Number geeks—and pie aficionados—around the world have an excuse to indulge themselves today. It’s Pi Day, when the calendar reflects one of the world’s most important mathematical constants. 

Pi is a circle's circumference—the distance around it—divided by its diameter, or the distance through its center. The relationship holds no matter the size of the circle: The rim of a soup can and a planet's equator both yield a ratio of pi. 

This rather simple-sounding description belies some rather fascinating qualities, says Ron Hipschman at the Exploratorium in San Francisco. Museum staff held the first Pi Day celebration in 1988 by eating pie, and have carried on the tradition every year since, adding a Pi Day "shrine" and a procession. 

"Pi is interesting