The straight line of sight distance d in kilometers to the true horizon on earth is approximately
d = √(13h)
where h is the height above ground or sea level (in meters) of the eye of the observer. Examples:
- For an observer standing on the ground with h = 1.70 m (average eye-level height), the horizon appears at a distance of 4.7 km.
- For an observer standing on a hill or tower of 100 m in height, the horizon appears at a distance of 36 km.
To compute the height of a tower, the mast of a ship or a hilltop visible above the horizon, add the horizon distance for that height. For example, standing on the ground with h = 1.70 m, one can see, weather permitting, the tip of a tower of 100 m height at a distance of 4.7+36 ≈ 41 km.
In the Imperial version of the formula, 13 is replaced by 1.5, h is in feet and d is in miles. Examples:
- For observers on the ground with eye-level at h = 5 ft 7 in (5.583 ft), the horizon appears at a distance of 2.89 miles.
- For observers standing on a hill or tower 100 ft in height, the horizon appears at a distance of 12.25 miles.
The exact formula for distance from the viewpoint to the horizon, applicable even for satellites, is
d = √(2Rh + h
2)
where R is the radius of the Earth [R, d and h in the same units].
...
As a final note, the actual visual horizon is slightly farther away than the calculated visual horizon, due to the slight refraction of light rays due to the atmospheric density gradient. This effect can be taken into account by using a "virtual radius" that is typically about 20% larger than the true radius of the Earth.
From 
Interesting question!
