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How far can you see on the horizon if it is a clear day, there are no mountains in the way, there isn't much smog, it's basically clear, and you have 20/20 vision?
Answer 1:

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-r^2)] where h = your distance above the Earth and r = the average radius of the Earth (6,380,000 m). If h = 2 meters the distance to the horizon is about 5052meters.

For details (they have a nice drawing) you can go to:
http://www.sailingissues.com/vier/mathproof2.html


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