So get this: A tower can be built directly under the sun's path so that there is no shadow when the sun is directly overhead, regardless of round or flat.
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At this point, you don't know anything. All you know is there is a single straight line of immeasurable distance between the tower and the sun.
But if you build a second tower, you will see a different shadow, one that does NOT point to the sun directly overhead.
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And you know the heights of the towers and distances between them, you can measure the shadows at the same exact time and you can use those two different points to triangulate.
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The differences in these are demonstrated by the length of the shadow cast. The shadows we have on Earth are demonstrated by the model on the right. The curve extends the shadow. Whereas, on a flat earth, they are not extended. The only ways they could be extended on the flat earth model are some shit that fucks with physics, like trying to talk about the bend of light, despite the zero-shadow model shows no bend, or we don't know the measurements of the towers or distances between, when we obviously calcuably do.
No, I getcha, I was just more joking at grammar at the start, that religion being just the system of believing obviously exists because there are systems that people believe.