Last night I was coming home from my regular job, and staring back at me in the southeast, was the moon. It was nearly, but not quite, full, slightly waning. It was around 10:30 at night, And I thought: "How can anyone who looks at the moon, I mean, really looks at it, believe for a moment that we live on a flat plane?"
Flat-Earthers have lots of different explanations for how the sun and moon work on a flat Earth. But in the light of the real, and quite beautiful, observations from the ground, they don't stand up.
While I was looking at the moon, the part of the Earth that was pointing at the sun was about 142 degrees away from me, which would mean it was solar noon in northeastern Russia. And if you look at, say, Gleason's Map, it's not too big a stretch to think that, perhaps, the sun could be lighting the moon at that angle. Well, nearly that angle.
But what if I was standing in, say, Ecuador? In real life, people in Ecuador see the same phase of the moon at the same time as I do, as the rest of the world does. But in the flat-Earth world, the moon would be lit from the side from that view. It would be a half-moon.
At this, some flat-Earthers will claim that the moon is a disc, or self-illuminating, or both. But that's another case of not trusting what you see. For if the moon were a disc, It would appear round only when it was directly overhead, and ovoid at any other time. And were it self-illuminating, the features would always look the same, without crater shadows, which show up only when the sun is shining obliquely on the moon.
Neither of these excuses match anything we observe in real life, even without the aid of a telescope.
Flat-Earthers will then say that they don't know the real model, because they don't have the vast resources to research it, but they've had at least 160 years, since Rowbotham, to work it out. And they don't really have to have a lot of money.
Just eyes, And brains. And a willingness to think through what you see without prejudice.