>>10894223One can falsify the flat-Earth concept on any clear night an hour or two after sunset by observing satellites in the sky, provided that one either accepts Occam's Razor or common sense as valid things. Plus sanity. Unless lots of people w/ lots of spare time & money are launching one-shot rockets every night to maintain the conspiracy or using holograms to project satellites onto the night sky, a flat Earth simply won't support a constellation of orbiting objects. They'd fall over the edge of the world, giving you nothing but blurry images of the underside.
Worth noting is that spherical trigonometry & Astro-trigonometry are exact sciences, which only work in a spherical environment.
Speaking of planes, aeroplanes that do long-distance travelling, if they can, will often fly near the poles to reduce the distance travelled rather than across, for example, the Atlantic Ocean. This assumes a spherical Earth, where distances near the poles are shorter.
& of course, there is the observation which first informed the ancients themselves - that ships sailing into the distance disappear hull-first.
