How did the Greeks prove that the Earth is round?
They measured the depth of the shadows in vertical wells at midday in 2 different locations.
The method works equally well for vertical poles, but requires you to be on the same line of longitude.
Imagine we are in Athens and agree that you will walk (exactly) south for 6 months, then find a well and on June 21st (the day the sun is highest at midday) measure how deep the sunlight reaches in your well.
I walk north for 6 months and do the same. We also agree to meet up one year from now and compare results. Clearly your shadow will be deeper down your well as the sun rises higher for you nearer the equator.
With a bit of Maths you can find out that the Earth is (approximately) a sphere e.g.
This shows the method using a vertical stick or pole, but the Maths is the same!