SAN FRANCISCO (USA TODAY) -- On Friday, millions of people worldwide will celebrate both a mathematical constant and a sweet filling in a wheaten crust. In other words—Pi Day.

Pi is the ratio used to compute the circumference, area, and volume of circles.

For those who didn't memorize it in middle school, that works out to 3.141592653 for the first ten digits.

It's written using the Greek letter π, pronounced "pie."

Schools often mark it by taking part in the math equivalent of the spelling bee—Pi recitation contests.

The winners, naturally, get pie.

Tech and science companies love Pi Day. In Massachusetts, Raytheon Company employees will deliver apple pies to middle and high school math teachers within 3.14 miles of company offices in Arizona, California, Florida, Maryland, Massachusetts and Virginia.

The Google Doodle (the art that sometimes graces the search engine's search box) typically honors Pi Day with a clever graphic representation of how it works.

Pi is interesting in part because it's an irrational number. That means it continues indefinitely, without repetition or pattern. Which makes memorizing it a fun challenge.

Thus far pi has been calculated out to more than two trillion digits, but of course its length is infinite.

That's how Mr. Spock in Star Trek managed to stop an evil entity that has taken over the Enterprise's computer system. He ordered it to "compute to the last digit the value of pi."

Pi Day gets extra credit because it's also Albert Einstein's birthday. The German theoretical physicist was born on March 14, 1879.

Pi Day was first celebrated in 1988 at San Francisco's Exploratorium science museum. There physicist Larry Shaw oversaw Pi recitations combined with people eating pie and walking around in circles. It's since spread to an international, math-focused celebration.

National Pi Day is actually a U.S. holiday. The House of Representatives passed House Resolution 224 in 2009, designating March 14 as National Pi Day. The resolution "encourages schools and educators to observe the day with appropriate activities that teach students about Pi and engage them about the study of mathematics."

Next year's Pi Day will be even more exciting. On that day, math geeks will get one, shining moment in which they can write the date as: 3/14/15; 9:26:53. Which, everyone knows, are the first ten digits of Pi in perfect order.

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