>>8792427Radio transmissions prove it.
If you set up a radio antenna that broadcasts in Ultra High Frequency or Very High Frequency, the transmission wave length will punch through the ionosphere and can only be received via line of sight. Once you move far enough away, you will lose the transmission due to the curvature of the earth. An identically powered High Frequency transmission on the other hand will bounce off of the ionosphere and can be received well beyond UHF or VHF range, including on the other side of the Earth.
If the Earth were flat, you would be able to pick up Denver UHF/VHF TV and Radio channels on the ground in New York.
