As we all know, Earth is rounded, but its surface is not smooth.
Europe – centered Earth globe
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.
Beyond Horizons 
Question… how can someone see a peak below 76000 ft. of curvature and the difference between peaks is only 1500 ft? This is the longest line of sight on your website.
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Because the Earth is not a sphere
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