Hi Serena.

No one invented it - it is a simple fact of nature that describes the ratio between the diameter of a circle and its circumference.

The name "pi" however is a greek letter and so we get its current name from the Greeks.

According to Wikipedia , we have estimates of pi dating back to 1900 BCE. Egyptian, Babylonion approximations were 25/8 and 256/81 which are both quite good. By the fourth century CE, Chinese mathematicians came up with 355/113 as a better approximation of pi, and this was the best widely-used approximation for about 900 years.

Today, we have done enormous advanced mathematical research and by using many computers working together for years, we have approximations of pi that are billions of digits long. You can even download these if you search on the internet, but I'm not sure you'd have much use for it!

Hope this helps,
Stephen La Rocque.

I would like to add:

While pi is a Greek letter, the Classical Greeks did not use it to represent the ratio of the circumference of a circle to its diameter; they used it to represent 80! (They used letters, in roughly aphabetical order, for 1,2,3,...,9,10,20,30,...90,100,...900. As they had only 24 letters, they brought back three obsolete letters "digamma", "koppa" and "sampi" to help out.)

Using it to represent 3.14159265358979... goes back only to the 18th century, when it was introduced by William Jones. (While Sir Isaac Newton was still alive at this time, he had basically given up mathematics in favor of theology and working as Warden of the Mint. So he probably never used the modern symbol.)

The ratio (as opposed to the name) was discovered by the ancient Bablylonians and/or Egyptians in about 1900 BC, long before Classical Greek civilization (or even the Homeric period.)

-RD