Sky is not shaped half of a ball if earth is flat
If earth is flat, then the sky is shaped straight line. And the sun isn't rise and set, but approach and leaves.
Flat earthers who said sinking appearance of leaving objects is optical illusion is not right. Because of earth shape, it is happening. Bigger planet, further horizon.
Note: This post is written lazily, and I'm too lazy to make quality posts now. :)