Pi is the ratio of the circumference of a circle to its diameter. Regardless of the size of the circle, pi is always the same irrational number: approximately 3.14. Twenty years ago, physicist Larry Shaw created a Pi Day celebration on March 14 at the Exploratorium Museum (see first site review) and it has grown into an international day of wacky mathematical celebrations at schools and universities around the world. July 22 (22/7 in European date format) is often also celebrated as Pi Approximation Day.
If you can't make it to San Francisco's Exploratorium for the twentieth anniversary of the first Pi Day, you'll find lots here to inspire your own celebration. Highlights include a Short History of Pi, Pi Limericks, Pi Haiku (called Pi-Ku), printable Pi Posters, and two pi activities. Discovering Pi is a hands-on activity measuring circles and recording their circumference and diameter. Cutting Pi is an exercise exploring the relation between circumference and hat sizes.
"For the sake of usefulness people often need to approximate pi. For many purposes you can use 3.14159, which is really pretty good, but if you want a better approximation you can use a computer to get it." Dr. Math answers basic questions about pi, and includes an extensive list of links to other pi-related questions and answers from the Math Forum archive. The little red star icons indicate "particularly interesting answers or good places to begin browsing."
Eve Andersson is a Google engineer with a soft spot for pi. Her homage to pi includes a Pi Trivia Game consisting of twenty-five randomly chosen questions ("Finally this is your chance to pay tribute to the magnificent transcendental number that we have all grown to love!) and a Pi Trainer to test your memory of the digits of pi. Under Aesthetics, she publishes a few pi poems and a small photo gallery. Nerd Stuff includes the digits themselves, and graphs of digit frequency.
"Besides being a center for teaching ideas and resources, we'll try to be your first stop for funny, smart, tongue-in-cheek tributes to the number pi." The most popular page on the site is the rap song Lose Yourself (In the Digits), a song about the pressure of reciting the digits of pi on stage in front of a crowd. Other crowd-pleasing clicks are the more than fifty ideas for celebrating Pi Day, stories about people important to the development of the concept of pi, and lyrics to three pi songs to be sung to familiar melodies.