# Pi Day Turns 25: Why We Celebrate an Irrational Number

Even if you hate math, you'll love Pi Day. Why? Because there's pie.

Pi Day is such a huge holiday that it's hard to imagine it didn't exist until 1988.

The first party in honor of the amazing mathematical entity known as "pi" began that year when Larry Shaw, a physicist at the Exploratorium, the San Francisco science museum, looked at the calendar and said, "March 14—it's the number pi [3.14, for those of you who have forgotten your grade-school math]. It's Pi Day. Let's celebrate!"

So he and his colleagues ordered some pie for the staff, and thus was a great tradition born. Today Pi Day is celebrated in schools, universities, and museums around the world.

(Note: You know this story must be true because it is in Wikipedia and nobody has dared to change it.)

Now Pi Day is marking its 25th anniversary. So it's a good time to catch up with Ron Hipschman of the museum to see what's up with pi, and Pi Day. Hipschman has worked at the museum for 41 years and describes his job there as "loose cannon."

Let's begin at the beginning. What is pi?

If you want to know how far it is around a circle compared to how far across, pi is your number. Pi is defined as the ratio of the circumferenc of a circle and divided by the distance across, which is its diameter.

I'm trying not to use mathematics words.

I'm very grateful for that. And how did it get the name "pi"?

It was first called "pi" in 1706 by [the Welsh mathematician] William Jones, because pi is the first letter in the Greek word perimitros, which means "perimeter."

Why does "pi" deserve its own day?

It's a special number. It shows up everywhere. In chemistry, physics, math, whether you're talking circles or cycles or anything to do with a curve, you're going to find pi in there somewhere. The surface area of a sphere is 4 pi "r" (for radius) squared.

What about for ordinary people who aren't chemists or physicists or mathematicians?

Take a string, stretch it around a can of Campbell's soup ...

Product placement, eh?

Or Progresso soup, which I like better. Or a can of gefilte fish. The distance around is 3.14 times the distance across. Stretch the string around a can and cut it, so you have one circumference of string. Stretch the string across the can and cut, stretch it across and cut again, and again. You'll have three pieces of string and a little bit more.

And it should be mentioned that pi is a never-ending number.

Pi is irrational, which means that you can't calculate pi by dividing an integer by any other integer. Pi never ends and it never repeats. People have to wrap their brain around that. Twenty-two over seven gives you a pretty good approximation of pi, but it's not pi.

And it's a pretty old thing, right?

It's in the Book of Kings, where they talk about a basin that is three times as far around as it is across. In the Bible pi equals three. Three is not a bad approximation, actually. As you move up through history people calculate it more accurately. [The Greek mathematician and astronomer] Archimedes got it between 3.141 and 3.143. The Chinese got it out to six digits around [the year] 480. In India, they got it out to 11 digits around 1400.

How far have we calculated pi now?

The most accurate is calculated to ten trillion digits.

I understand Pi Day is not the only day that honors pi.

July 22 is Pi Approximation Day. Sometimes we also celebrate Two Pi Day: June 28 [because the date 6/28 is 2 times 3.14, the approximate value of pi]. Because we're wacko and love pi, sometimes the Exploratorium celebrates Three Pi Day on September 42. You have to carry a few days into October.

I understand the museum is celebrating pi at its soon-to-open new location this year, and that you are contractually obligated to state many things about the nature of your celebration and also about the museum's new building.

Yes. We will be serving our usual array of pies [to the public] at Pi Day, tossing pizza dough, and having our famous pi procession, with pie plates attached to yardsticks that have the number pi out to a couple hundred digits, all at the Exploratorium's new location on Pier 15 on the San Francisco waterfront. It's halfway between the ferry building and Fisherman's Wharf, and officially opens on April 17. It'll also have two restaurants, one of which is named the Seismic Joint.

Mother Nature has apparently put the kibosh on plans to have a plane write "pi" in the sky.

We're looking at the weather forecast. It's supposed to be overcast all day. We may have to cancel the flight, but maybe we'll do it on Two Pi Day or Pi Approximation Day.

One last question: Does the magic of pi make you believe in a higher power?

No. Nothing makes me believe in a higher power. But pi does make me believe in irrationality and the wonders of mathematics.

P.S. For readers who'd like a personal pi experience, there's a website that will help you find the first time your birthday appears in pi written as month/date/last two digits of year. And we'd love to hear from you about how you celebrated pi day.