A flat planet (ours or any other planet) would be such an incredible observation that it would pretty much go against everything we know about how planets form and behave. It would not only change everything we know about planet formation, but also about star formation (our sun would have to behave quite differently to accommodate the flat-earth theory) and what we know of speeds and movements in space (like planets' orbits and the effects of gravity). In short, we don’t just suspect that our planet is spherical. We know it.
20. The common sense of man tells him - if nothing else told him - that there is an "up" and a "down" in -nature, even as regards the heavens and the earth; but the theory of modern astronomers necessitates the conclusion that there is not: therefore, 'the theory of the astronomers is opposed to common sense - yes, and to inspiration - and this is a common sense proof that the Earth is not a globe
78) From Anchorage, Alaska at an elevation of 102 feet, on clear days Mount Foraker can be seen with the naked eye 120 miles away. If Earth were a ball 25,000 miles in circumference, Mount Foraker’s 17,400 summit should be leaning back away from the observer covered by 7,719 feet of curved Earth. In reality, however, the entire mountain can be quite easily seen standing straight from base to summit.