They invent new scientific rules. With two caveats: first, the new rules are not independently tested by experiment (maybe they are vaguely justified by thought

They invent new scientific rules. With two caveats: first, the new rules are not independently tested by experiment (maybe they are vaguely justified by thought experiment; thought by them); second, their rules are just qualitative, nearly never quantitative, so that they basically everything can be true. So, measuring angles is off limits. Then they admit you cannot see things that are far away and close to the horizon, but not by curvature. By perspective: perspective laws, they say, mandate that far away objects become very small, too small to be seen. Do you basically can see only a limited part of the sky, at a sufficiently high angle from the horizon on the flat earth. That is why you can't see the whole sky.