Eristothenes measured the circumference of the earth by measuring the shadow of two poles, miles apart, at the exact same time. So how did he know the poles were being measured at exactly the same time in an era when people used the sun for timekeeping?

Question

Eristothenes measured the circumference of the earth by measuring the shadow of two poles, miles apart, at the exact same time. So how did he know the poles were being measured at exactly the same time in an era when people used the sun for timekeeping?

In: Mathematics

Answer 1

He didn’t measure two poles. He measured one pole and knew that on one day out of the year where the sun fell directly in a well in a distant city at solar noon. So on that day, he measured the length of the shadow at solar noon then did his trig calculations.

He knew it was the same time because it was the right day (via a calendar) and the right time (the solar noon/highest point in the sky the sun would reach).