Refuting the Flat-Earth theory

[FlatAssembler]

It's not obvious. If it were obvious, Aristotle would not need to make his famous three arguments for that: ships disappearing bottom first, shifting constellations as one goes north or south, and the shadow on the moon during lunar eclipse always being round (rather than sometimes round and sometimes elliptical).

It may not have been obvious twenty four centuries ago, it's pretty obvious now. A little thing called 'accumulation of evidence'.

Boru

The problem is that not everybody knows how to interpret the evidence.
When I first heard the "ships disappearing bottom first" argument, back when I was a 5th-grader, I thought "Waves seem like an obvious explanation, not the Earth being round.". Somebody had to explain to me why waves are not a good explanation for that.
For the constellations shifting as you go north or south, Flat-Earthers often say "Well, that can also be explained by stars being very close to us, not just by the Earth being round.". That is, of course, incorrect, because if that were caused by the stars being close to us rather than the Earth being round, constellations would have different shapes depending on where we look at them. But not everybody realizes that.
The Aristotle's argument from lunar eclipse seems to be rather weak (How do you know the moon get its light from the Sun? How do you know it's the Earth's shadow on the moon, rather than the shadow of some other celestial body?), and Flat-Earhters probably cannot be blamed for not accepting it.
Fast-forward to the modern age when we can see time zones and GPS working and the dip of the horizon from an airplane... Not everybody realizes that time zones, the way they are, can only be explained by the Earth being round. Not everybody even knows about polar day and polar night at Antarctica, much less has thought about it to realize that it proves the Earth to be round. Not everybody understands how GPS works well enough to understand how it proves the Earth is round, how we can prove that it receives signals from the satellites rather than land-based emitters (that, if GPS devices received signals from land-based emitters, they would need more than 3 signals to determine their location). And not everybody knows about the dip of the horizon (which is, in my opinion, enough to prove the Earth is round to any remotely reasonable person).