>>6088938You need a stick (arranged perfectly vertically) and a tapeline. At a certain time during the day (say noon) you measure the stick's shadow. With the length of the shadow and the length of the stick you can calculate the sun's altitude.
Now get together with at least 2 other people around the world (preferably at wildly different latitudes, but same hemisphere: no measurement possible during the night) who do the same at exactly the same time.
So you you get at least 3 different sun altitudes.
Now 1. try to calculate the Sun's height above the Earth if the Earth was flat: the geometry does not work because the lines do not meet in one spot when using 3+ measurements.
Now 2. assume that the Sun is so far away that the sun beams arrive virtually parallel (for our crude measurement devices). Calculate the Earth diameter from that.
It's all high school math. Pythagorean theorem and trigonometric functions (you may use a calculator for this).
