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.