One of the major holes in the flat-Earth model is the apparent motion of the Sun. The sun rises and sets once a day. But if the Earth is flat, then sunrises and sunsets should never happen, and the Sun would be visible all the time, from anywhere on Earth. In order to plug the hole and ‘explain’ this problem, they invented an ad-hoc hypothesis that the Sun sets because of perspective.
The pretext is that an object appears closer to the horizon as it is moving away from the observer. They extrapolated this fact and wrongly assume that if it keeps going, then eventually it will appear to go down crossing the horizon line and is no longer visible.
The claim is obviously contrary to what we experience every day. Any object that appears above the horizon will stay above it no matter how far it is. If the object is far enough, then it will appear as a single dot, coinciding with the vanishing point.
Conversely, the same happens with objects that appear below the horizon. For example, a long and straight rail track will appear to converge to a single dot if it is far enough. It will not appear to ‘go up’ and cross the horizon line no matter how far it is.
Sunset has a much simpler and plausible explanation. When sunset happens, the sun is physically no longer above the horizon. It is not visible because it is obstructed by the curvature of the Earth.
- Perspective (graphical) – Wikipedia