A "holiday" celebrated each year on 14 March. Reason is that March 14 looks like 3/14 --> 3.14 when expressed in numerals. I like the holiday. My high school physics teacher encouraged everyone to celebrate it. Some students in Universities and high schools alike take the time to get some chalk and write out PI to as many decmials as they can in one day, all over sidewalks and streets.

Quite the geek holiday.

(To the tune of O, Christmas Tree)

Oh, number Pi Oh, number Pi Your digits are unending, Oh, number Pi Oh, number Pi No pattern are you sending. You're three point one four one five nine, And even more if we had time, Oh, number Pi Oh, number Pi For circle lengths unbending. 1

Well, we’re gearing up for Pi Day at school. March 14 is almost upon us once again, and students are memorizing as many digits as they can of the revered irrational number. One of the fun things about working with kids is that you’re always on the lookout for things to celebrate, and holidays, major and minor, are seen as teachable moments. Teachers are the people dressed in pink for Valentines day and green for St. Patrick’s; we are the adults who can tell you all about Black History Month and how to properly fold and cut a six-sided paper snowflake.

The lowercase Greek letter π is used in mathematics to represent the ratio of the circumference of a circle divided by its diameter. It’s an irrational number; expressed as a decimal, it does not terminate or repeat. Most people know π as 3.14, but at one point the Indiana legislature actually tried to pass a bill making π = 3.2, to simplify things. 2

2007 marks San Francisco’s Exploratorium science museum’s 19th annual pi day celebration; celebrants will be eating pizza and fruit pies, learning about π, and singing happy birthday to Albert Einstein as they parade 3.14 times around the π shrine at exactly 1:59 in the afternoon (making the date and time 3.14 1:59...) What could be more fun? Go, if you're in the area. 3

So, what do we do to celebrate Pi Day? Well, the wonderful man who runs the cafeteria will be serving all manner of round foods. In all of my classes we will talk about the amazing fact that no matter what the size of the circle, if you measure around the outside and then divide that number by the distance across the middle, you’ll always get the same number—or at least, you’ll get close to 3.14159…, depending on how accurate your measurements are. We’ll measure a few things—I keep some hula hoops on hand for just this occasion—and we’ll talk about how, although the decimal digits in pi keep going for ever and ever, pi falls in-between 3 and 4 on a number line. (I have the first 207 digits written on cash register tape, wrapping around two walls of my classroom; my younger kids don’t recognize the decimal point, and think that pi is a very BIG number.)

After that, we’ll do different things depending on the age of the student. My Algebra students will write expressions for areas of different shapes in terms of pi; my PreAlgebra students will plot the diameters of coins compared to their circumferences (guess what the slope of that line is ), my Geometry students may try to approximate the circumference of a circle using polygons with increasingly large numbers of sides. To my younger students I will read Cindy Neuschwander’s "Sir Cumference and the Dragon of Pi", a charming tale about the knight, his son Radius, and Radius’ mother, the Lady Di(ameter). EVERYONE will sing the pi day song, loudly, joyfully, repeatedly. All who care to will have a chance to demonstrate how many digits they have memorized—I handed out lists of the first 160 digits, separated into groups of 10, a week ago.

Last year one of my youngest students, a 6th grader, memorized 102 digits. It was quite impressive. Some kids hadn’t worked to memorize any digits, but decided at the last minute that they wanted to try; some managed as many as 40 digits in a half hour of class. Many of my students recited the digits, having learned them in clumps of five or ten; a few preferred to write them down. Some students learned them just by listening to others recite. Everyone who participated got certificates with their names written in calligraphy, commemorating their achievement; we passed out “Happy Pi Day” buttons to the school. This year I have an actual prize for the first place winner—my brother gave me a t-shirt with the symbol π on the front. My kids are horrified that I’m re-gifting, but they want the shirt.

Websites listing Pi Day activities suggest encouraging students to write poetry about pi. I tried that last year, but aside from my own terrible attempt, the only submission I got was from my teaching assistant, who composed this elegant pi-ku:

Seasons ebb and flow
consistent meaning of life
unlike pi’s digits 4

If I had web access in my classroom (which I don't), I'd take my kids to PiNation (http://www.pination.com/), where they could see color representations of the digits of pi and listen to "tubular bells" ring the digits. This year, we are going to try another suggestion from the web sites—creating a paper chain where each colored link corresponds to a digit of pi. I'm looking forward to it. Of course, Maui’s Lokelani Intermediate School holds the current record—last year, “the school’s 700-member student body created a 21,682-piece, multicolored paper chain.” 5 Zeesh, I only cut up enough paper to form 480 links…

Oh, number Pi, Oh, number Pi You're truly transcendental. Oh, number Pi, Oh, number Pi You're physical and mental. You stretch the bounds...of all we know, And tell our circles where to go Oh, number Pi, Oh, number Pi Your digits are so gentle.

Oh, number Pi, Oh, number Pi Why can't I learn you faster? Oh, number Pi, Oh, number Pi You're really hard to master. Just when I think...I've got you down I flip a 6 and 5 around Oh, number Pi, Oh, number Pi Numerical disaster!

Oh, number Pi, Oh, number Pi Why are you so specific? Oh, number Pi, Oh, number Pi Your digit growth's terrific. Ten years ago, you had a Mill And now you're at a couple Trill Oh, number Pi, Oh, number Pi You stretch to the Pacific!

If you're bummed out because there’s not enough time left to plan a Pi Day celebration of your own, don’t despair. You could always celebrate on Pi Approximation day, which according to Wikipedia, is July 22 (making it 22/7 if you use the European date format). 7

1 According to http://www.winternet.com/~mchristi/piday.html , this Pi Day song was written by LaVern Christianson, when he was a math teacher at Windom Area High School, Windom, MN, USA, with some help from his colleagues. The version we sing in class substitutes “Circumference divided by diameter” for the last line. It doesn’t rhyme or anything, but it gets the meaning across to my kids.

2This is almost as bad, (or maybe worse, since it's real life) than Professor Frink’s announcement on The Simpsons that pi is exactly 3; fortunately, the Indiana bill never passed. http://www.straightdope.com/classics/a3_341.html , 3/11/07

3 http://www.exploratorium.edu/pi/pidaysched.html ; more information about the museum at http://www.exploratorium.edu/visit/index.html

4 By Will McD, March 2006

5 http://www.mauinews.com/story.aspx?id=17784 , 3/11/07

6 http://www.teachpi.org/music.htm

7 http://en.wikipedia.org/wiki/Pi_day


Update 3/13/10: still going strong on Pi Day. We started Monday 3/8 this year, with a review of how to find perimeter and area of rectangles, parallelograms, and triangles, before moving on to how to find circumference of a circle. Once we had all that down, I drew a circle on the board, cut up into (pie) wedges, and arranged the triangles crust up/crust down to make a sorta-parallelogram shape. My students recognized the paralleogram; from there it's a short step to explain that the height of the pie wedge parallelogram is r and its base (half the circumference of the circle it came from) is π r , so the area is π r 2.

Actual Pi Day is Sunday this year, so we celebrated on Friday. It never ceases to amaze me, how my kids go about memorizing digits. Some put it to a beat, and recite the digits while standing up, rocking slightly. Others write it down--one student this year traced the numbers over and over until he had learned 87 digits. My favorite was the student who announced that he was good at memorizing phone numbers. He drew a keypad on the board and tapped out digits of pi; in less than an hour he had memorized 51 digits.

Three one four, the ratio of a diameter
It's 10:34 and I just met her
March One Four, the day of π
Certainly not a day to act so shy
Cause it's cream π day
And there's only one way
To make math useful in my day-to-day
When I walked up to her with and told her what
I asked if she'd have a cream π in my hut
But the only pie I had was walnut
And walnut pie sucks.
