Why do you need to prove them wrong? Have they given you doubt the world is round?
https://physics.stackexchange.com/questions/26427/what-is-th...
https://physics.stackexchange.com/questions/26427/what-is-th...
Have to convert the radicals.
One argument is when the plane lands why doesn't it curve. Another is why can we see over 1500km with a telescope.