The distance in km of the horizon on earth, in a plain (standing on the ground or on a tower, or from a plane) or on a hill or mountain surrounded by plains is approximately √(13h), where h is the height in meters of the eyes.
Examples:
These figures indicate theoretical visibility (what can be seen depends also on how clear the air is, of course) of objects at ground level. To compute to what distance the tip of a tower, the mast of a ship or a hill is above the horizon, add the horizon distance for that height. For example, standing on the ground with h = 1.70 m, one can see, weather permitting, the tip of a tower of 100 m height at a distance of 41 km.
In astronomy the horizon is the horizontal plane through (the eyes of) the observer. It is the fundamental plane[?] of the horizontal coordinate system, the locus of points which have an altitude of zero degrees. The regular horizon is a little below that.
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