The distance travelled between the bearings is the same as the distance from the visible wreck. Isosceles, so d 2 is also 10 nm. Isosceles, so d 2 is also 4 nm. Hence, of these three fixes the four point fix is the most precise one. On a flat world there would be no difference between the visible and sensible horizon.
Atmospheric refraction bends light rays passing along the earth's surface toward the earth.
If an object is observed to be just rising above or just dipping below the visible horizon, its distance can be readily calculated using a simple formula. Click on the image on the right to view a magnificent lighthouse. Finally, take a bearing on the object to get a second LOP and a position fix. 72' total, and corrected for index error. Water height in meters above or below chart datum of object.