In article ,
Roger Smith writes:
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.

That's fairly small for visible light, isn't it? I know that when
working with radar, refraction is commonly allowed for by assuming an
Earth radius of 4/3 times the true value.
--
John Hall
"Acting is merely the art of keeping a large group of people
from coughing."
Sir Ralph Richardson (1902-83)