View from a satellite looking roughly westwards over a computer simulated landscape of the Preseli Mountains:

View from a satellite looking roughly eastwards over a computer simulated landscape of the Preseli Mountains:

Both of these pictures look fairly ordinary. But one of the two must be looking down: Feddau is below Eryr in the first picture and Eryr is below Feddau in the second picture. But if one is looking down, where is the horizon?

This is a question I was asked on the Simon Mayo show: Can you see the curvature of the Earth from a mountain? The answer is that you can see a disk shape, but the disk you can see is not necessarily the curvature of the Earth: You could be on a flat disk. However, from two tall mountains of the same height, you can prove that the Earth is curved if you can see the horizon (below the slope of the other mountain) at sunrise: Sunrise or sunset is the time that the horizon can be precisely seen.

The angles involved are tiny: This experiment with mountain peaks only seems to work at a place called Preseli in Wales where two high mountain peaks are approximately of the same height, are aligned approximately east-west and have no obscured views for a long distance in either direction: The angles to the horizon at this particular height would show that Ireland and England would not exist if you are on a flat disk.

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