Last year, the “Pi Second” occurred on Saturday, March 14, 2015, at 9:26 and 53 seconds. Does that sound crazy? Sure it does. It’s irrational! Pi is the world’s most famous “irrational” number.

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 is a question that people have been working on 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 0.3333…, 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 mysteries of pi to over a trillion digits. The digits keep going. No pattern has ever been found. (Written in an ordinary font, a trillion digits of pi would go around the world 50 times.)

But the endless, patternless nature of pi enchants many minds and some delight in memorizing the digits. A Chinese man, Chau Lu, set the record after studying for four years: he recited 67,890 digits of pi.

Can you see a date in the first three digits: 3.14? Since 1988, March 14th of every year has been celebrated as Pi Day. Students, teachers and math enthusiasts worldwide enjoy pi-themed activities, clothing, jokes and food (namely pie). Can you think of another “holiday” that has gone international in just 27 years?

This year is an ordinary year as far as Pi Day is concerned, but last year was really special. After 3.14, the next two digits of pi are 15. So March 14, 2015, was not just any old Pi Day. It was the “Pi Day of the Century.” You’ll have to wait until March 14, 2115 for another one.

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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