It's not quite that easy. One could suppose that light falls due to gravity. So the light from the bottom of a far-away ship falls into the ocean before it reaches you, leaving only the top of the ship visible to you.
I vaguely recall that historically the round shadow of the earth during lunar eclipses was one good piece of evidence.
I vaguely recall that historically the round shadow of the earth during lunar eclipses was one good piece of evidence.