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Horizon Dip

When the elevation of the observer is higher than sea level, due to the Earth’s curvature there is a dip of the horizon that can amount to a few degrees provided the elevation is high enough and the view unobstructed down to the geographical horizon.
Of course it all depend on the local slope of the terrain and generally only locations by the sea, at sea or where the terrain is completely flat need that correction. Another good example is when viewing the eclipse from a jet aircraft at high altitude. When there are mountains visible, you should retrieve their apparent height to the dip of the horizon generated by the curvature of the Earth.

The table below will give you an idea of what to expect:

Elevation
(m)
  Horizon
dip
(°)
Mountain at 30 km
as seen from sea level
(°)
0 0.0 0.0
10 -0.1 0.0
500 -0.7 +1.0
1,000 -1.0 +1.9
3,000 -1.8 +5.7
6,000 -2.5 +11.3
9,000 -3.0 +16.7
12,000 -3.5 -