How do you prove that the world is round?

When you think about it, so many people must take this for granted, just like people used to believe that the earth was flat. I know that there is a proof, or at least some evidence. Photos can't count as evidence anymore, not in this day and age. They can be tampered with quite easily. So, how do you... show more When you think about it, so many people must take this for granted, just like people used to believe that the earth was flat. I know that there is a proof, or at least some evidence.
Photos can't count as evidence anymore, not in this day and age. They can be tampered with quite easily.
So, how do you actually prove the world is round?
Update: If the Earth were a disk, then you could get a round shadow as well. Not a great proof. Going by the ancient idea that the sun and moon rotated around the disk of the world, that could account for the difference in shadow lengths. Different places will be at slightly different angles to the sun. No proof. The... show more If the Earth were a disk, then you could get a round shadow as well. Not a great proof.
Going by the ancient idea that the sun and moon rotated around the disk of the world, that could account for the difference in shadow lengths. Different places will be at slightly different angles to the sun. No proof. The sun orbiting the disk explains day and night as well.
Jared's is looking best. The only thing I can think of to discredit that is that the atmosphere could possibly deflect enough light over a large distance so as to let absolutely no light through. That could explain it.
Travelling the world is certainly a way to prove it, but it's not very practical. Lots of people would have to base it on faith that you can do that.
Update 2: This is much harder to prove than I'd expected. How amusing.
Update 3: Arthur, I haven't asked this question before. Check before you make insulting comments.
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