Many of you are probably aware of "Pi Day", an annual celebration of the number π (pi) that takes place on March 14 (3/14). Some of you may also be aware that there are serious problems with π, which is defined as the ratio of a circle's circumference to its diameter. This definition is in many ways a confusing and unnatural choice for the circle constant. I have just launched a new project called The Tau Manifesto, which argues in favor of the true circle constant, the ratio of a circle's circumference to its radius, and proposes that we call this number τ (tau). And, since numerically τ is 2π, you might note that today is June 28 (6/28)—that is, today is Tau Day.
The Tau Manifesto was written using the same typesetting system (which I call PolyTeXnic) used to write the Ruby on Rails Tutorial book, and PolyTeXnic itself is written in Ruby. I hope the more mathematically inclined Rubyists out there will be interested to read the Tau Manifesto, which can be found at tauday.com.
Happy Tau Day!