Today is 22/7. No prizes for guessing what that means – yes, its Pi Approximation Day! March 14th (3.14) is also celebrated as Pi Day, but I kind of prefer the 22/7 version.

Pi, that curious little number that seems to pop up every time we start going in circles. A number so important that it even got its own name – not many numbers can claim that distinction!

Pi, or *π*, is a mathematical constant that represents the ratio between a circle’s circumference and its diameter, or *π* = C/d. It is what’s known as an irrational number – a number that cannot be expressed as a ratio between two integers. Being irrational, it has an infinite number of digits in its decimal representation, and it does not end with a repeating sequence of digits. It is also a trancendental number – a number that cannot be expressed with a finite sequence of algebraic operations.

In addition to its application in geometry and trigonometry, the constant *π* is found in many formulae, in a variety of sciences, including physics, number theory, thermodynamics, statistics, electromagnetism and mechanics.

The value of *π* (to 5 decimal places) is 3.14159, which is also approximately the value of 22 divided by 7. Calculating the value of *π* to higher and higher degrees of accuracy have been a challenge to mathematicians and computer scientists through the ages. Utilising the latest computing technology, the digital representation of *π* has now been calculated to more than 10 trillion digits. Memorising *π* to a large number of digits (a practice called *piphology*) is another challenge taken up by many pi-fanatics, and the current record stands at an astounding 67 890 digits, recited in 2005 in China by Lu Chao over a period of more than 24 hours. *(Wow, he probably doesn’t get out much!)*

A nice trick to remember the first few digits of pi is to use a poem or sentence where the lengths of the words correspond to the digits in pi. A well-known example, courtesy of English scientist James Jeans, is *“How I want a drink, alcoholic of course, after the heavy lectures involving quantum mechanics”*, cleverly representing pi’s first 15 digits.

Such is the pervasiveness of the number *π* that it can even boast numerous appearances in modern popular culture, from TV series (Simpsons, Twin Peaks) to novels (Carl Sagan’s “Contact”) to pop music (Kate Bush’s “Pi“).

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