The Flat-Earthers are unable to account for many simple observations.
For example:
1) The fact that the stars all appear to revolve around Polaris (North Star) and AT THE SAME TIME around a point with no bright star in the southern hemisphere.
2) The fact that at the equator, you can take one unchanging heading by gyroscopic reference and if you continue it you will eventually end up where you started without ever changing heading according to the gyroscope.
3) At the equator you can see stars SIMULTANEOUSLY revolving around BOTH Polaris and the dark area off the southern axis at the same time.
4) As a ship distances itself from a stationary observer, it appears to sink into the water until only the tip of the mast is seen as it follows the curvature of the Earth. If the observer rises to the top of a building, the ship is again visible.
5) If you move north in an unchanging heading by gyroscopic reference, Polaris will move up in the sky by the exact angle to distance ratio predicted by simple high school polar trigonometry. If you move south from the equator, the same thing happens to the southern celestial pole.
No flat-earth theory can account for these observations. There are many, many more that are more subtle and harder to understand, but the first four are simple for any layperson to understand and #5 can be understood by anyone with a high school trigonometry education.
Edit: Just realized this is a cross-post. That sucks.
