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'Tau day' marked by opponents of maths constant pi | 'Tau day' marked by opponents of maths constant pi |

(40 minutes later) | |

By Jason Palmer Science and technology reporter, BBC News | By Jason Palmer Science and technology reporter, BBC News |

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

Not all fans of maths agree, however, and pi's rich history means it will be a difficult number to unseat. | Not all fans of maths agree, however, and pi's rich history means it will be a difficult number to unseat. |

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

"If you define your circle constant as the ratio of circumference to the diameter, 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." | "If you define your circle constant as the ratio of circumference to the diameter, 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." |

The discrepancy is most noticeable when circles are defined not as a number of degrees, but as what are known as radians - of which there are two times pi in a full circle. With tau, half a circle is one-half tau. | The discrepancy is most noticeable when circles are defined not as a number of degrees, but as what are known as radians - of which there are two times pi in a full circle. With tau, half a circle is one-half tau. |

Dr Hartl reckons people still use degrees as a measure of angle because pi's involvement in radians makes them too unwieldy. | Dr Hartl reckons people still use degrees as a measure of angle because pi's involvement in radians makes them too unwieldy. |

He credits Bob Palais of the University of Utah with first pointing out that "pi is wrong", in a 2001 article in the Mathematical Intelligencer. | He credits Bob Palais of the University of Utah with first pointing out that "pi is wrong", in a 2001 article in the Mathematical Intelligencer. |

But it is Dr Hartl who is responsible for the Tau Manifesto - calling tau the more convenient formulation and instituting Tau Day to celebrate it. | But it is Dr Hartl who is responsible for the Tau Manifesto - calling tau the more convenient formulation and instituting Tau Day to celebrate it. |

Kevin Houston, a mathematician from the University of Leeds, counts himself as a convert. | Kevin Houston, a mathematician from the University of Leeds, counts himself as a convert. |

"It was one of the weirdest things I'd come across, but it makes sense," he told BBC News. | "It was one of the weirdest things I'd come across, but it makes sense," he told BBC News. |

"It's surprising people haven't changed before. Almost anything you can do in maths with pi you can do with tau anyway, but when it comes to using pi versus tau, tau wins - it's much more natural." | "It's surprising people haven't changed before. Almost anything you can do in maths with pi you can do with tau anyway, but when it comes to using pi versus tau, tau wins - it's much more natural." |

Dr Hartl is passionate about the effort, but even he is surprised by the fervent nature of some tau adherents. | Dr Hartl is passionate about the effort, but even he is surprised by the fervent nature of some tau adherents. |

"What's amazing is the 'conversion experience': people find themselves almost violently angry at pi. They feel like they've been lied to their whole lives, so it's amazing how many people express their displeasure with pi in the strongest possible terms - often involving profanity. | |

"I don't condone any actual violence - that would be really bizarre, wouldn't it?" | "I don't condone any actual violence - that would be really bizarre, wouldn't it?" |