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When you measure a circle, its width always comes out at just over 3 times its circumference. Don’t ask us why, it just does. Whether it’s a button, a plate, or a car tyre, no matter the size of the circle, the ratio is always the same – around 3.14159265, commonly known as Pi.

Pi pops up in all sorts of formulas across the worlds of physics and maths - you remember learning about it in school, right? Academics the world over see it as perhaps the most important and intriguing irrational number that’s ever existed (i.e. it never terminates or repeats). And we agree with them.

Geeks everywhere celebrate Pi Day on March 14 (written in the US as 3/14) by getting involved in good stuff like pie eating, pie throwing and games to see who can memorise the most digits of the infinite number.

So, how much do you know about pi?

**1. Origins in Babylon**

The first written record of pi is attributed to the ancient Babylonians. A tablet (of the stone variety), dating from between 1900 and 1680 B.C., calculates pi at 3.125. Impressively close to the modern calculation^{1}.

**2. A Greek perimeter**

Pi (π) is the 16th letter of the Greek alphabet (just as p is the 16th letter of the English alphabet), chosen because it’s the initial letter of the Greek word ‘perimetros’, meaning ‘circumference’.

**3. Einstein’s birthday**

It feels quite fitting that legendary theoretical physicist, Albert Einstein - who probably knew more about pi than most people ever will - was born on Pi Day: March 14, 1879.

**4. Trillionaire’s row**

As an infinite decimal, pi is a number with no end. Super-geeks have managed to calculate it to over two trillion digits past its decimal point^{2}.

**5. It’s universal**

Only 39 digits past pi’s decimal are needed to accurately calculate the volume of our observable universe, to within the width of a single hydrogen atom^{3}. Now reread number 4 again. Mind, literally, blown.

**6. A game of Pi**

Memorising the digits of pi in the correct order has become a global phenomenon. The world record holder is Rajveer Meena of India, who memorised the first 70,000 decimal places in 2015^{4}. It took him 10 hours and he was blindfolded the entire time.

**7. Fun with mnemonics**

Techniques for memorising pi include grouping the digits into sets of four to make them easier to remember, like a telephone number. But a poem like this could be equally helpful – count the letters in each word:

*Sir, I bear a rhyme excelling. In mystic force and magic spelling.*

**8. The Pyramids are made of pi**

The vertical height of the Great Pyramid at Giza has the same relationship to the perimeter of its base as the radius of a circle has to its circumference – a link to pi that has fascinated Egyptologists for centuries.

**9. Pi and the death of Archimedes**

Greek mathematician, Archimedes, devoted much of his time to pondering pi. In fact, he was musing towards his next discovery when a Roman soldier burst in and bumped him off. His last words are said to have been, ‘Noli turbare circulos meos’ (do not disturb my circles).

**10. Pi me a river**

If you divide the length of a naturally flowing river from source to mouth across a gently sloping plane, by its direct length as the crow flies, the answer is always approximately 3.14.^{5}

We’ll be celebrating this marvel of mathematics with the most appropriate thing we can think of... a slice of pie (we could even use pi to figure out the circumference and divide the slices equally).

And what better pie than like your nan used to make with big chunks of Bramley apple in rich shortcrust pastry? We’ve even found a failsafe recipe for you.

Happy Pi Day.