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