From an observer on a shore, the distance to the horizon is 5 km (3 miles), or more if the observer is higher. Therefore, Earth’s curvature obscures objects starting from that distance. If the objects are not large enough, we need an optical aid, like zooming in using a camera, to see them in the first place.
Flat-Earthers often show us wide-angle photos to demonstrate that objects are not obscured by Earth’s curvature. In reality, in such photos, objects far enough to be obscured by Earth’s curvature cannot be resolved. It is hard to see objects obscured by Earth’s curvature if the objects themselves are not visible in the picture.
The Lake Ponchartrain Causeway and transmission lines are two objects where the effect of Earth’s curvature can be easily observed. To see it, we need to use an optical aid, like a long-zoom camera. If we were to use a wide-angle lens —or the widest zoom of a zoom camera— the curvature’s effect will not be visible. The scene we are looking for will appear compressed as a tiny dot in the middle of the image. To see it, we have to increase the zoom factor, in effect enlarging the center portion of the image, so that the effect of Earth’s curvature is visible.
Using a wide-angle lens to demonstrate the ‘lack of curvature,’ flat-Earthers are simply hiding the effect of Earth’s curvature in the middle of the picture. We cannot see objects obscured by Earth’s curvature if the objects themselves cannot be seen.
- Flat-Earth: Finding the curvature of the Earth – Walter Bislin