People who like math and also enjoy eating pie are celebrating today, because it’s Pi Day — a holiday commemorating the mathematical constant symbolized by the Greek letter “pi”. The number’s first three digits are three-point-one-four, which is why it’s celebrated on March 14th. As you probably learned in school, pi is the ratio of a circle’s circumference to its diameter. U.W. Madison math professor Jordan Ellenberg has these thoughts on what makes pi worth celebrating.