Final answer:
The concept relates to visual perspective on a spherical Earth, where a ship's lower parts disappear first because of the curvature, and lookouts on masts can see farther due to an increased visible range. There would be no advantage for mast lookouts on a flat Earth.
Step-by-step explanation:
The question is addressing the concept of visual perspective and how it relates to the curvature of the Earth. This is an observation that has been used historically to argue for a spherical Earth. When a ship sails away from an observer on a spherical Earth, the lower parts of the ship disappear before the upper parts due to the curvature hiding them from view. Additionally, from a higher vantage point, such as the masthead of a ship, a lookout can see farther than from the deck. This is because being higher up increases the horizon's distance, allowing for a wider field of vision over the Earth's surface.
If the Earth were flat, there would be no advantage to posting lookouts on the mast, as their range of vision would not be significantly different from the deck. In practice, lookouts were placed high on masts to maximize visibility due to the Earth's curvature.
To illustrate, consider a diagram where a ship is moving away from the observer. As the ship progresses, it seems to sink lower and lower until it disappears completely from the bottom up. When represented in a diagram, the curvature causes an obstruction which cannot be seen through. The lookouts on old sailing ships utilize this principle to maximize their range of visibility by stationing themselves at the highest practical point.