As we all know, Earth is rounded, but its surface is not smooth.
Mountains along Earth surface allow us to observe distant horizons, even as the further surfaces start to disappear as a result of Earth’s curvature.
The first requisite for a mountain to be observed from another distant summit is to have direct visibility, or in other terms, to have a clear view between each other:
If the mountains are too far away, Earth’s curvature starts to take its presence, and it is not possible to see distant mountains:
We will see later that under certain conditions, atmospheric air can bend light trajectories and this will make hidden mountains visible. This phenomenon is called refraction.