You don't need to communicate in real time. You just need to agree on which date to do the measurement and that is determined by reference to the stars. Pries

You don't need to communicate in real time. You just need to agree on which date to do the measurement and that is determined by reference to the stars. Priests had been maintaining the calendar already for quite some time. Then you just wait for the sun to reach the zenith, noon. Now you can measure the angle of the sun from the vertical then it's relatively simple trigonometry to calculate the circumference. See https://en.wikipedia.org/wiki/Earth%27s_circumference https://en.wikipedia.org/wiki/Earth%27s_circumference