- Question
- Is the Earth flat?
- Position‹2 of 2
- No, the Earth is not flat
- Argument1 of 3›
Ships appear to sink as they move closer to the horizon and emerge as they move closer to the observer
As a ship travels toward the horizon, the ship appears to sinking until it disappears altogether. The curvature of a spherical Earth explains why the ship disappears.
The argument
What happens to a ship as it approaches the horizon is one of the oldest observations bearing on the shape of the Earth, and it is decisive because the two competing models predict different things. The observation itself is specific. As a ship moves away from an observer toward the horizon, the hull disappears first, then the mast and the main structure, and lastly the antennas or sails. When ships return from sea the sequence is exactly reversed: the antennas and sails are seen first, followed by the mast and main structure, and finally the hull. The order is consistent, and it is always the lowest part of the vessel that goes first and returns last. If the Earth were flat, this is not what would happen. On a flat surface, an object receding from an observer simply subtends a smaller and smaller angle: the ship would shrink uniformly until the entire craft was too small to discern, with every part of it fading together. Ships do shrink as they move away from an observer, but that is not the whole of what is seen — eventually parts of the vessel disappear piece by piece, from the bottom up, while the parts still visible remain perfectly clear. Distance alone cannot produce that, because distance does not select which end of an object to remove. A globe Earth explains the observation exactly. A ship that is slowly disappearing has travelled far enough that the curvature of the Earth has come between it and the observer, and the surface rises between them. It is impossible for an observer to see beyond that curvature, so the hull is occluded while the mast is still above the line of sight, and the ship appears to sink rather than to shrink. The reversal on the return journey follows from the same geometry.
Premises
Counter-arguments
There are many reasons why the ship appears to sink as it moves toward the horizon. First, the hull disappears first because its angular size reaches the limit of the human eye before any other part of the ship. Second, the sinking ship effect can be caused by bulges on the surface of the ocean. Third, the sinking ship effect could be a result of refraction. Mirages can appear and disappear over water, which causes the sinking ship effect.