Why was the Mercator projection useful to sailors and navigators?

The Mercator projection, introduced by Gerardus Mercator in 1569, became an essential tool for maritime navigation because it uniquely represents rhumb lines—paths of constant compass bearing—as straight lines.12

For sailors, this feature is highly practical. By drawing a straight line between two points on a Mercator chart, a navigator can determine a constant heading to follow, which simplifies the process of plotting a course. While other map projections might preserve distances more accurately, they often distort directions, making them less reliable for maintaining a steady bearing.13

The projection achieves this by spacing parallels of latitude farther apart as they move away from the Equator. This mathematical adjustment compensates for the convergence of meridians toward the poles, ensuring that the east-west and north-south scales remain consistent at any given point. Although this causes significant distortion in the size of landmasses at higher latitudes, navigators historically accepted this trade-off to ensure they could reliably reach their destinations.13