- Pi is the ratio of circumference to diameter of a circle
- Tau, twice pi, is the ratio of circumference to radius of a circle
- Pi Day is celebrated widely on March 14; Tau Day is June 28
- See how iReporters celebrate Pi Day, and share your story

**(CNN)** -- Ask Michael John Blake how old he is, and says "I am 35, I think, maybe 36" and then tries to do a subtraction involving his birth year.

"I'm not really good with numbers," he says, finally.

There's one particular number he does remember, though -- the one that has made him an internet sensation. Pi, the ratio of circumference to diameter of a circle or, approximately, 3.14, is the basis for a song he composed.

It's gone viral on YouTube and other websites, garnering the praise of many people fascinated by the number pi.

Over the last several years there's been an explosion of online geekery about numbers, but especially pi around Pi Day, March 14 (3/14 ... get it?).

Blake had no idea he would achieve such online fame by tapping into this curiosity.

"I've been a musician for 20 years. Nothing I've ever done has ever approached this amount of exposure," he said.

On Pi Day, schools, science museums, educational websites, and groups of general enthusiasts from around the world will commemorate the number.

See how iReporters celebrate Pi Day, and share your story

There will be pi(e)-eating, pi-digit memorizing, pi songwriting and poem-writing, and exchanging of pi greeting cards. You can find dozens of online resources like Pi Across America and the San Francisco Exploratorium that have other suggested educational activities. Since Pi Day is also Albert Einstein's birthday, the town of Princeton, New Jersey, where he lived for more than 20 years, is having a Geek Freak Weekend culminating on March 14.

Source: "The Joy of Pi," Guinness World Records

But amid all this celebration, there lurks a force attempting to disrupt the revelry and push self-proclaimed pi nerds in a new direction.

It's called tau.

Tau, technically, is just pi multiplied by 2, so about 6.28. But Michael Hartl, a physicist by training who's now an educational entrepreneur, considers this number a more elegant and appropriate circle constant than pi and thinks pi should be replaced by tau across the field of mathematics (with the proper factors of 2, of course).

He's not denying the historical importance of pi, but he thinks it's time to set the record straight.

"I want to hack geek culture. I want to add this new number to the world of computer and math geeks," he said.

So what's all this about? The idea of using twice-pi as the circle constant arose in a 2001 essay called "Pi is wrong!" by Bob Palais.

Hartl has expanded on those ideas and chose "tau" to represent this number. Palais has since written on his website that he's "pleased to lend my support" for tau. In fact, says Palais, twice-pi may have been treated as a single symbol as early as 1889 by French mathematician Paul Matthieu Hermann Laurent.

(Warning: Hefty math ahead!)

According to Hartl, circles are most naturally defined by their radius. Tau would be the ratio of the circumference to radius of the circle, while pi is circumference to diameter.

In mathematics, diameter is rarely used, meaning the number 2 is often used in formulas involving pi. When you think of dividing a circle, you probably know that a quarter-turn is 90 degrees and a half-turn is 180 degrees. But trigonometry uses a unit called "radians," where a quarter-turn of a circle is pi/2 and a half-turn of a circle is pi, and so on -- which is confusing for new learners. Tau makes this more intuitive, says Hartl: A quarter turn of a circle is tau/4, and a half turn is tau/2.

Still, Hartl really wants you to read his entire manifesto before passing judgment.

"If you want pi to be an ambassador for mathematics, you have to come to terms with pi being, fundamentally, a stupid choice for the circle constant," he said.

For Hartl, March 14 is "Half Tau Day," which has its own website complete with tau merchandise. And there must be a proper Tau Day, too, he says. June 28 (6/28) marked the launch of Hartl's "Tau Manifesto" detailing all the reasons that tau makes more sense as a mathematical constant than pi. He's planning to have a big party on Tau Day this year.

Michael Hartl, author of "The Tau Manifesto," is calling Pi Day "Half Tau Day."

"If you think the circular baked goods on Pi Day are tasty, just wait: Tau Day has twice as much pi," he said.

Hartl gets e-mails almost every day from people who are excited about his ideas and say they're "converting to tauism." But, of course, there are a lot of pi loyalists out there, too.

"Some people react with mild hostility, because people care about pi. It's skewering a sacred cow. But, on balance, there's just been a tremendous amount of support," he said.

Leading pi experts say twice-pi (tau) is important in mathematics, but pi itself isn't going away anytime soon.

After all, pi as a symbol has been in use for 3.14(etc) since the 1700s, and has been memorized to more than 67,000 decimal places (Here's an iReporter doing the first 100). Tau and pi both have infinitely many digits that go on indefinitely in a seemingly random fashion, but it's pi that even non-math-geeks like Blake remember from school.

"Sure, 2pi shows up a lot. But I dare you to memorize 100 digits of tau... or, if you do, I dare you to find anyone who cares," jokes David Blatner, author of "The Joy of Pi," in an e-mail.

Tau is actually another example of how anything involving pi "just captures the imagination and makes people sit up and notice," Blatner says.

"There's definitely interest in pi, and multiples and fractions of pi, but it's still that fundamental constant of pi," said Ivars Peterson, director of publications and communications at the Mathematical Association of America.

At the Exploratorium, which credits itself with having invented Pi Day in 1989, there have already been pie-eating celebrations for what Hartl would deem "Tau Day" for several years, to celebrate pi-times-two, said Ron Hipschman, who's been with the museum for four decades. But the museum's big blowout activities for the public take place on March 14 (although not this year, since the museum is closed on Mondays).

As for Blake, who spent about a month making the viral pi song, he hadn't heard of tau. He remembered pi vaguely from school, even though he's "basically bad at math."

But based on his addicting song being shared and praised across the internet, he earns an A for pi music.

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