I've been hanging out in a Flat-Earther-forum for a while. Let me tell you how these people think: [View all]

1.
Their knowledge of physics and math is below-average. They can parrot talking-points, but when you try to get into the details, they get all confused and angry.
e.g. When you point out that one of their Flat-Earth-models (the "Universal Acceleration" model) is wrong because of Special Relativity, it becomes clear pretty fast that they don't know how Relativity works.



2.
Every single shred of evidence that Earth is round has been fabricated by an all-powerful cabal that has existed since nobody-knows. Every single astronomer, seafaring captain, cartographer and long-distance pilot is part of this conspiracy.

How do ships and planes still find their way, if all these Spherical-Earth-maps are wrong?
The answer is easy: They all have a super-secret set of accurate Flat-Earth-maps.



3.
The Flat-Earthers have not performed A SINGLE RIGOROUS EXPERIMENT that could prove whether Earth is flat or not. They have run some experiments, but these experiments are so full of mathematical misconceptions and ridiculously inaccurate measurements that the results are entirely meaningless.

For example, one of their favorite arguments is how the "spherical dropoff" isn't real. How a ship that supposedly disappeared beyond the curvature of the horizon can still be seen with a telescope.
And yet NOT A SINGLE FLAT-EARTHER could have been bothered to conduct this experiment in a clean fashion with a telescope, a camera and spirit-level for leveling the setup horizontally.

The only evidence Flat-Earthers have is eyewitness-testimony from fellow Flat-Earthers...



4.
The Flat-Earthers do not have an agreed-upon Flat-Earth-map. When you start asking questions about that map, they wiggle out by saying that nobody knows what the actual geography of Earth is. (But it's definitely flat.)



5.
They debunk your Spherical-Earth-theory with meaningless, faulty experiments full of mistakes.
If you try to prove your Spherical-Earth-theory, then you are fabricating evidence.

When you try to debunk one of their Flat-Earth-theories, they simply come up with a more convoluted and ridiculous one.
When they try to prove... just kidding. Flat-Earthers don't prove anything. They love to wax about the big bad conspiracy and what a Flat Earth could possibly be like, but an absolutely tiny minority is actually interested in running experiments and even then they are not thought-through.

For example, one of them is trying to calculate a map of Earth by comparing how long it takes for commercial flights to go from one city to another.
Ignoring such things as different velocities of the planes, climate, mountains, local air-travel regulations, how long it takes to reach travel-altitude, how long it takes to land...



6.
In order to attack Spherical Earth, some have started attacking the scientific method itself and proposing the zetetic method instead.
https://rationalwiki.org/wiki/Zetetic
The zetetic method postulates that there is no such thing as a finite, constant fact that your investigation could reveal. Your investigation never ends. Which means that Earth cannot be declared spherical on principle.