The distance to the horizon at sea depends primarily on the observer's height above sea level. A commonly used approximate formula for the distance to the horizon is:
d≈3.57hd\approx 3.57\sqrt{h}d≈3.57h
where ddd is the distance to the horizon in kilometers and hhh is the observer's height above sea level in meters. For example, for an average person standing at sea level with eyes about 1.75 meters above the surface, the horizon distance is about 4.7 to 4.8 kilometers. This distance increases with height: a person at 10 meters above sea level will see the horizon roughly 11.3 kilometers away, while a height of 100 meters extends that to about 36 kilometers. These calculations assume a spherical Earth without atmospheric refraction effects. Thus, the horizon's distance at sea is not fixed but depends on how high the observer's viewpoint is above the water level.
