哥本哈根的环形桥，丹麦 The Circle Bridge in Copenhagen, Denmark (© Lingxiao Xie/Getty Images)

哥本哈根的环形桥，丹麦 The Circle Bridge in Copenhagen, Denmark (© Lingxiao Xie/Getty Images)

A circular celebration

It's Pi Day, the day we celebrate the mathematical constant π. This day was chosen since today's date (March 14) expressed numerically (in US format) matches the number's first digits, 3.14. Pi represents the ratio of a circle's circumference to its diameter. It can be used to compute the area and circumference of a circle, such as the circles that make up this bridge we're looking at here in Copenhagen. (Who said math wasn't useful in the real world?) Many towns, schools, and museums mark the day with festivities based around mathematics and, of course, serving pie. We can't think of a more delicious way to study geometry.