I don't know if someones posted this yet, but there's a simple way to prove RET. A lunar eclipse occurs when the moon passes behind the earth so that the earth blocks the sun's rays from striking the moon. A side effect of that is the fact that you can actually see the shadow of the Earth in the moon. When it begins to eclipse, we can see the reflection of the curve in the earth.
Now, if you have the time and are in the right place, turn off all your lights except for one desk lamp or the light of your computer. Then hold out your hand. The shadow of your hand looks like a hand (obviously). However, if you rotate your hand (to simulate looking at it from a different angle) it looks like a thick, imperfect line.
Now apply this to the Earth. If you look at the Earth's shadow on the moon from any angle it looks like a a curve. However, if we accept that its a disk, then from the south it should look like a distorted disk. This never looks like this at any angle. This has never been recorded.
Therefore, the Earth cannot be a disk.