N.b. for the moderators - I'm dealing with this central issue in digestible chunks. This is not low-content posting, and it's all relevant to this topic. I could stick it all in one fat, indigestible post, but that would make it more difficult to follow.
So what is the flat Earth "perspective" explanation of the sunset? Here's one example, from one of the leading intellectuals of the movement pbrane.
https://www.youtube.com/watch?v=Vq5ixQytLXE This is tough watching. I have to say, that about a minute in, my teeth started to hurt. Basically, pbrane is making up laws of perspective which have nothing to do with any kind of reality. His claim is that objects up in the sky don't get smaller as they are further away. How does this happen? Well, it just does.
This is such obvious, palpable nonsense that actually refuting it seems redundant. There is no reason whatsoever why looking at objects in the sky, or objects far off, should operate on different visual laws to anything else. The reason that objects far away appear smaller is obvious if we simply draw them, and draw lines from them to an observer. The angle becomes increasingly smaller.
However, even if we were to accept pbrane's nonsensical claim, this would in itself destroy the use of perspective. The entire principle of perspective rests on objects becoming smaller as they move away. That is the principle from which the concept of the "vanishing point" arises. If objects subtended the same angle as they moved away from the observer, there would be no vanishing point.
The reason we are unable to see objects when they are far away, even when there is a clear, direct line of sight to them, is that our eyes aren't able to resolve an image of the object. In general, the smaller the angle subtended by the object, the harder it is to see. However, other factors come into play. A dark object against a dark background can be harder to make out at distance than a bright object.
But this is the Sun! It's undoubtedly the brightest object that any of us regularly see. Indeed, it's too bright, even at sunset, to look at directly without damaging our eyes! So why does it disappear? It's not at the vanishing point, because objects
cannot reach the vanishing point. The vanishing point is at infinity. Not only that, since the Sun remains the same size in the sky, if we were to trace perspective lines,
they would not meet. pbrane simply superimposes a picture of a wall and assigns vanishing lines.
You can't do that. To establish the vanishing point of an object, you trace lines that connect successive images
of that object. pbrane doesn't use lines to connect successive positions of the Sun, because
they wouldn't meet. (Nor would they form a straight line, but that is a moot point).
I have assumed that pbrane is not an outlier on this matter of flat-Earth perspective "theory", but I'd welcome any clarification which would try to make something coherent out of the nonsense which seems, to me, to be the most absurd and most obviously wrong part of the whole business.