One way to calculate this is to assume the earth
is spherical. Based on how high above the earth's
surface you are, you can calculate the distance to
the horizon by measuring the length of a line than
starts at your eye and is tangent to the earth's
surface. As it turns out (can you figure out
why?), the length of the line equals:
sqrt[(h+r)2 - r2)]
where h = your distance above
the earth and r = the average radius of the
If h = 2 meters, the distance to
the horizon is about 5052 meters.
details (they have a nice drawing) you can go
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