1. There is no feasible way to explain our observations of the sun in a flat-earth model. Firstly, the sun would start small, grow larger, and then get smaller again every single day as it passes overhead. In addition, if it travels at a constant speed over the whole disc-shaped earth, it would appear to speed up as it passed overhead then slow down again as it moves away (in the same way, if you watch a fast car drive past, your head turns fastest at its nearest point). Finally, there could be no seasons in the way we observe them - for the sun to make a full circle of the flat-earth in one day, it would have to move faster then further away it got from the us (in winter) as it has a bigger circle to move around.