How did the Mercator projection help navigators plot their courses?

The Mercator projection, introduced by Gerardus Mercator in 1569, revolutionized maritime navigation by allowing sailors to plot courses as straight lines.12

On the spherical Earth, a path of constant compass bearing—known as a rhumb line—is actually a spiral that does not follow a straight line. Navigators previously struggled because these paths were difficult to calculate on flat charts.32

The Mercator projection solves this by representing meridians as equally spaced vertical lines and parallels of latitude as horizontal lines. To compensate for the fact that meridians converge at the poles on a sphere, the projection spaces the parallels of latitude farther apart as they move away from the Equator. This mathematical adjustment ensures that any straight line drawn on the map represents a constant true bearing. Consequently, a navigator can simply draw a straight line between two points on the chart and follow that constant compass heading to reach their destination, prioritizing reliable direction over accurate distance.12