The mathematical constant pi (π), approximately equal to 3.14159 is celebrated annually by math enthusiasts around the world on March 14th. Pi Day is held on that date because the number is commonly rounded to 3.14. The official celebration begins at 1:59 pm, to make an appropriate 3.14159 when combined with the date.

## Why Pi?

The symbol π (the 16th letter of the Greek alphabet) represents the ratio of a circle’s diameter to its circumference. More specifically, it’s 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 goes on and on in a seemingly random sequence that never ends or repeats, i.e. 3.141592653589793238462643383279502… etc. Consequently, all the digits can never be fully known (the record for calculating pi is 5 trillion digits using a computer), and for that reason it’s impossible to find a circle’s exact circumference or area.

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 make 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. It’s been used in hundreds of equations including those describing the DNA double helix, a rainbow, ripples spreading from where a raindrop falls into water, waves, superstrings, general relativity, normal distribution, distribution of primes, geometry, navigation, number theory, probability, and chaos theory.

A cheer at the Massachusetts Institute of Technology (MIT) goes like this: “Cosine, secant, tangent, sine, 3.14159!” 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, i.e. “Math may not be a piece of cake, but it can be as easy as pi.” It can even be a palindrome: “I prefer pi.” (Palindromes read the same forwards and backwards.)

## Fun Fact

Would you believe, Pi Day is Albert Einstein’s birthday! The famous physicist and mathematician was born on March 14, 1879. Here’s an idea: have a birthday party on this date with a round cake and decorate it with a π symbol!

## Pi Day Activities

- Celebrate with fruit pies, cream pies, and pizza pie! You can also have Oreos, cookies, cupcakes, doughnuts, pineapple rings, and 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 measure their circumference and diameter, and see what you get.
- See how many digits you can memorize and recite. (Hint: divide it into 5-digit “zip codes” for easier remembering.) Multiple students can team up and each do one 5-digit segment.

## NASA Pi Day

Pi Day gives us all a reason to celebrate the mathematical wonder that helps NASA explore the universe. Students will have a chance to join in the fun by using pi to explore Earth and space themselves in the U.S. space agency’s annual NASA Pi Day Challenge. K-12 students and educators can explore a collection of Pi Day Challenge problems, lessons, articles, infographics, and free downloads, including mobile phone and desktop backgrounds. All of these resources are accessible at the following link: