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

Following the introduction of the marine compass in Europe during the last decades of the thirteenth century, an intimate – and sometimes problematic – relationship between the Earth’s magnetism, navigation and nautical cartography developed. Because charts were constructed using uncorrected compass courses collected by pilots at sea, their coastlines often appeared distorted, reflecting the effect of magnetic declination. Although the phenomenon was acknowledged by the late fifteenth century, serious consideration of its effect on charts was slower to come. This status quo was first put in question after Magalhães and Elcano’s voyage to South-East Asia (1519–1522), when it was realized with surprise that, contrary to contemporary nautical cartography, the coveted Spice Islands were not situated in the Spanish hemisphere, according to the terms the Treaty of Tordesillas. The issue triggered a long discussion between politicians, cosmographers, chart-makers, and pilots, not only about the necessity of correcting the mistake but also about how navigation should be conducted and charts made. At the core of the debate was the inescapable clash between the contemporary cartographic model, based on routes, and the Ptolemaic map prescriptions, based on geographical coordinates. This article explores how nautical cartography responded to such issues. By analysing the geometry of Africa in early modern charts of different origins, the author first clarifies whether and when they began to be corrected for magnetic declination and then offers hypotheses about how and why those changes were made. The results indicate that rather than correcting the orientation of coastlines on charts to conform with either true geographical directions or up-to-date compass readings, the adjustments involved astronomical measurements of longitude.