Pi Day – on March 14 – will be particularly memorable this year: the date can be written 3/14 by those who opt for the month then day format, which is Pi to two decimal places, 3.14. If you include the year this year then that gives 3/14/15, which is Pi to four decimal places, 3.1415.

This happens only once a century, and the Museum of Mathematics in New York City, among others, is taking Pi Day 2015 one step further, by celebrating at 9:26pm, adding three more digits to Pi, 3.1415926.

You can personally celebrate the event 12 hours earlier at 9.26am, wait a further 53 seconds to get 3.141592653 Pi to nine decimal places. That’s probably the best time and date approximation to Pi you can get with your typical time piece, although the digits of Pi continue on indefinitely, but more on that later.

Chicagoans plan to celebrate Pi Day this year by running in a Pi-K race of 3.14 miles. Numerous city bakeries are offering special pies for the occasion at US$3.14 per slice.

## Another celebration

Not as well known perhaps is the fact that March 14 this year is also the 136th birthday of physicist Albert Einstein, and that 2015 is the 100th anniversary of the publication of Einstein’s paper on general relativity.

To commemorate this doubly significant event, Princeton University is planning a its usual gala event, including a pie eating contest, a performance by the Princeton Symphony, a contest to see who can recite the most correct digits of Pi (the current Guinness world record is 67,890 places), a guided Einstein tour and even an Einstein look-a-like contest.

## Pi in the popular culture

Pi Day long ago extended its reach beyond a handful of mathematical zealots, to become a widely celebrated, even the subject of a resolution to mark the day each year that was passed by the US House of Representatives in 2009.

This may well be the first legislation on Pi Day to have been adopted by a national governmental body. Pi Day even has its own following on Twitter through the hashtag #piday.

In general, Pi is much more in the public eye than it was even five or ten years ago, as we wrote last year.

Pi continues to fascinate and made another appearance on the US quiz show Jeopardy! on May 9, 2013 when it featured in an entire category of questions. The clues provided were:

- (US$200) Pi is the ratio of this measurement of a circle to its diameter.
- (US$400) Numerically, pi is considered this, like a type of “meditation”.
- (US$600) For about $19,100 x pi, this “Black Swan” director made “Pi”, his 1998 debut film about a math whiz.
- (US$800) In the 100s AD this Alexandrian astronomer calculated a more precise value of pi, the equivalent of 3.14166.
- (US$1,000) You can find the area of this oval geometric shape with pi x A x B, if A & B are half of its longest & shortest diameter.

The clues and the answers (all were answered correctly by various contestants) are given here in the J-archive, an independent repository of clues and answers maintained by Jeopardy! fans.

## Current record for computing Pi

Ever since the dawn of the computing age, researchers have plied their craft at computing Pi, by a variety of often exotic techniques.

As we explained earlier, if we count things to the second, this year’s Pi Day gives the number down to nine decimal places, at 3.141592653. But this is still only an approximation to the true value of Pi.

Pi is a transcendental number which means you can continue to expand the number of decimal places of Pi forever and there is no repetitive pattern.

The current record for calculating digits of Pi is 13.3 trillion decimal digits, which has been ascribed to someone known only as “houlouonchi” and Alexander J Yee.

## What is Pi good for?

So what is Pi good for, anyway? Does Pi or the digits of Pi ever really enter the day-to-day world? It does, actually, quite a bit.

For example, Pi is central to digital signal processing, which is pervasive in our modern wireless world. The digits of Pi (in binary) are probably somewhere in the programming of your smartphone, used in digitally decoding multichannel, gigahertz signals while you casually chat with your friend about the local weather and politics.

Mathematically speaking, your smartphone is performing a fast Fourier transform, which involves Pi.

Pi even appears in the field equations of Einstein’s general relativity. So when you read reports about tests of Einstein’s general relativity, such as the recent dramatic discovery of a four-way gravitational lens, keep in mind that Pi is behind the equations governing these mind-blowing phenomena.

For those interested in looking over some of the original technical papers on Pi that have appeared over the past 120 years in the American Mathematical Monthly, see the Pi Day anthology by one of us and Scott Chapman in the March 2015 Monthly. While many of these articles are targeted to mathematical researchers, quite a few are readable by those with relatively modest mathematical training.

For the rest of us, perhaps it is enough just to know – for this year’s Pi Day purposes – that Pi = 3.141592653 so set your alarm clocks now for 9:26am, Saturday March 14, 2015, in your favourite timezone, and enjoy the 53 second countdown.