Eratosthenes calculated the circumference of the Earth without leaving Egypt. Maybe history used Pyramids bases and shadows to make the measurements of time and distance.If a rod has a diameter of 1 unit it has a radius of .5 units. The rod has a circumference. The circumference is measured in the same units. If you measure the circumference with a length of (theoretical) string, the length of the string is always 3.1416 (rounded off) times 1 unit of diameter. So 29.072 units of diameter means the circumference of the rod is 29.072*3.1416 units. Pi*D=Circumference.

If in reality you measure the string length and it was 3.64 times the length of 1 unit of diameter then pi would be 3.64, but that's not the case in reality. In reality if you measure the diameter and then measure the circumference, it's always circumference/diameter=3.1416 (rounded off).

Indeed, wrap a piece of string, around a circle and the distance is always the value of 2Pi*2r, and this is why the values work.

They are not numeric values just pulled from fresh air, they are measurable values that always ''predict'' the correct results.

I never personally use to see the true value of maths, but I am starting to see why the importance of the values match Physical process.