Think back to your school days, sitting in front of a chalkboard as your math teacher wrote out a long, mysterious number that went on forever — literally.

That number of math classes past is pi. No, not apple pie like your grandmother makes but the number pi, which represents the ratio of the circumference of any given circle to its diameter.

A little rusty on your math skills? Pi is an irrational number, which means it can't be represented as a simple fraction of two integers, and its decimal representation goes on indefinitely without the digits repeating in a pattern like other infinite decimals do (1/7 expressed as a decimal, for example).

Regardless of the size of the circle, the circumference will always equal pi multiplied by the diameter. It's also used to calculate the area of a circle, which equals pi multiplied by the radius squared.

Pi is an irrational number, which when expressed as a decimal goes on indefinitely without its digits repeating in a pattern as other infinite decimals do. To date, it has been computed to 10 trillion decimal places. (iStock)

Today marks the mathematically quirky Pi Day, celebrated because the month and day of today's date — 3/14 — correspond to the first three digits of pi: 3.14. Observed in science and math communities around the world, Pi Day has become an annual event that is recognized on university campuses, in particular.

Larry Shaw, a physicist at San Francisco's Exploratorium science museum was the first to celebrate this day in 1987. Shaw says he decided to honour the irrational number when thinking about "the possibility of entering other dimensions through a rotational motion and contemplated the relation of the linear measurement of pi to a sphere.''

After realizing that March 14 was also Albert Einstein's birthday, Shaw had all the more reason to celebrate the day. Pi Day has since gone mainstream, and in 2009 the U.S. House of Representatives officially recognized March 14 as Pi Day.

1. The word pi is taken from the Greek letter π, which is the 16th letter in the Greek alphabet. In the Latin alphabet, the letter "p" is also the 16th letter. Mathematicians and physicists have a tradition of using Greek letters to represent constants, variables or certain functions and assigned π to stand for the unique mathematical constant that is the ratio of a circle's circumference to its diameter.

Students at Evergreen School in Shoreline, Wash., hold up nearly 5,000 digits of the number pi, each digit written on a link of a paper chain, during the 22nd annual Pi Day on March 14, 2011. (Joshua Trujillo/Seattlepi.com/Associated Press)

2. The ancient Greek mathematician Archimedes is credited with documenting the first calculation of pi in about 250 BC. His approach consisted of inscribing and circumscribing regular polygons with many sides in and around the circle and computing the perimeter of these polygons.

3. If you were to print pi calculated to a billion decimal places in average font size, it would stretch from New York City to the approximate midpoint of the state of Kansas.

And for the other 0.14: visit CBC's Storify looking at how the online world is celebrating Pi Day.

Whether you are a math nerd or not, it is hard to deny the remarkable nature of this famous number. Although usually rounded off to just two decimal places, as of October 2011, pi had been calculated to 10 trillion decimal places — by Japanese systems engineer Shigeru Kondo and U.S. computer scientist Alexander Yee, who already held the Guinness World Record for computing pi to five trillion decimal places.

Honour the nifty number in your own way this Pi Day, but stay tuned for an even more coincidental date. On Pi Day 2015, when the clocks show precisely 9:26:53, the date and time written in succession will correspond to the value of pi calculated to nine decimal places: 3.141592653.