Alpha2Omega, observations of the stars are not observations of the Earth. Likewise, observations of the sun are not observations of the Earth. They are observations from the Earth, thus invalid as examples of the chosen option.
Because astronomical observations are observations from the earth, the shape of the earth is absolutely a factor in how the stars appear to move relative to the earth, which depends on its shape.
For instance, having the sun high in the sky at one location, on the horizon in another, and below the horizon in yet a third, all at the same time is easily explained by the earth being a globe, but is difficult, if possible at all, to explain if it is flat.
For another example, if you measure the elevation angle of the celestial pole (the point that stars appear to revolve around through the night (in the northern hemisphere, Polaris is a useful proxy for the celestial pole for many purposes since it it close to the actual location of the pole), then travel 60 nautical miles toward the pole and measure its elevation angle again, you will find that it has increased by 1°. Continue traveling toward the pole and the elevation angle changes by 1° for every 60 nautical miles until it reaches 90° at the geographic pole. This, again, is very easy to explain with a spherical earth. If the earth were flat, the change in angle would not remain constant; it would change very slowly near the equator and increase dramatically as you approached the point on the surface directly below the celestial pole. In fact, if the earth were flat, the poles would not be on the horizon at the equator, as they actually are, unless light behaves in ways that are not easily explained and have not been observed in other situations.
I stand by my statement. I can see why you might want to simply dismiss all of these observations, because they are contrary to the flat earth model, but you cannot dismiss them without a rational reason, and you have none.
This leaves only your remaining horizon examples. A distinct line, as in a distinct, straight, flat, line, is also a flat earth example. So, invalid. Nice try!
What would the horizon look like on a flat earth? Examples proposed here often rely on atmospheric extinction (i.e. absorption and scattering of light as it passes through air) to limit how far we can see, and explain why you can't see, for instance, the lights of London from New York. This, however, would not produce the sharp horizon line we see on clear days at the seashore, the surface would just gradually fade in the distance. That's not what we see. Again, this is evidence for a spherical world.
If you want to suppose the horizon is the edge of the disc of the earth, then, if you're standing on a beach, it should be possible to walk along the shore from where you're standing the relatively short distance to the "edge" and peer over. Of course, if you try to do that, you will find that you're still standing at the center of your disc. If you try to get to the "new" edge, you'll be disappointed again to see that you're still right in the center. That is exactly what would be expected if you're on the surface of a large sphere, but not what you'd expect if you're on a large flat plane unless something is limiting the distance you can see and at the same time creating the illusion of a sharp horizon.
Your two remaining horizon examples are worthy of discussion, though, as is your example 4, which is also about the horizon.
Horizon, horizon, horizon.
I can understand why this bothers you. Tough!
When you go to night clubs, do you do the horizontal tango?
What?
But, are you absolutely certain these horizon examples aren't flat earth savvy?
Yes.
If you go lay down in the middle of an Olympic sized ice skating rink, and put your eyes as close to the surface, and look around - there's your horizon across a flat surface. (Just don't put your globe licking forked tongue on the ice)
Are you absolutely sure? Have you tried this?
So, your remaining examples are also looking.......you guessed it......invalid
Sorry, your reasons are not sufficient to reach that conclusion.
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