I haven't taken it all in yet myself, but the part about perspective affecting observed slope makes sense to me. It's only difficult to accept because you can't see a physical line connecting the sun and moon. The part about following the perceived slope of the line where a wall meets a ceiling is a perfect example.
Not only do you think the earth is flat, you are also unable to think in three dimensions at all.
This image makes perfect sense in the heliocentric model.
I found an interesting paper that explains the phenomenon: https://www.seas.upenn.edu/~amyers/MoonPaperOnline.pdf
I haven't taken it all in yet myself, but the part about perspective affecting observed slope makes sense to me. It's only difficult to accept because you can't see a physical line connecting the sun and moon. The part about following the perceived slope of the line where a wall meets a ceiling is a perfect example.