Curvature of Earth's Sea-Level Horizon, to Scale

“Flat Earthers” often claim that the curvature of the Earth's surface is “8 inches per mile squared” (that's true enough, over distances of several miles), and that the horizon should therefore have a noticeable curvature when viewed from low altitudes, instead of appearing to be a straight line. Here's your chance to test that latter claim yourself. By clicking on the check box and buttons, you can see for yourself how difficult it would be to tell, by eye or in a photo, whether the horizon is a straight line or an “8 inches per mile squared” curve.