"Two Pi Day", also known as "Tau Day" is celebrated on June 28. 2π, also known by the Greek letter tau (τ) is a common multiple in mathematical formulae. Some have argued that τ is the more fundamental constant and that Tau Day should be celebrated too. Celebrations of this date jokingly suggest eating "twice the pie".
History
The first known use of tau was approximated by Archimedes in the third century. He was born in the Greek city-state of Syracuse on the island of Sicily. Archimedes was known as one of the greatest scientists of the classical age, who, among many other things, calculated pi to the most precise value known. Pi is defined as the ratio of a circle’s circumference to its diameter as is tau, but some believe that Archimedes’ work would have been all the better if he had relied more on τ than π.
During the eighteenth century, Leonhard Euler used the Greek letter π to represent the ratio of pi itself and for many years it has remained as the quintessential constant circle. This helped people understand circles, triangles, and other principles of geometry. It was a useful device that served its purpose well, but a new voice would define it as inelegant a couple of centuries later.
The tau movement was founded by the former University of Utah math professor Robert Palais who believed that τ simplified the math. Palais noticed that something was off with pi when calculating the sine of π/2 and the image he saw didn’t add up to the calculations. From this, he knew that pi wasn’t the way. Palais published his findings in a 2001 article titled “π is Wrong” in the “Mathematical Intelligencer.” He noted that Euler went back and forth between tau and pi, but pi became the accepted constant. Palais then proposed tau was superior and he used the pi symbol with an extra leg to represent it, and it eventually became an uppercase T.
On June 28, 2010, “The Tau Manifesto” launched at the same time as the first Tau Day. “The Tau Manifesto” was a book written by Michael Hart 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.