3. Star Trails. If Earth is flat, how can billions of people see two different sets of constellations rotating in opposite directions, around two opposite poles? How can people in the southern hemisphere essentially stand in a circle with their backs to each other, and all be facing the same southern constellation?
Anyone in the Southern Hemisphere may observe how stars rotate clockwise around the South Celestial Pole, where Polaris, Ursa Minor, Ursa Major, and Cassiopeia are absent - while anyone in the Northern Hemispshere sees stars rotating counter-clockwise around the North Celestial Pole, where Crux, Centaurus, and Carina are absent -
On flat Earth, it is geometrically impossible to see these two completely different skies, rotating in opposing directions around two different poles.