​What is pi—and why is there an entire day devoted to it?

​The day celebrates one of the world’s most important mathematical constants. And who doesn’t love pie?

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 because it goes on forever [and its sequence] never repeats," Hipschman says. That's probably part of the reason why people are so enamored of it, he adds.

The history of pi

People have known about this number for thousands of years—it even shows up in the Bible. And the fascination cuts across cultures. The Babylonians, ancient Greeks, and even the Chinese have tried their hand at calculating ever more accurate versions of pi.

The famous mathematician Archimedes attempted an exact calculation of pi in 250 B.C. using two 96-sided polygons—one drawn inside a circle and a second one drawn outside. He argued that the value of pi would lie between the lengths of each polygon's perimeter. His was the first theoretical, and most accurate, calculation of pi at the time.

Through the years, people have calculated pi to greater accuracy. But the hunt for even more decimal places really got cracking with the development of computers. Two American mathematicians calculated pi out to 1,120 digits in 1949 using a desk calculator. Currently, pi has been calculated out to 13.3 trillion decimal places, Hipschman says.

How Pi Day is celebrated

People express their love of this unique number in myriad ways, from memorizing and reciting as many digits as they can—the current record-holder got to 70,030 decimal places—to composing haikus and songs.

In 2009, the U.S. Congress declared March 14 as Pi Day with a vote of 391 for and 10 against. "Ron Paul voted against it," says Hipschman, "among others."

Even bastions of higher learning celebrate in their own way. The Massachusetts Institute of Technology releases its admissions decisions every March 14.

Meanwhile, many others around the world honor Pi Day in the most delicious of ways: by baking pies

"People like celebrating," says Hipschman, "especially when pie is involved."

Editor's note: This story was originally published on March 13, 2015. It has been updated.