Our eyesight, is limited (at least at ground level) to around 3ish miles, weather dependent. At that distance, laws of perspective cause a horizon line to be drawn.
This, to some people, is proof of our curvature. But when you view the same scene with an optical aid, the horizon line is drawn further into the distance.
'But, the bottom of objects dissapear at distance!'
Or another one that has repeatedly been used, 'boats dissapear behind the curve!'
No they don't.
The horizon line happens at eye level always. Laws of perspective split our scene in to two main images, lower and upper. At the very center (terminator line?) The images converge. The lower portion of your upper image, reflects the upper portion of your lower image. So when the object or boat is at distance the horizon is actually lower than it, but laws of perspective create an optical illusion that causes the strongest reflection to be reflected, just above the horizon line and converging into it.