This got buried in the questions thread, and I never got a satisfactory answer.
When I look at the moon from Australia, I see it as upside down to the moon I see when I'm in the northern hemisphere. It's trivial to understand why this is so on a globe. But no so trivial on a flat earth.
Let's see if we can explain why the flat earth model fails, and incidentally, I will also show how it could succeed.
Imagine the flat earth model, the moon orbits roughly over the equator, which is a circle on the flat earth. For the sake of orientation, I am going to paint a big red dot on the bottom half of the moon, by bottom, i mean the part of the moon that is closest to the horizon as I see it.
Now looking at the moon from Australia, I see the big red dot at the bottom of the moon, so now I go to England, and look at the moon, lo and behold if the earth is flat, the big red dot is still at the bottom of the moon. So the moon is the same way up when seen from either hemisphere. The flat earth model fails to explain why, in reality i see the moon upside down.
Now, let's throw the flat earthers a lifeline to cling to. Imagine the same scenario, as before but this time the moon is a flat disk oriented horizontal to the earth, I place a red dot on the part that looks to me like it's the bottom of the moon, now when I go to England, lo and behold the image has flipped and the red dot now appears on top. Finally we have a workable explanation.
So, in conclusion, in order for the earth to be flat, the moon must be flat as well, if the moon is a sphere then so is the earth. QED.