How did Gerardus Mercator create his famous world map in 1569?

In 1569, the Flemish cartographer Gerardus Mercator published his famous world map using a revolutionary projection designed specifically to aid navigation.12

While Mercator did not disclose the specific mathematical details of his method at the time, the projection functions by representing meridians as equally spaced vertical lines and parallels of latitude as horizontal lines. To compensate for the distortion caused by flattening the Earth's surface, he spaced the parallels increasingly farther apart as they moved away from the Equator. This technique ensured that any straight line drawn on the map represented a constant true bearing, allowing mariners to plot courses over long distances without needing to constantly adjust their compass readings.321

Although this innovation was invaluable for navigation, it resulted in significant scale distortion, particularly in regions far from the Equator, where landmasses appear disproportionately large. A formal mathematical explanation and correction tables for this projection were not provided until 1599 by the English mathematician Edward Wright.32