And as it's also the anniversary of Albert Einstein's birth, and now, sadly, the date of Stephen Hawking's death, it puts maths and science firmly into people's minds - often helped by a Google doodle.
Pi (Greek letter π) is the symbol used in mathematics to represent a constant - the ratio of the circumference of a circle to its diameter - which is approximately 3.14159.
The diameter of a circle is the distance from edge to edge, measuring straight through the centre. The circumference of a circle is the distance around.
Who invented Pi?
Devised by Ancient Greek mathematician and general polymath Archimedes (287-212 BC), pi is an irrational number, meaning it continues infinitely without repetition or pattern.
Calculating pi
Since November 2016, we have been able to write more digits of pi than ever before. An extra 9 trillion digits after the decimal point have been discovered, smashing the previous world record set back in 2013.
The extra digits were discovered by R&D scientist Peter Trueb, who used a computer to calculate pi to 22,459,157,718,361 digits.
"I was really surprised that it worked so smoothly, I was so happy," Trueb told New Scientist.
How 22/7 fits in
Pi Day should not be confused with Pi Approximation Day, which falls on July 22 (22/7 for Americans). The fraction 22/7 is a common approximation of Pi.
The mathematical magic of Pi
Mathematician Steven Strogatz, writing in the New Yorker, explains why pi continues to fascinate:
"The digits of pi never end and never show a pattern. They go on forever, seemingly at random - except that they can't possibly be random, because they embody the order inherent in a perfect circle.
"Pi touches infinity in other ways. For example, there are astonishing formulas in which an endless procession of smaller and smaller numbers adds up to pi.
"One of the earliest such infinite series to be discovered says that pi equals four times the sum 1 - 1/3 + 1/5 - 1/7 + 1/9- 1/11 and so on."
Pi has been calculated to over one trillion digits beyond its decimal point.
Here are some incredible facts about the ‘most important number in maths’:
1. Origins of the symbolThe symbol (Greek letter “π”) was first used in 1706 by William Jones. A ‘p’ was chosen for ‘perimeter’ of circles, and the use of π became popular after it was adopted by the Swiss mathematician Leonhard Euler in 1737.
2. Google gets ahead
If you type ‘pi’ into google the calculator will appear with 3.14159265359 already entered.
3. Thanks a million
Want to see pi calculated to one million digits? Click here.
4. What a coincidence...If you hold a mirror to a circle, it looks like a circle. If you hold a mirror up to 3.14, it spells PIE!
5. Blow your mind
Only 39 digits past the decimal are needed to accurately calculate the spherical volume of our entire universe.
6. Useful formulae with pi
The number pi is extremely useful when solving geometry problems involving circles, such as:
The area of a circle: A = πr2
Where ‘r’ is the radius (distance from the centre to the edge of the circle). Also, this formula is the origin of the joke “Pies aren't square, they’re round!”
The volume of a cylinder: V = πr2h
To find the volume of a rectangular prism, you calculate length × width × height. In that case, length × width is the area of one side (the base), which is then multiplied by the height of the prism.
Similarly, to find the volume of a cylinder, you calculate the area of the base (the area of the circle), then multiply that by the height (h) of the cylinder.
7. Anyone for pi-ball?
A sport based on pi has was invented in South Africa in 2008 – supposedly on Pi Day.
It’s played on a circular court around a central circular ball strike area. It’s comparable to beach volleyball, but where two players on a team are separated by a net, pi-ball players instead stand diametrically opposite one another within the confines of a circular court.
8. It's official
Pi Day is officially recognised by Congress in the US.
9. No nothingIn the first 31 digits of Pi, there is not one zero.
10. A repeating pattern
314159, the first six digits of Pi, appear in order at least six times among the first ten million decimals of Pi.
11. Illogical, captain
In the Star Trek original series 'Wolf in the Fold' episode, Spock destroys a computer taken over by Jack the Ripper (yep, really) by commanding it to compute the last digit of Pi.
12. Pi recall world recordA Japanese man holds the world record for remembering the value of Pi to 111,700 digits. Akira Haraguchi spent 16 hours, 30 minutes reciting the numbers in an epic public event near Tokyo - although his record has not been validated. China's Chao Lu is the official holder, with just 67,890 places.
13. The full 360The number 360 (as in degrees of a circle) is the 359th digit position of Pi.
14. A biblical reference
Pi is mentioned in the Bible , although slightly imprecisely. The relevant passage is 1 Kings 7:23, which states “Now he made the sea of cast metal ten cubits from brim to brim, circular in form, and its height was five cubits, and thirty cubits in circumference.” This verse describes a cylindrical vessel built at the order of Solomon.
Dividing the circumference (30 cubits) by the diameter (10 cubits), gives 3.