Curvature of the horizon

Is this an optical trick, or is the curvature really that noticeable? It would be about 35 miles to the horizon from the altitude at which I was standing.

lead_mines_eastwards-2012-12-08.JPEG

If it hadn't been so foggy then from left to right on the horizon you'd be able to see the Mull of Galloway and the Isle of Man.

Next task: tell Hugin to stop downsizing images to 1600 pixels.