circle is the set of points a particular distance from the center. That distance is the radius.

YOUR TAKE: What are you eating?

"When you divide by the diameter, you're really dividing by twice the radius, so that factor of 2 shows up everywhere," Hartl said in an interview with USA TODAY Network.

Hartl isn't the first person to dispute pi. In 2001, Bob Palais, a math professor at the Utah Valley University, wrote an article titled, "π is wrong!" in The Mathematical Intelligencer.

Palais points out that Leonhard Euler, the mathematician who popularized pi, actually used the 6.28 value first.

Euler went back and forth between the 3.14 and 6.28 values, "so he was sensible and used it for what seemed appropriate for a particular setting," Palais said in an e-mail to USA TODAY Network.

To better understand the tau vs. pie debate, Vi Hart, known for producing YouTube videos about math, created this segment that explains tau, using pies.

Pi is still the dominant constant, but tau is gaining popularity.

Last year, the University of Oxford hosted a seminar called "Tau vs. Pi: Fixing a 250-Year-Old Mistake."

In 2012, the Massachusetts Institute of Technology started a tradition of releasing its admissions decisions on Pi Day at Tau Time (March 14 at 6:28 p.m.)

There's even a tau watch, created by Cadence Watch.

Still not convinced? Hartl writes in the manifesto, "If you think that the circular baked goods on Pi Day are tasty, just wait — Tau Day has twice as much pi(e)!"

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