Celebrate Tau Day in style with a Tau tee shirt, available only for a limited time from Teespring. Offered in both men’s sizes as well as fitted women’s.

Buy the downloadable ebook versions of The Tau Manifesto in EPUB, MOBI, and PDF formats, along with downloadable versions of the Tau Talk videos!

State of the Tau 2020

Happy Tau Day 2020, everyone! If you’re celebrating Tau Day on social media, don’t forget to use the hashtag #TauDay. :-)

On June 28, 2010 (6/28/10 in the American calendar system), I published the first edition of The Tau Manifesto, thereby giving the circle constant $C/r$ a name and inaugurating the very first Tau Day. On this 10th anniversary of that fateful day, I’d like to thank the thousands of math fans, science nerds, computer geeks, and fellow tau travelers who have joined me in this quirky, quixotic quest to recognize, honor, and celebrate the true circle constant, $\tau = C/r = 6.283185\ldots$

Amazingly, even after ten years, tau isn’t remotely out of steam—if anything, it continues to gain strength. Indeed, this past year saw the biggest media coverage of tau yet, a new round of Tau Manifesto translations, and endorsements from some of the biggest institutions in science and mathematics.

N.B. You can still get the latest version of the Official Tau Shirt for a limited time. You can also sign up for the The Tau Manifesto email list here.

Tau in The Wall Street Journal

On the occasion of Half Tau Day (a.k.a. Pi Day) this year, tau reached a new milestone: full-length coverage in one of the best-known mainstream news sources. In honor of that day, The Wall Street Journal published “For Math Fans, Nothing Can Spoil Pi Day—Except Maybe Tau Day” by Robert McMillan, which prominently featured both me and original “Pi Is Wrong!” author Bob Palais. (In case you don’t subscribe to The Wall Street Journal, you can find an archived copy of the article here.)

Participating in the story was terrific fun, and I’d like to thank Robert McMillan for his great work. Although in many ways we’re still in the beginning of this little movement, tau has already exceeded my wildest expectations—when I first published The Tau Manifesto in 2010, I never would have expected that someday I’d get excited messages from far-flung family and friends saying, “Hey, I just saw you in The Wall Street Journal!”

Italiano, на русском языке, 简体中文

Although recognition by a major media outlet like The Wall Street Journal is a welcome confirmation of our efforts, tau has always been a true grassroots movement. One sure indication of such grassroots support emerged in the past year: full (and 100% volunteer) translations of The Tau Manifesto into not one, not two, but three additional languages!

Adding to the excellent Spanish translation by Juan Guijarro Ferreiro, the Tau Day website now features translations of The Tau Manifesto into Italian, Russian, and simplified Chinese:

I’m deeply grateful for these wonderful efforts, which underscore how mathematics—and hence tau—really is a global community. Thanks for all the great work! (By the way, if you’re interested in translating The Tau Manifesto into another language, please let me know!)

Site Redesign

With links to all the new translations, the menus on the Tau Day website were getting awfully cluttered, and the site was long overdue for a redesign. In particular, while the original Tau Manifesto launched in a world where the iPhone was only three years old, in today’s world pretty much every website has to look good on mobile devices to remain relevant.

Accordingly, I and my Learn Enough cofounder Lee Donahoe (but mostly Lee) redesigned the site to work better on smaller devices by increasing the font size, breaking up longer equations, and switching to a menu system that works well whether you’re browsing on a small smartphone or on a giant desktop. There are a lot of different devices and design constraints, so we’ll no doubt continue tweaking things for a long time to come, but it’s already a major improvement over the previous design. Thanks to Lee for the help!

New Computer Language and Other Public Support

The last year saw the addition of tau to many computer languages and frameworks (adding to a list that includes Python, Julia, and many others):

Other support came both from an unexpected direction and from an old friend. Vitalik Buterin, cofounder of the Ethereum smart-contract platform, unexpectedly boycotted Pi Day because “tau day is better”. Thanks for the support, Vitalik! Fellow tauists everywhere appreciate it.

Meanwhile, known Friend of the Tau Saturday Morning Breakfast Cereal (SMBC), a popular webcomic that has featured tau before, included a sly tau reference in the May 20, 2020 comic “Social”. In the final panel, a smart and savvy computer user sports a T-shirt bearing the statement $\text{“}\tau > \pi\text{”}$—which is both metaphorically and mathematically true.

Incidentally, that shirt is real! Indeed, as yet more evidence of tau’s grassroots nature, there are many different variations, and I had absolutely nothing directly to do with any of them.

Tau Day 2019 Roundup

Finally, I’d like to review some of the highlights from Tau Day 2019.

Found on Instagram: a version of the popular Drake meme template, in this case featuring Drake looking on with disdain at a bunch of pi formulas and then with approval at the same formulas featuring tau.

The Massachusetts Institute of Technology (MIT) continued its long-running tau support with “Three cheers for double Pi!”

Meanwhile, the Wolfram|Alpha Twitter account offered a tau-tology in honor of the day. As an enthusiastic user of Wolfram Mathematica back in my graduate-student days, this one meant a lot to me!

Although the Wolfram Language doesn’t (yet) support tau natively, the Julia language does, and @_cormullion put it to good use by making this beautiful animation using overlapping circles to draw a representation of tau. Fantastic work!

Thanks!

Thanks again to everyone who has supported tau these last ten years. Here’s to many more years to come!

Michael Hartl

Founder, Tau Day

Author, The Tau Manifesto

State of the Tau 2019

Happy Tau Day 2019, everyone! On this June 28 (6/28 in the American calendar system), thanks for joining me in the annual celebration of the true circle constant, $\tau = C/r = 6.28\ldots$

A special thank-you goes out to anyone wearing an Official Tau Shirt today. You can post pics or any other tau-related material on Instagram #TauDay or Twitter #TauDay.

Spanish Version

The tau news I’m most excited about this year is a full translation of The Tau Manifesto into Spanish by tau supporter Juan Guijarro Ferreiro. You can find the result, El Manifiesto Tau, on the Tau Day website. ¡Muchísimas gracias, Juan!

Tau Birthdays

Some pi partisans occasionally brag about how “Pi Day” (3/14) is the birthday of Albert Einstein, so it’s perhaps worth noting that Tau Day is the birthday of another great 20th-century physicist: Maria Goeppert Mayer, who won a Nobel Prize in Physics for her groundbreaking work on the nuclear shell model.

Long-time tau supporter Joseph Lindenberg has pointed out that Tau Day is also the birthday of famed entrepreneur and literal rocket scientist Elon Musk.

On this 9th annual Tau Day, please join me in wishing happy birthday to Professor Goeppert Mayer and Mr. Musk!

A Tau Conversion

Tau Manifesto reader Jason Brown passed along a delightful tau essay that I’d somehow missed all these years: “My Conversion to Tauism” by Stephen Abbott, which appeared in the magazine Math Horizons in 2012. Professor Abbott is a mathematician at Middlebury College and author of the outstanding textbook Understanding Analysis, making his endorsement especially gratifying. I particularly appreciated this line from the essay:

“The Tau Manifesto” is an entertaining read, so entertaining in fact that you don’t immediately notice how utterly compelling it is.

Thanks, Professor Abbott! I’m glad you liked it. :-)

Internet Trivia Famous

My friend Alex Wood reports on an exchange involving an Internet trivia game:

My girlfriend, reading me trivia questions: “According to a manifesto written by Michael Hartl—” Me: “Tau” You’re internet trivia famous, man 😂

A Comics Theme

Continuing a theme set by xkcd & SMBC, on October 13, 2018, the comic strip Sally Forth prominently featured tau:

Thanks to alert reader and long-time tauist Skona Brittain for the heads-up!

Speaking of SMBC, tau also makes a cameo appearance in the strip “Mega-Pi” about a proposed mathematical constant equal to two million pi (i.e., one million tau).

Another Computer Language

The popular Unity video game engine now includes tau as a pre-defined constant, thereby joining languages such as Python, Processing, and Modula-2. (Still waiting for Ruby to join the party, though!)

Tau Day 2018 Celebrations

Finally, here’s a short summary of some of the high-profile celebrations of Tau Day from last year.

Museum of Science

Joseph Lindenberg passed along this report of the Museum of Science in Boston holding a Tau Day fundraiser, with a benefactor offering to match donations. (The campaign is from last year, but appears to be active, so I think you can still donate if you like.)

The Museum of Science also sponsored tau-themed yoga classes at 6:28, both a.m. and p.m.!

Massachusetts Institute of Technology

Speaking of the Boston area, the official MIT Twitter account sent out a tweet in honor of Tau Day:

Thanks, MIT! Now if only either of my own almae mātrēs would show tau such love…

SciShow

The hugely popular science-themed YouTube channel SciShow (5.7 million subscribers as of this writing) featured an outstanding “Happy Tau Day!” video. Thanks to SciShow for the support!

SciPress

Swiss open-access academic publisher SciPress ran a Twitter poll on Pi vs. Tau in which, appropriately enough, tau was around twice as popular as pi!

(I neither retweeted nor voted in the poll, so that’s about as unbiased as a Twitter poll can get!)

Onward & Upward

That’s it for this year! Thanks again for all the enthusiasm and support for this quirky little project.

Happy Tau Day, everyone!

State of the Tau 2018

Happy Tau Day 2018, everyone! Thanks for joining me in this annual celebration of the true circle constant, $\tau = C/r$.

I hope you’re all prepared with twice as much pie. 😋🥧🥧

As is my usual custom, I’ve prepared a State of the Tau update summarizing the various tau happenings over the past year.

I’ve substantially revised and updated “Getting to the bottom of pi and tau”, especially the section on hyperspheres, including a brand-new section on recurrences. It’s more mathematically challenging than the rest of The Tau Manifesto, but of course this can be a feature. Go ahead and take a look if you’re in the mood for a challenge!

Tau Day 2017 Celebrations

June 28, 2017 proved to be the most eventful Tau Day since the original Tau Talk in 2011. First of all, I was hosted by Googler Kevin Oberlies and the other gracious folks at Google’s Los Angeles office as part of their “Talks at Google” series. The result is a beautiful HD update to the 2011 version of the talk:

After finishing the presentation, I and some fellow tauists adjourned for lunch, where the Google dining staff had prepared a special surprise for us—tau-themed cupcakes!

Perhaps the greatest highlight of the day was finally meeting Bob Palais, author of the original “π Is Wrong!” article that planted the seed for The Tau Manifesto back in 2001. Bob attended the talk at Google and then came to a Tau Day party I hosted that evening. Here we are in our matching Tau Shirts:

We celebrated late into the night with twice as much pie. Thanks to everyone who made Tau Day 2017 so memorable!

The Chad Hartl–Palais Constant

In March 2018, just before Half Tau Day, I was pleased to learn (via Joseph Thiebes) about a fun new tau meme making the rounds online. Created by mathematician Diego Vera, it’s titled “The Virgin Archimedes’ Constant vs. The Chad Hartl–Palais Constant”, and is based on the famous Virgin vs. Chad meme template. Here it is, lightly edited to keep in line with Tau Day’s family-friendly G rating:

You can find Diego’s original version (rated PG for language) here. Thanks to Diego for the great meme work, and to Joseph for giving me the heads-up about it!

Euler’s Near Miss

Did you know that the great Leonhard Euler almost settled on using $\pi$ to mean $C/r$ instead of $C/D$? Oh, how much confusion that would have spared us! Check out this fascinating video on the subject:

Breaking Mainstream

Recognition of tau as the true circle constant continues to grow. OG tauist Wyatt Green sent this encouraging field report:

I just wanted to report to you that my dad emailed me this link: “Forget Pi Day. We should be celebrating Tau Day.”

Tau Day has crossed the barrier from esoteric geek conversation to links that family members forward to each other. I feel this is a special accomplishment—a breakthrough into the mainstream.

I also learned via a tweet from Sarah Jeong about an article at The Verge called “Stop celebrating Pi Day, and embrace Tau as the true circle constant”. It’s exciting to see this idea taking root!

Here are the full attributions for reference:

Tau Shirt Sighting

Finally, as usual I’ve relaunched sales of the Official Tau Shirt for a limited time (through Monday, July 2 at 8 p.m. Pacific). It’s an international phenomenon! For example, Johann Swanepoel from South Africa writes:

See attached, me proudly wearing my Tau shirt at the weekly parkrun in George, Western Cape, RSA.

Thanks to Johann and all the other tauists around the globe for making Tau Day and The Tau Manifesto a continuing source of amusement and joy!

Happy Tau Day!

Till next year,

Michael Hartl

Founder, Tau Day

Author, The Tau Manifesto

State of the Tau 2017

Happy Tau Day, everyone!

We’ve come a long way since the launch of The Tau Manifesto in 2010, and this year saw continued progress and interest in τ, the true circle constant.

In case you’re short on time, here are three quick action items:

Join me and fellow tau enthusiasts at the official Tau Day 2017 party at Urth Caffé in Santa Monica, California. Support Ryan Brancheau’s Tau Euro Magnet Kickstarter. (I’m a backer, but I don’t have any financial interest in the project.) Get your Official Tau Shirt now for a limited time.

There are many ways to celebrate Tau Day, of course, and I hope you’re having fun.

I’m celebrating this year by giving an updated version of the original Tau Talk at Google’s Los Angeles office in beautiful Venice, California. Joining me there will be original “π Is Wrong!” author Bob Palais as a special guest!

The Google event is closed to the public, but (as noted in the action items above) anyone in the Los Angeles area is welcome to join me and fellow tau enthusiasts for the official Tau Day 2017 party.

The plan, as usual, is to eat twice as much pie!

Tau in Scientific American

Just in time for Tau Day 2017, Scientific American has published “The Tao of Tau”, an exploration of the use of the letter tau in science, engineering, and mathematics. The article opens with an acknowledgment of the true circle constant:

“It is lamentable that there’s no famous dessert named ‘tau,’” Michael Hartl told me recently at a sunny, stylish café in Venice, California. He reluctantly admitted that pi, the constant approximately equal to 3.14, has this one advantage over tau, a number he introduced to replace it.

Read the whole thing there. Congratulations to author Elizabeth Landau on a job well done!

Tau in Python

One of the most exciting tau events this year involves the Python programming language, one of the most popular and powerful languages in use today.

Although nowadays I’m better known for my contributions to the Ruby community, Python was actually the first programming language I really loved.

It is therefore with great pleasure and pride that I announce the inclusion of tau in the latest version of Python. That’s right—after some lively debate, the BDFL of Python, Guido van Rossum, decided to include tau as part of the official math library in Python 3.6:

>>> from math import tau >>> tau 6.283185307179586

Go forth and wield tau, O brave Pythonistas!

Tau N Pi

Another welcome tau contribution came via the son of reader Bobby Cieszki, who created a haunting parody of Kid Cudi’s song “Day ‘N’ Nite” called “Tau N Pi”. Give it a listen—it’s really quite extraordinary.

Key Lime Tau

Last year I mentioned that two breweries (Hawkshead Brewery in the UK and Crooked Stave here in the US) had collaborated to bring the world Key Lime Tau beer. Sounds great, right?

Alas, I soon discovered that it is rather hard to come by.

Well, tauist Richard Soderberg took this as a challenge. He diligently tracked down a vendor, and procured for me a generous supply.

Verdict: Deliciously sour, and worthy of its name!

Thanks, Richard!

Tau art

Tom Magliery reports on an art project he created involving a collage of numbers—found in the wild, so to speak—assembled to form the digits of tau:

Cool!

Los Alamos National Labs

On Pi Day (or, as I prefer to call it, Half Tau Day), Los Alamos National Labs acknowledged tau in its Bradbury Science Museum newsletter article called “Celebrate Pi Day on March 14 (3.14)”:

Pi and controversy (Didn’t see that coming, did you?)

Back in 2001, Bob Palais, a math professor at the University of Utah, wrote an opinion piece for The Mathematic[al] Intelligencer called “π is Wrong!” Apparently, the counterargument against pi is that it is often used as 2π for equations, so why not just use 6.28 instead of 3.14? That call reemerged again in 2010, when Michael Hartl wrote a book [sic] called The Tau Manifesto making a similar argument and offering up the name of “tau” as the shorthand of 2π or just τ as its symbol. He’s called for an annual Tau Day on June 28 (6/28 on the American calendar) to raise awareness of the tau concept. In case you don’t have enough interest to absorb the contents of his book [sic], Michael’s website includes two videos explaining the concept of tau in both short (14 minutes) and long (51 minutes) versions to bring you up to speed.

Dutch newspaper article

Alert reader Wouter Vink passed along this remarkable sighting of tau in an article from a Dutch newspaper:

I can’t read Dutch, but this has to count as a good sign. Tau continues to be an international phenomenon!

Tau Euro Magnets

Tauist Ryan Brancheau has launched a tau Kickstarter to make European-style oval magnets with 6.28 in the middle and further digits around the side. Let’s see if we can get Ryan’s project to tip!

Tau Shirts

Finally, as usual I’ve relaunched Tau Shirt sales as part of the Tau Day celebrations.

To paraphrase Yogurt from the movie Spaceballs:

Merchandising! Merchandising! Where the real money from the Manifesto is made!

Supporting tau is a labor of love, of course, and it isn’t exactly a major profit center. But you keep asking for tau shirts, so I keep selling them!

Tau Day 2017

Thanks to all the tauists out there who continue to make this quirky project so much fun. Hope to see you at the party.

Happy Tau Day!

Michael Hartl

Founder, Tau Day

Author, The Tau Manifesto

State of the Tau 2016

Happy Tau Day, everyone! It’s hard to believe it’s already been six years since The Tau Manifesto launched and the real battle against pi’s supremacy began.

I don’t keep careful statistics, but I can confidently state that awareness of tau is enormously widespread. As an active member of the Caltech alumni community, for example, I frequently meet Caltech undergraduates, and they’ve virtually always heard of Tau Day.

Similarly, I’ve found in the programmer community that a fair fraction have heard of tau. In my own little corner of tech, every so often someone will freak out upon realizing that the author of the Ruby on Rails Tutorial and The Tau Manifesto are the same person. Indeed, today I hope to blow more than a few such minds, because I’ve just launched the new edition of the Ruby on Rails Tutorial book—on Tau Day!

One cool thing is seeing people wearing the Official Tau Shirt, which as usual is available for a limited time (sales expire tomorrow). For example, my mother reported seeing one on an elementary school student in Orange County, California, last year. And just a couple of weeks ago I was driving through downtown Los Angeles when I saw a woman wearing a Tau Shirt. I recognized her as my friend Carol Hoffstedt, so I rolled down the window and said, “Hey, Carol! You win!” I mean, what are the odds!?

As in previous years, people continue to amaze me with the incredible variety of creative projects inspired by tau. For example, an alert tauist passed along this remarkable Tau Day Sonnet, written for Tau Day last year:

As a big fan of the sonnet form myself, I can hardly imagine a better Tau Day present than this. Well, maybe beer (see below).

A second tauist, this one at MIT, told me about a “Tau Day Hack” involving “Tau on the Cow” (don’t ask me exactly what it means!):

I’m glad to see MIT picking up the tau ball and running with it—and challenge my fellow Techers at Caltech to do the same. (Hint, hint!)

Finally, the ever-enthusiastic Joseph Lindenberg passed along this fantastic bit of tau culture found in a most unexpected place: a specialty beer from Hawkshead Brewery called Key Lime Tau!

As described on its website:

The [Key Lime Tau] beer, originally a collaboration brew with Crooked Stave (USA), is our take on a key lime pie. It was brewed on both sides of the Atlantic, hence two ‘pies’ and is 6.28% in strength.

6.28% in strength! How can you not love that?!

Alas, it looks like Key Lime Tau beer is pretty hard to come by, and I haven’t yet procured any for myself, but for next year I’ll do my best to fulfill the promise implicit in this lovely poster, and drink Hawkshead Key Lime Tau on International Tau Day!

Until next year—Happy Tau Day!

Michael Hartl

Founder, Tau Day

Author, The Tau Manifesto

State of the Tau 2015

Happy Tau Day 2015! Interest in the true circle constant (τ = C/r = 6.283185…) and The Tau Manifesto continued unabated this year, highlighted by a surge of attention on the “Pi [Half Tau] Day of the Century” (3/14/15). (Tau will have its revenge on 6/28/31—party at my place!) As one of the leaders of the “opposition”, I was invited to the Pi Day festivities at the Exploratorium in San Francisco—the organization that originally created Pi Day—but I was on vacation in Barcelona at the time and was unable to attend. (I know, rough life!) That the invitation was proffered in the first place is an excellent sign, though, as it serves as proof that even the Paladins of Pi recognize tau as a legitimate rival.

Here are some of the highlights since last year’s Tau Day:

Using tau instead of pi makes math clearer, and thus easier to understand.

Using pi is like having a weird car whose odometer and speedometer display half-miles and half-miles-per-hour, while all the road signs show miles and miles-per-hour. (The road signs of math are naturally in units of tau.) So you constantly have to convert between what your car says and what the road signs say. 55 mile-per-hour speed limit? Make sure your speedometer needle doesn’t go over 110. But instead of nice round numbers like 55, imagine the sign says 68.7 miles-per-hour. So your speedometer needle shouldn’t go above… how much? Your trip odometer reads 35.7. So you’ve traveled… how many miles? Sometimes you must multiply by 2. Sometimes you must divide by 2. And before doing either, you must always stop and decide which to do in this particular case. If you’re driving in heavy traffic, or bad weather, or you’re lost, you don’t want that distraction. The same is true if you’re lost while trying to learn trigonometry.

Thanks for all the support! For me, that makes for a very happy Tau Day.

Michael Hartl

Founder, Tau Day

Author, The Tau Manifesto

State of the Tau 2014

Happy Tau Day, everyone! This was a huge year for $\tau$, with continued adoption in programming languages and classrooms, appearances in a couple of prominent webcomics (xkcd & SMBC), and a big endorsement from Google. I also received an amazing parable about pi and tau from a precocious high-school student—seriously, give it a read.

Here’s a list of some of the highlights since Tau Day 2013:

I’ve been gratified by the continued enthusiasm for $\tau$ since the launch of The Tau Manifesto in 2010. I especially appreciate the support of Robert Palais, Joseph Lindenberg, Peter Harremoës, Robin Whitty, Vi Hart, and all the tauists who’ve reached out about $\tau$ in the past four years. Here’s to another great year ahead for $\tau$!

—Michael Hartl, Tau Day 2014

State of the Tau 2013

It’s been a big year for $\tau$. When I launched The Tau Manifesto back on Tau Day 2010, I hoped that $\tau$ might strike a chord, but its popularity has exceeded my wildest expectations—due in large part to the efforts of the many ardent tauists who picked up $\tau$ and ran with it. Now three years on, activity surrounding both the constant and the notation continues apace. In honor of Tau Day 2013, I’ve listed below some of the highlights from the last year. Enjoy, and Happy Tau Day!

Michael Hartl

Founder, Tau Day

Author, The Tau Manifesto

Note: I’d like to thank Joseph Lindenberg, Robert Palais, Robin Whitty, and Peter Harremoës for their help in compiling this list.