I don't know if this question's for here, physics, maths, or geometry.
I know that this or something similar has been asked a million times, but I've recently come to the concept that since the horizon is a circle centered at the viewer's position, the whole horizon is at the same level, hence why it's flat.
So what is it that we're seeing when we start seeing the curvature once we're high enough? I have a feeling that the answer to this has to do with the angle that the globe takes in your "camera", but the question still stands, what is this picture looking at? Why does it show a curve in the horizontal?