úÎõÓverybyorgey@cis.upenn.edu*The circle constant, the ratio of a circle's circumference to its  radius . Note that pi = tau/2. @For more information and a well-reasoned argument why we should ' all be using tau instead of pi, see The Tau Manifesto,   http://tauday.com/. ATo hear what it sounds like (and to easily memorize the first 30  digits or so), try  http://youtu.be/3174T-3-59Q. 8A convenient synonym for those wishing to avoid Unicode  identifiers.  tau-6.283Math.TauÏ„tau