Sailors sometimes need to estimate the range of a distant contact. Without radars, the only readily available reference point for estimating ranges is the horizon.
By knowing the height of the observer from the waterline, it is possible to calculate the distance to the horizon, and thus, it is possible to determine the distance to a remote contact, relative to the horizon. It is possible to do this only because the Earth is spherical.
- Lookout Training Handbook – United States Navy