public void UpdateSunDirection(Light sunlight)
        {
            MyContract.RequireField(PlanetNumber != OrbitingBody.SUN,
                                    "this must not be the sun",
                                    "PlanetNumber");
            MyContract.RequireFieldNotNull(Maths, "Maths");

            Vector3    current_position = Maths.current_location_game_coordinates();
            Quaternion Rotation         = Quaternion.LookRotation(current_position);

            sunlight.transform.rotation = Rotation;
        }