It’s Pi Day, the calendar date that when written (albeit in the American format) resembles the first digits of the mathematical constant Pi – 3.14.

## Pi Day

Although Pi itself has been discussed and used in mathematics since the Babylonian Empire and the Ancient Greeks, 2018 is the 30th anniversary of Pi Day, an annual event first recognised in 1988 by physicist Larry Shaw.

In America, in particular, maths fans celebrate by eating a slice of their favourite pie and for Pi Day 2018, Google has created a Google Doodle alongside a Salted Caramel Apple Pie recipe. The Doodle uses ingredients from the recipe to form the letters of the word Google, and it was baked and built by pastry chef Dominique Ansel.

Google last celebrated Pi Day with a Doodle in 2010. Pi Day, or 14 March, is also the date of Albert Einstein’s birthday which Google celebrated with a Doodle in 2003. Einstein would have been 139 this year. It is also the date Stephen Hawking died.

Elsewhere, to mark Pi Day, NASA has released its Pi In The Sky challenge – a series of maths problems NASA scientists and engineers have to solve using Pi.

The first, titled Solar Sleuth, asks you to use Pi to determine the radius of the exoplanet Kepler-186f. In its description, NASA explains: “Scientists can then determine the size of the exoplanet based on how much the star’s light dipped when the planet passed in front of it. If the Kepler detects a 0.042% drop in brightness from the star Kepler-186, which has a disk area of 416,000,000,000 km2, what is the radius of Kepler-186f?”

The second, called Helium Heist, asks you to establish how many Earth-size spheres of helium have been rained out of Jupiter’s molecular hydrogen layer.

There is a total of 20 challenges to solve and NASA said it will put the answers to the 2018 NASA Pi Day Challenge on NASA’s Pi In The Sky challenge hub on 15 March.

**Take the NASA Pi In The Sky challenge**

And of course, Raspberry Pi Foundation got in on the action with the launch of its Raspberry Pi 3 Model B+.

### What is Pi?

At its most simple, Pi is a number that is a little larger than 3.14, but of course it’s more complex than that. Pi is used to work out the area of a circle (A=Pir²) and is the number you get if you divide the circumference of any circle by its diameter, meaning it’s the same for all circles.

Mathematicians in the Babylonian Empire, in around 2000 BC, established that Pi was around 25/8, or 3.125. Egyptian mathematicians calculated Pi to be approximately 3.16. In the 3rd century AD, Chinese mathematician Liu Hui calculated Pi was 3.14, and this has been built on ever since. The name comes from the Greek letter **π**, or “p”, because it’s the first letter of the Greek word “perimetros” or perimeter.

Since it was first discovered, Pi has been calculated to more than one trillion digits beyond its decimal point. It’s classed as an irrational, and transcendental number, meaning it will “continue infinitely without repetition or pattern”. You can see Pi to ten million digits here.

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