How far away is the horizon from a given height, and how far can two raised points see each other.
Work out how far away the horizon is from a given eye height, taking Earth's curvature into
account. Add a target height (a hill, tower, or lighthouse) to find the maximum line-of-sight
range at which the two points can see each other. Atmospheric refraction is applied by default,
which extends the geometric horizon a little.