Hello, I am a 3rd year physics student, and I'm here to put and end to this discussion...
The reason why earth looks round from up in the sky is as follows. I'm sure you all have seen that when you put a pencil halfway in a glass of water, it appears to be broken. This is because water is denser than air and light rays bend when the density of the medium changes. This is known as the Snell Law.
Have you ever seen on airplanes' info channels the outside temperature? It goes all the way down to a few degrees, because atmosphere gets colder as you ascend. Cold air is denser than hot air - this is actually why hot air balloons fly! So, when you are looking down from a plane to the surface of the earth, you're looking through the atmosphere that gets warmer (and therefore less dense) as you descend. This causes light rays from the earth to bend and makes the surface look curved - just like the pencil in the water getting broken. It's not a sharp break as in the pencil example but a smooth curve, because the temperature and so the density changes gradually, not suddenly.
This was one of the things that fooled our ancestors into believing earth is round. The so called "fact" really shook the religious world because it was against what was believed so far. Then, when the scientists realized they were wrong, they just decided to play along with the whole scam because if they revealed their mistake, religion would get back its power and they would lose the eternal battle. Please, think about it, it's all out there, it's the biggest scam on earth's history!