How long has pi been around?

For at least 3,000 years.

'pi' is the small case Classical Greek letter 'p', and means 'proportion'.

The Sumerian civilisation is thought to have calculated it.

They noticed that when a donkey is tethered to a pole to drive up water, it went round in a circle. The distance around the circle(circumference) was greater , the longer the tether was made (radius).

From this they deduced that the circumference was directly proportional to the diameter * 2 radii).

In algebraic terms

C is directly proportional to d

Equating

C = d * k ( k is a constant)

Hence dividing by 'd'

k = C/d

K was found to be always 3.141592.... and was given the name 'pi' , for proportion.

Hence today we use C = pid or C = 2pi*r

If you are learning about this in school , you will probably be given pi = 3.14, but this is only an approximation. A closer approximation is pi = 3.1416

However, 'pi' is an IRRATIONAL number, which casually means, the decimals go to infinity and are not in any regular order. Supa Dupa computers have calculated 'pi' to billions of places and still going.

A Classical Greek mathematician , Eratosthenes, in approximately 220 BC and using 'pi' calculated the Earth's circumference to a very high degree of accuracy. It took Christopher Columbus 1600 more years to actually prove the point.