Due to the curvature of the earth, a ship's upper rigging will be visible at a much greater distance than its lower hull: for example, in clear air a lookout at the top of a mast 130 feet (40 m) above the water will be able to see the top of another 130 ft mast from over 24 nautical miles (44 km) away, but will be able to see the full hull of the other ship from only 12 nautical miles (22 km) away.[1] The discovery of the Hull-down phenomenon in sailing was essential to disproving the Flat Earth theory.