RE: I'm Going To Keep Talking About Flat Earth
And dear fellow, the video you sent me is shot with a fish-eye lense (GoPro), you can see the Earth being spherical at one time, then concave and then flat. So which one is it? The answer is: when the camera is pointing straight ahead and the lense is level with the horizon - that is the correct shape of the Earth. In the video you sent me, the camera is clearly pointing down. It needs to point straight forward and be level because otherwise the fisheye lense will distort it. And even if you would set the camera straight and level, there would be a slight distortion because of the lense. You need to film the horizon with a non fisheye lense that is wide enough but not so wide to turn into a fisheye. A neutral lense that is closest to the human eye.
Well, but if you do that, you will see a "flat" horizon whether or not the Earth is a sphere or flat. This will be the case for any round object no matter how high you go until, eventually, it all disappears from view.
Why? Because the horizon line you see against the blackness represents the points furthest away from you. If the Earth is indeed round, all of these points will be equally far away from you and also equally far beneath you. In order to see curvature to either side, the centre of your view would have to be higher or further away. WHich is not the case when you stare directly at the horizon. So no, the fact that the horizon is a straight line is not evidence against a round Earth nor evidence for a flat one.
What is, however, quite good evidence for a round Earth is the fact that no matter where you send the balloon on Earth, it always provides you with the same circular view. Meaning you always see as far in all directions 360 degrees around you as you turn the camera, and the increase in viewing distance always scales with the assumed rate of curvature no matter where you are in the world. This would require a very good explanation for a flat earth model to be supported.
Anyways, I don't expect this will convince you of anything, but I hope that you now understand that the level view of the horizon is not evidence against a round Earth. It is actually exactly what you would expect to see if the Earth was round.