Globes, rhumb tables, and the pre-history of the Mercator projection

Abstract

In this paper we show that, for historical reasons, the most likely way for Mercator to have drawn his map was by using a table of rhumbs. This is argued by examining in some detail the development of the concept of a rhumb line from its initial articulation by Pedro Nunes in 1537 to its advanced treatment in 1566. We also describe the intellectual debate that took place around this topic in Europe during that period. A hitherto neglected Portuguese source of about 1540, where tables of rhumbs are discussed for the first time, has been found to be an important link in the chain connecting the initial idea of a rhumb line to Mercator’s achievement of 1569.