@freemo mirage?

@freemo I don't think the earth needs to be curved to have that effect though - even on a plane, if you have a thin layer of less-dense air along the ground, the ray from any distant point on the surface to your eye will originate at a shallow (nearly horizontal) angle, then be refracted upward to strike your eye at a steeper angle. So the horizon will appear depressed, because a ray originating at a point on the horizon strikes your eye along a line which intersects the ground at a point nearer to you than the true direction of the horizon. The ship, on the other hand, sits mostly above the density gradient, so rays originating from it travel a more or less straight path and strike your eye from an angle above the apparent horizon.

@freemo this is how ships travel when they are at the limit of the plate and you observe them hidden behind a rock!