No, not Towel Day, but Tau Day, 28 June.

You’ve probably heard of Pi Day, which is celebrated on the 14th day of March (using the date format 3/14).

However, according to some people, pi is overrated, and there is a ‘more natural’ number that can be used to simplify many different formulae.

This number is tau, or τ, which was proposed in the 1950s to represent the ratio of a circle’s *radius* to its circumference (rather than π which represents the ratio of the circle’s *diameter* to its circumference).

So therefore, τ = 2π. In short, proponents of tau believe it’s twice as good and we should be celebrating Tau Day instead.

And you guessed it, just like Pi Day falls on 3/14, Tau Day falls on 6/28.

But even if you don’t agree that tau is twice the Greek letter that pi is, you can always join in the celebrations today and appreciate the other ways that it can be used.

**Four things you might not know about tau:**

**Astronomers** also use it as a measure of Optical Depth, which is a measure of the ‘optical thickness’ or opaqueness of a medium, like planetary rings.

**Particle physics**: the discovery of the Tau Lepton resulted in the 1995 Nobel Prize for Physics.

**Tau Ceti**: In December 2012, evidence suggested possibly five planets orbited the Sun-like star Tau Ceti, with one of these being potentially in the “habitable zone”. At just 12 light years away from Earth, Tau Ceti makes for an interesting target for the Search for Extra-Terrestrial Intelligence.

Lastly, **it sounds beautiful**:

Still not convinced? Then at the very least, take a leaf from the Pi Day enthusiasts, but instead of eating just one pie today, treat yourself to two!