Different reasons can cause a distant object to be not visible:
- The angular resolution limit of the observer.
- The visibility limit imposed by the atmosphere.
- Obstruction by another object, including by Earth’s curvature.
Flat-Earthers incorrectly presumed “a distant ship is not visible only because of Earth’s curvature.” Incorrectly concluded if we can bring the ship back into view, the curve must not exist. In reality, Earth’s curvature is not the only thing that can cause a distant ship to be not visible; other reasons can also cause it.
Our eyes cannot resolve an object with an angular size of less than one arcminute. We might be able to reveal the object by using an optical aid like a telescope or binocular or by zooming in with a camera.
Our atmosphere attenuates and scatters light. Beyond a certain distance, light from the object will be too weak to be discerned; or too scattered & do not form an image. We might be able to improve the visibility by using selective yellow headlights, a thermal camera, or an infrared camera.
If the object is obstructed by another object, it will not be visible, including if it is obstructed by Earth’s curvature. We might be able to reveal it by increasing the height of the observer.