Very easy to notice proofs that the Earth is flat:
>The sun doesn't just rise and set in a perfect easy/west angle, it actually curves in from the northeast when it rises and curves into the northwest when it sets. It also doesn't visibly go over the horizon; it just gets further away and obscured by mountains, forests, mirages, etc. If it was round the east/west setting pattern would be perfectly a straight line, but it actually does turn to the north, indicating a circular pattern, over us, rather than us turning relative to it.
>None of the oxygen on the earth should be able to stay within the established boundaries if it's just blatantly exposed to outer space. Gases always try to equalize with their surroundings, and try to escape their containers. "Gravity" doesn't prevent air from escaping into outer space. There must be some physical barrier and we must be in an enclosed ecosystem, AKA the firmament, being this barrier in question, which is a FE concept.