## The mystery of pi

Pi is a mathematical mystery that’s captivated people for thousands of years. There’s even an entire holiday dedicated to this mystery, called Pi Day (which is on March 14). There are numerous ways to celebrate Pi Day, from cracking pi jokes to learning more about the never-ending number. We couldn’t believe some of these fascinating pi facts – how many do you know? Read on to unlock parts of the mystery.

## People have been using pi for thousands of years

Pi (the Greek letter π, pronounced like the word “pie”) is the ratio of the circumference of any circle to the diameter of that circle, explains maths instructor Steven Bogart. It equals roughly 3.14. No matter what size a circle is, the circumference will always be 3.14 times bigger than the diameter. Over 4,000 years ago, the ancient Egyptians and Babylonians had figured out this constant and were using it to make calculations. In the 18th century, mathematicians gave the number the name “pi.”

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## We celebrate pi on March 14

Back in 1988, Larry Shaw of San Francisco’s Exploratorium science museum started observing March 14 – get it? 3/14! – which also happens to be Albert Einstein’s birthday, as Pi Day. By 2009, the celebration had grown so big that US Congress passed a resolution to make the designation official. The resolution states: “The House of Representatives supports the designation of a ‘Pi Day’ and its celebration around the world … and encourages schools and educators to observe the day with appropriate activities that teach students about Pi and engage them about the study of mathematics.” In another stamp of approval, in 2010, Pi Day got its own Google Doodle.

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## Pi is a never-ending number

Pi is an irrational number. It can’t be expressed as a fraction; it doesn’t end with a repeating pattern (like the decimal expression of 1/3, 0.33333…, in which the threes repeat forever), or terminate after a certain number of decimal places (like 3/4, or .75). It just keeps going, going, and going. So far, pi has been calculated to over 22 trillion digits. It took a computer with 24 hard drives, working nonstop for 105 days, to make that calculation.

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## The digits of pi after the decimal point are random

Here’s a mind twisting pi fact: the trillions of digits of pi that have been calculated continue without any discernible pattern. Mathematicians have been looking for those patterns for centuries, but as far back as 1768, a self-taught Swiss-German mathematician and astronomer named Johann Lambert proved that pi is irrational.

## More pi isn’t necessarily better

While we know pi to more than a trillion places, we really don’t need them. Scientists can determine the spherical volume of the entire universe using just 39 places past the decimal, according to piday.org. NASA’s Jet Propulsion Laboratory only uses pi up to 15 decimal places for its robotic space and earth science missions. “For JPL’s highest accuracy calculations, which are for interplanetary navigation, we use 3.141592653589793,” explains engineer Marc Rayman. “There are no physically realistic calculations scientists ever perform” that would require more decimal points than that.

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## “Pi” is much shorter than the number’s previous name

The number’s connection with the Greek letter π is actually pretty recent, considering that people have known about the quantity since the time of the ancient Babylonians. A British mathematician named William Jones was the first person to call the quantity π, in 1706. People theorise that he chose pi because it represents the Greek letter P, and pi can find the perimeter of a circle. People have found maths books from before 1706 that refer to the number as a lengthy Latin phrase that translates to “the quantity which, when the diameter is multiplied by it, yields the circumference.” Anyone who talks about math for a living, owes this Jones fellow one!

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## Someone has memorised 70,000 decimal places of pi

The Guinness Book of World Records recognises Rajveer Meena as the champion memoriser of digits of pi. On March 21, 2015, at VIT University in Vellore, India, Meena recited pi to 70,000 places past the decimal point. A 21-year-old student at the time, Meena proved his powers of memory by reeling off the numbers while wearing a blindfold. It took him more than nine hours to do it.

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## You can borrow expert memorisers’ techniques

How in the world does someone memorise a string of 70,000 random numbers? Most record-holders (or just interested hobbyists) use an association technique. They bunch smaller groups of numbers together and memorise those: 14, then 15, then 92, then 65, and so on. Or they may look at each set of nine digits as a telephone number and memorise them that way. Another strategy is to match each digit or small groups to a word, then make a story out of those words. Yet another method is spatial visualisation, in which you picture a familiar place, then assign numbers to different spots in that place. To recall them, you mentally walk through the space and see the numbers as you go.