Eratosthenes Experiment

In 205 BC, Eratosthenes determined the circumference of the Earth by measuring the length of the shadow cast by a rod. He did the measurement in Alexandria and timed it to coincidence when the Sun is directly overhead Syene.

Using the flat-Earth model, the length of the shadow will have a linear response. On the other hand, using the spherical Earth model, the length of the shadow has a non-linear response.

Eratosthenes did his experiment by taking measurements in two locations: Alexandria and Syene. But we can easily modify his experiment by making measurements in three or more places, and the flat-Earth model is instantly proven wrong.

We can also observe that in near-polar locations, the experiment will result in a very long shadow. The flat-Earth model cannot explain this phenomenon.