The easiest way to prove that the earth is not flat

Compass, type measurer, pen and paper. Everything you need.

Observe the shadow of an utility pole in your neighborhood at high noon, and when the shadow is moved to its shortest position, call your friends live at different time zooms and ask them to observe their poles and measure the length of the poles as well as the length and direction of the shadows. Measure the length of the poles on the ground at a 45-degree angle.

Analyze the data. Draw your own conclusion and try to calculate the size of the earth.