Circular logic

The mathematical constant pi is under threat from a group of detractors who will be marking "Tau Day" on Tuesday.

Tau Day revellers suggest a constant called tau should take its place: twice as large as pi, or about 6.28 - hence the 28 June celebration.

Tau proponents say that for many problems in maths, tau makes more sense and makes calculations easier.

"I like to describe myself as the world's leading anti-pi propagandist," said Michael Hartl, an educator and former theoretical physicist.

"When I say pi is wrong, it doesn't have any flaws in its definition - it is what you think it is, a ratio of circumference to diameter. But circles are not about diameters, they're about radii; circles are the set of all the points a given distance - a radius - from the centre," Dr Hartl explained to BBC News.

By defining pi in terms of diameter, he said, "what you're really doing is defining it as the ratio of the circumference to twice the radius, and that factor of two haunts you throughout mathematics."

[...] —BBC (via Jane)