There are quite a few threads on here discussing that. Here are six of the best ways to easily demonstrate that the horizon is below eye level: 1. Use a professional theodolite. Eye level is where the crosshair is: 2. Download and properly calibrate a theodolite app. Eye level is where the crosshair is: 3. Make your own theodolite using a spirit level: www.metabunk.org/posts/204999 4. Use parallel lines to find the vanishing point (which is always at eye level): www.metabunk.org/posts/209251 5. Take a picture of eyes, with the camera at eye height, and see where the horizon is: www.metabunk.org/posts/207592 6. Use a water level - after all, all flat earthers know "water always lies flat". www.metabunk.org/posts/207891 Number 5, by the way, is an example rather than a demonstration - the camera in that photo is probably slightly below eye level, therefore his eyes appear even more above the horizon than they normally would..