Observe the sun (via a filter for safety) or moon fairly high in the sky for a few hours on a single clear day. As it crosses the sky, it remains the same size and shape (or any change is too small to be noticed easily). This indicates that it maintains about the same distance from the observer as it crosses the sky, which is geometrically impossible if the earth is flat and the sun or moon is anything like as close as 73 miles.
How do you know that the moon is real? It could be, but what's to say that it's not a reflected image which has been projected onto the underside of the earth's 'firmament'? Maybe it remains the same size because it's a projected image, the source of which is two or more locations on the flat earth! Off the top of my head, I can think of three such locations where this technology could be being sourced and implemented from - the hole near the south pole, the other hole at the so called top of the world in the region of the north pole, and the Alien base, in AREA 51, in Nevada!
Now I stumbled upon some very interesting images at Area 51, Nevada, which could purport to show, how images of the moon and the other planets and stars may get projected so that they appear to an observer to be up inside the earth's 'firmament', when the bottom line is that these could simply be illusions! Do you think the powers that be aren't capable of projecting an image of the moon into the 'firmament' using technology?
Anyhow, I think the population at large is the subject of brainwashing, when it comes to the orientation of the earth which is most definitely flat in orientation, and the sun and the moon! The latter two are much or less the same in size and shape as they appear to us as observers! Calculating how high the earth's 'firmament' stretches up in altitude above the inner wall (if that's what we should call it) which commences at around 73 mile's in altitude, and stretches all the way upwards to the edge of Van Allen's (radiation) belt!