Of all the discussions I've encountered in the stream of flat Earth theory, I assumed that the retrograde motion of Mars would be one topic that flat-earthers would have left alone. My assumption was based on the fact that basic geometry can easily show that Mars' retrograde motion is impossible on the flat Earth model.
Spend enough time talking to flat-earthers and you'll find that they are easily fooled by their own senses. For example, in the live feeds from the ISS and some satellites, it's hard to see the clouds move, so naturally the images/videos must be fake, right?
The curve of the Earth can be observed by experimenting with the horizon over varying distances. However, we have additional proof of Earth's roundness in the rising and setting of the sun. If the Earth was flat and the sun was always above the plane of the Earth, then why is it's light not always visible and why can we not see it?