Today is Pi Day 2019, when we celebrate the 'most important number in maths.'

Pi Day is celebrated on March 14 every year, due to the fact that this is depicted as 3.14 (the first three digits of Pi) in the American calendar.

In a rather strange coincidence, March 14 is also the anniversary of Albert Einstein's birth, as well as the Stephen Hawking's death.

The number is used in maths to represent the ratio of the circumference of a circle to its diameter.

While this is roughly 3.14, Pi is actually irrational, meaning there's not a finite number of numbers when written as a decimal.

### Who invented pi?

Pi was first devised by Ancient Greek mathematician Archimedes (287-212 BC).

However, it wasn't widely used until 1737, when it was adopted by Swiss mathematician, Leonhard Euler.

### What is pi used for?

Those of you who did GCSE maths may remember some of these handy equations:

**The area of a circle = πr2**

Where ‘r’ is the radius (distance from the centre to the edge of the circle).

**The circumference of a circle = πd**

Where ‘d’ is the diameter (distance from one edge of the circle to the other).

### How does 22/7 fit in?

You may have heard about Pi Approximation Day, which falls on July 22.

The fraction 22/7 is a common approximation of Pi.

### The mathematical magic of pi

Writing in the * New Yorker * , mathematician Steven Strogatz said: "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 three trillion digits beyond its decimal point.