Take a circle, any circle, and divide the circumference by the diameter. The quotient is the number called pi, represented by the Greek letter π. It is a little more than three. How much more? That’s a question that people have pondered for centuries.

Pi is an incredibly useful number in mathematics, physics and engineering. It helps us understand things from the shape of an apple to the energy of stars. It helps us design things, from buildings to spaceships.

Pi is an irrational number. That means when you write it as a decimal, its digits do not just end (like 3.5) and they do not repeat in a pattern (like 0.333, where the 3s go on forever).

Here is a slice of pi: 3.141592653… The “dot-dot-dot” means the digits keep on going. How far? Is there a pattern?

With supercomputers, mathematicians have probed the mystery of pi to over a trillion digits. No pattern has ever been found. Written in an ordinary font, a trillion digits of pi would go around the world 50 times.

The digits of pi go on forever. That does

*not*mean that pi is an infinitely large number, as people sometimes think. Pi isn’t a large number at all. It’s only a little bigger than 3.

The endless, patternless nature of pi enchants many minds, and some people delight in memorizing the digits. Children have memorized hundreds of digits of pi. After studying for four years, a Chinese man, Chau Lu, set the record: he recited 67,890 digits of pi!

Can you see a date in the first three digits of pi? March 14th of every year has been celebrated as Pi Day since 1988. Students, teachers and math enthusiasts worldwide enjoy pi-themed activities, clothing, jokes and food (namely pie).

March 14, 2015, was not just any old Pi Day. It was the “Pi Day of the Century" because, after 3.14, the next two digits of pi are 15. So 3.1415 denotes March 14 of the year 2015. You’ll have to wait until March 14, 2115 for another Pi Day so special.

Wait! There's more!! Take pi out five more digits and you get 3.141592653. That means that on March 14, 2015, at the time 9:26 and 53 seconds, we had a very special moment: the "Pi Second." I hope you enjoyed your Pi Second!

Happy Pi Day, everybody!

David Schwartz probes many mathematical mysteries in his books and school presentations given all over the world. He wrote this Nonfiction Minute while celebrating Pi Day at Tashkent International School in Uzbekistan. He is a member of iNK's Authors on Call and is available for classroom programs through Field Trip Zoom, a terrific technology that requires only a computer, wifi, and a webcam. Click here to find out more.

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Some of the Native American languages were so difficult that our enemy code breakers could not figure out how decode our wartime messages. It was many years before the heroic activities of the Native American Code Breakers were declassified, but tomorrow Jan Adkins can tell you their story.