Magnetism, Navigation, and Cartography in the Early Modern Period: An Elusive Relationship.

The article examines how early modern European nautical charts, particularly those depicting Africa from the sixteenth to the early eighteenth centuries, addressed the challenges posed by magnetic declination—the angle difference between magnetic north and true north—and its impact on navigation and cartography. It finds that until the seventeenth century, charts were constructed using uncorrected compass courses, resulting in distorted coastlines reflecting magnetic declination, and that deliberate corrections to charts did not align them with compass readings but rather involved astronomical measurements of longitude to improve longitudinal accuracy. These adjustments led to charts approximating the plate carrée (cylindrical equidistant) projection, prioritizing longitudinal correctness over preserving compass-based directions, thereby facilitating navigation despite the inability to determine longitude at sea. The study also highlights the political and navigational debates triggered by the misplacement of the Spice Islands after Magalhães’s voyage, which underscored the limitations of contemporary cartography and spurred efforts to reconcile navigational practice with geographic accuracy.