Final answer:
The Earth is considered spherical for navigation purposes and this shape is corroborated by observations such as ships disappearing bottom first over the horizon and the increased visibility from higher vantage points. Ancient Greeks and navigational practices have long recognized the Earth's roundness, which is important for using tools like Mercator maps effectively.
Step-by-step explanation:
The statement that the Earth is spherical is used for navigation purposes is true. This is because the Earth is indeed not a perfect sphere but an oblate spheroid, which means it is slightly flattened at the poles and bulging at the equator. However, for simplification in navigation, it is often considered spherical. This shape is vital for understanding why objects like ships disappear from the bottom up as they move away and why lookouts posted higher on ship masts can see further.
A simple diagram to illustrate this shows a ship moving away from an observer on the shore. As it sails away on the curved surface of the Earth, the bottom of the ship is obscured before the top, due to the curvature of the Earth. This is because the line of sight from the observer to the ship is interrupted by the Earth's bulge. In contrast, from the masthead, which is higher up, lookouts can see over the curve to a greater distance. When envisioning this on a flat Earth, there would be no advantage to posting lookouts at the masthead, as their line of sight would not be significantly improved at that slight elevation.
This understanding predates Columbus, with ancient Greeks like Pythagoras theorizing the Earth's roundness even earlier. Nautical observations have long provided practical evidence for a rounded Earth, and without these, navigation with tools like the Mercator maps would be far less effective. Gerardus Mercator's map, developed in 1569, showed latitude and longitude lines on a flat projection and required an understanding of the Earth's spherical nature to correct for distortions that arise from representing a round object on a flat surface.