There's one phenomenon that's common with the world today that we're all familiar with, and that's time zones. Different parts of the world have the sun shining over it at different times. It could be miday for the US while being night time for asia. This is something that the flat earth model has struggled to explain. I've seen some theories explain that the sun is a direct flashlight who's light points straight down on different parts of the world as it moves across the globe, but this is a weak explanation.
Here's why I think that's wrong:
1. Horizontal Difussion of Light vs Vertical Diffusion of Light:
If the earth was flat where the sun directly shined it's light on different parts of the world, the amount of light that diffuses across the north/ south direction should be the same amount that is diffused across the east/west direction. But this doesn't line up with the way time zones work, as they are drawn out Vertically for a reason. This explains why the time of day for someone in the east coast US would be the same for someone in Brazil since they are getting about the same amount of sunlight, yet someone on the west coast has to be 3 hours behind because of the curvature of the earth and the distance between the earth and the sun.
1.1 Relative Perspective of the Sun:
On the other hand, if you pinned the sun above the flat earth model above the east coast, technically someone there should notice that the sun is straight above them. At this exact moment, someone on the west coast should notice that the sun is not exactly above them but slightly at an angle from the horizon because of the time zone they are in. If you went to Brazil, which is at least 3x times the distance away from the east coast as the east coast itself is from the west coast, the angle that that somebody should see the sun from the horizon should be a lot narrower. But we know this isn't true, and I could prove it as well. The angle that someone sees the sun at from Brazil and from the east coast of the US are close to each other then the angle from the west coast, even though both coasts in the US are much closer to each other.
2. "Why does the sun disappear over the horizon?":
This is an interesting question that I haven't found an answer for from a flat earther. If we live on a flat earth, then why does the sun disappear over the horizon? Where does the sun go when it appears to be dipping below the edge of the earth?
The only explanation I can think of for the flat model is that the sun is past the edge of the earth and is moving below it to move across to the other side. Yet, there's a big issue with that explanation. The whole earth would have to be dark until the sun could come up on the other side. However, we know this isn't true and can be proven otherwise.
And finally, this is what I mean by the final nail in the coffin: An experiment - that can show how the earth is rounded rather than flat. It wouldn't have to be sponsored by NASA or the government to be able to pull off, just a lot of cameras recording at the same time and correlation between a lot of people.
If you get a person for every timezone to point two cameras, one at the horizon where the sun rises and the other one pointing at where it sets, and told everyone to hit the record button at the same time for 24 hours, then synchronized the recordings and played them side by side, you'd notice that the sun only moves in one direction across each horizon. On the flat earth model, this would be impossible for the sun to accomplish. This I believe could shatter the flat model I would hope.
If you have any counter claims, go ahead and share them here.