How it began
Pedro Nunes was appointed royal cosmographer of Portugal in 1529. His first published book, the Tratado da Sphera of 1537, included two works of his own on navigation. In 1544 he took the new chair of mathematics at the University of Coimbra, created to teach the technical knowledge that navigation required. According to the MacTutor biography, around 1550 he made the discovery for which he is best known, his investigation of the curve he named the rhumb line. The Columbia Encyclopedia calls him its reputed inventor.
Before it existed
Control of sea trade was the chief source of Portuguese wealth, and pilots steered across the ocean by holding compass courses. Nunes devoted himself to problems of maps and map projections at a time when charts did not agree. It had not been made clear that a course of constant bearing differs from the shortest route between two points on a globe, which is a great circle.
How it works
A rhumb line is the path a ship follows if it always keeps the same compass setting. In geometric terms, it is the curve that cuts every meridian on the sphere at the same angle. Unless the course runs due north, south, east or west, the line does not close on itself. It spirals toward the pole.
How it spread
Nunes's distinction between rhumb lines and great circles came before the better-known use of it by Gerardus Mercator in map projection. On the Mercator projection every rhumb line becomes a straight line, so a navigator can rule a course on the chart and read off a single bearing to steer. The Dutch scientist Willebrord Snell later gave the curve the name loxodrome.
What lasted
A Springer reference work describes Nunes as one of the most significant scientists of the 16th century, best known for his contributions to navigation and cartography, and credits him with the idea of the rhumb line that Mercator eventually perfected. Britannica calls him the chief figure in Portuguese nautical science.
