Eratosthenes
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Actually, Eratosthenes of Cyrene (276? - 195? B.C.) was the first to calculate the Earth's circumference accurately. He is supposed to be the inventor.
Pi is the number of times the diameter of a circle will fit into the circumference. Pi is not a circle, and does not have a circumference or area. To calculate a circumference USING pi, the circumference is diameter * pi. To find the area using pi, area = pi * radius * radius
Multiply the diameter by pi to find the circumference
The circumference and area of a circle, with radius r is: circumference = 2*pi*r and area = pi*r2 Use the first to calculate r and then the second to calculate the area.
The Equator is a real world example, being the circumference of the Earth.