**Tau Day** an anti-Pi Day holiday is an unofficial observance held every year since 2010 on 28 June. The day began to recognize tau, τ, a ratio constant describing the relationship between the circumference and the radius of a circle. In recent years, some mathematicians and physicists have argued for the replacement of pi (π) by tau.

An unofficial holiday that operates on a quite simple premise: that PI is wrong. This holiday celebrates Tau—a replacement for PI that sets the circle constant at approximately 6.28. It is anti-Pi because Pi equals a circle’s circumference divided by its diameter (3.14), while Tau represents a circle’s circumference divided by its radius. Tau is considered by some to be a lot more accurate way to find a circle’s circumference, but not every in the mathematics community agrees. As a result, this holiday is still somewhat on the fringes of mathematical holidays. Regardless of whether this holiday is used as a bookend with PI Day or whether it is celebrated as a holiday.

**History of Tau Day**

On 28 June 2010, “** The Tau Manifesto**” launched at the same time as the first Tau Day. “The Tau Manifesto” was a book written by Michael Hartl that was dedicated to the lesser-known number. In it, pi is referred to as unnatural and confusing. Pi compares a circle’s circumference with its diameter, and many mathematicians are disinterested in this quantity whereas tau is the number that connects a circumference to that quantity. The day is used to celebrate all of mathematics, but tau specifically still has an uphill battle in receiving recognition.

**Replacing Pi**

The concept of replacing pi with tau as a better way to describe the relationship between a circle’s circumference and its radius was first brought up by mathematics professor, Bob Palais in the Mathematical Intelligencer in an article called π is wrong! This was followed by the publishing of the Tau Manifesto by physicist Michael Hartl. Both articles called for the replacement of pi by tau. Tau, the ratio of a circle’s circumference to its radius, is equal to 2 π or 6.283.

**Wider Acceptance**

Tau is not yet widely accepted within the scientific community, though there is a large community of scientists and technologists who have called for the replacement of pi by tau.

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