Question

An observer watching a sailing ship at sea notes that the ship appears to be "sinking" as it moves away. What would best explain this observation?

Answer 1

This is a classic proof that the Earth is round.

Explanation:

In a sense the ship actually is sinking, as the curvature of the round Earth carries the ship below your field of view. Of course, the ship isn't sinking into the water because the sea, following the curvature of the Earth, goes below your field of view too.

Alternatively, if the ship is approaching you it "un-sinks" into your field of view, as first the mast and then the hull come "up" the curved surface of the sea/Earth.