The circumference of a circle is its perimeter or the length around it. The distance from the center of a circle to its edge is the radius. The distance from one side of a circle to the opposite side (twice the radius) is the diameter.
Pi Day is a day devoted to the famous mathematical ratio between a circle’s circumference and its diameter (3.14159265…), currently celebrated on March 14. Regardless of the size of the circle, pi is always the same number. So, for any circle, dividing its circumference by its diameter will give you the exact same number: 3.14159…or pi.
Some mathematicians argue, however, that pi is less appropriate for characterizing a circle than another number, namely the ratio of the radius to the circumference. That number is called tau, and it is equal to 2 x pi, or or 6.28318530…..
The radius is more useful in trigonometry and especially in calculus. The unit circle, i.e., a circle with a radius of one unit is ubiquitous in calculus and trigonometry. Thus many important equations use 2π, or tau, anyway, instead of pi. So why not just use tau instead of having to double the number of pi?
Mathematicians may be divided on the issue, but to me it makes infinite [sic] sense to celebrate Tau Day, because honoring the occasion requires TWO pieces of pie. TWO pies. This sounds like a good thing. Since Tau equals 6.28… (etc.), Tau Day is celebrated on June 28.
For those who miss the reference, this is a play on the famous 1929 picture by Rene Magritte:
Note: Some fans of Tau Day just make one pie, but call it a tau, as at the website NotAboutApples: