Tau Day, in honour of the constant some say should replace pi, is being celebrated by those who agree with US mathematicians Bob Palais and Michael Hartl. The two argue that tau, twice as large as pi at approximately 6.28, makes many calculations easier to perform. In The Tau Manifesto, Hartl says that although pi is well- defined, it's "a confusing and unnatural choice for the circle constant...Pi is half of something. It's the something that is fundamental." Whereas pi defines a circle as the ratio between circumference and diameter, tau defines a circle in terms of its circumference and radius. Hartl and Palais note that tau shows up in several major mathematical equations - an observation, they say, that isn't simply a coincidence, but rather justifies adopting a new constant. Supporters have taken the concept one step further by declaring June 28 Tau Day, meant to rival the more familiar tradition of Pi Day, held on March 14 each year. Patrick Speissegger, associate professor at McMaster and Canada Research Chair in Model Theory, doesn't buy into the idea. "It's not a question of right or wrong, but a matter of opinion," says Speissegger. "Philosophically speaking, changing the constant from pi to tau makes no difference." Except perhaps on the issue of practicality. - www.genesisgrid.co.uk