The majority of the flat Earth models place the Arctic Ocean in the middle of the flat Earth, and Antarctica at the edge of it. The Sun is pictured floating and moving in a circle above it. The Sun’s area of light is limited to a circular area below it, like a spotlight.
A problem: a simple observation of day and night cycles in a different area of the world cannot be explained in this flat Earth model.
In such model of the Earth, a day near the equator will last only 8 hours, and night would have been about 16 hours long. Obviously, this is not consistent with real-world observations.
Furthermore, the southern part of the Earth like Australia, New Zealand, South Africa and South America would never experience a day that is longer than the night. And thus, those who live there would never experience summer time.
These simple observations are inconsistent with what would happen if the Earth were flat. And the Earth can’t be like what is described by these flat-Earthers, not even close.
Some of them tried to ‘plug the hole’ by assuming the Sun’s area of light is not circular, but elliptical, or even half a circle. But the followup questions can’t be avoided: “How is it possible that the Sun’s area of light is half a circle?”
Rather than answering the question with just another ad-hoc hypothesis that will eventually lead to another objection that needs to be plugged with just another ad-hoc hypothesis, and so on and so forth; it is easier to take the simpler explanation, that is consistent with every single observation: the Earth is in fact spherical.