"Joe Hunt" wrote in message
...

"Roger Smith" wrote in message
news
Joe

"On a clear day you can see for ever"

Do you really think so? The Earth is NOT flat. Work it out.

Regards, Roger

Roger,

I fully understand the Earth is not flat, however, what equations are used
by websites such as the one quoted earlier in the post ?

Joe
Joe, taking a diametral section through the Earth as being circular, you can
use very simple geometry to determine the approximate distance of a point on
a tangent from the nearest point to it on the circle as a function of the
distance of that point from where the tangent is in contact with the circle.

A simple but fairly good approximation is

h=d^2/D

where

D is the diameter of the Earth
h is the distance of a point on the Earth's surface below the observer's
horizon plane at a distance d from the observer.

For determining the visibility of distant objects this formula ignores the
effects of refraction.

Regards, Roger