Calculator for calculating the curvature of the earth, ie the visibility of an object distant (d) from the observer at altitude (Vp). The height of the hidden part of the object (Vo) is that part which is below the horizon, which is not visible. In the example, the height of the observer is 2 m and the distance from the object is 50 Km, the radius of the earth (R) is 6371 Km.