Why Pi?

Pi or is a number, approximately equal to 3.14159 (often shortened to 3.14). It represents the ratio of a circle’s circumference to its diameter.

More specifically, pi is the number of times a circle’s diameter will fit around its circumference. This is the same value as the ratio of a circle’s area to the square of its radius. The decimal in pi goes on and on in a seemingly random sequence that never ends or repeats. Consequently, all the digits of pi can never be fully known, and for that reason it is impossible to find a circle’s exact circumference or area. Still, we can get close enough. If the circumference of the earth were calculated using pi rounded to only the ninth decimal place, an error of no more than one quarter of an inch in 25,000 miles would result. The record for calculating pi is 5 trillion digits using a computer.

Pi is such a unique number that it has fascinated mathematicians for centuries, and in fact its history dates back to ancient times. By 2000 BC, Babylonians had established the constant circle ratio as 3-1/8 or 3.125. The ancient Egyptians arrived at a slightly different value of 3-1/7 or 3.143. Pi is even alluded to in the Bible, where 1 Kings 7:23 describes a cylindrical vessel built for the altar of Solomon’s temple: “And he made a molten sea of cast metal, ten cubits from brim to brim: it was round all about, and its height was five cubits: and a line of thirty cubits did compass it about.” These measurements procure the following equation: 333/106 = 3.141509.

Also called the “circular constant,” pi is the most recognized mathematical constant in the world. Many formulas from mathematics, science, and engineering use pi. Since the twentieth century, pi has been used in hundreds of equations in many sciences including those describing the DNA double helix, a rainbow, ripples spreading from where a raindrop falls into water, superstrings, general relativity, normal distribution, distribution of primes, geometry problems, waves, navigation, number theory, probability, and chaos theory.

Fascination with the number pi has even carried over into non-mathematical popular culture. There’s just something intriguing about a number that’s so complex and yet associated with a geometric object as simple as a circle. Plus it’s a nice play on words! If something is easy as pie, it is very easy indeed. Math may not be a piece of cake, but it can be as easy as pi. That’s how I hope my students will feel about math following their lessons.

The first 36 digits of pi are called “Ludolph’s Number” after German-Dutch mathematician Ludolph van Ceulen (1540-1610), who is famed for his calculation of pi to 35 decimal places:

3.14159265358979323846264338327950288

Pi Day

Pi Day is celebrated by math enthusiasts around the world on March 14. The official celebration begins at 1:59 pm, to make an appropriate 3.14159 when combined with the date.

Celebrate with fruit pies, cream pies, and pizza pie! You can also have Oreos, cookies, cake, doughnuts, pineapple slices, and any other foods that are round.

Read Sir Cumference and the Dragon of Pi, by Cindy Neuschwander and Wayne Geehan. (This picture book is like a children’s fairy tale, but can be used with older kids since it involves division with fractions.)

Go on a scavenger hunt to find objects shaped like a circle. On the circles you’ve found, use a piece of string to “prove” pi by measuring their circumference and diameter.

See how many digits of pi you can memorize. (Hint: divide it into 5-digit “zip” codes for easier remembering.)

Recite pi as a team by dividing it into 5-digit segments, one for each student in the class to memorize.

Look for numerical patterns in pi.

Have a “Pi-athalon” featuring a “pi-mile run” (3.14 miles).

Did You Know…?

Pi Day is also Albert Einstein’s birthday! The famous physicist and mathematician was born on March 14, 1879.

In the Star Trek episode “Wolf in the Fold” (original series), Spock foils the evil computer by commanding it to “compute to last digit the value of pi.”

A cheer at the Massachusetts Institute of Technology goes like this: “Cosine, secant, tangent, sine, 3.14159!”

Here is a pi palindrome: “I prefer pi.” (A palindrome reads the same forwards and backwards.)

Comedian John Evans once quipped: “What do you get if you divide the circumference of a jack-o’-lantern by its diameter? Pumpkin pi.”