1 Rupe nigra: Mercator and magnetism by Giorgio Mangani (Università Politecnica delle Marche, Ancona, Italy) Full edition of the paper discussed in the International Conference, upon the anniversary of Mercator’s 500th birthday “Gerhard Mercator: Wisseschaft und Wissentransfer” University of Duisburg – Essen, Germany, 29th February – 2nd March 2012).* A synthetized edition, in German, has been published as Rupes Nigra: Mercator und Magnetismus, in Ute Schneider, Stefan Brakensiek, eds, Gerhard Mercator: Wissenschaft und Wissentransfer, Darmstadt, WBG, Wissenschaftliche Buchgesellschaft, 2015, pp. 116-131 (Isbn 9783534264513). Quamobrem satis sit nobis, a rebus aut sensu, aut certa historia notis, rerum verarum scientiam petere, non opinabilium, quorum non est scientia, sic enim non aberrabimus, si in modo iusto ordine progrediamur. (Gerhard Mercator, Atlas. Prolegomenon Fabricae Mundi, Caput I, 1595, p. 3) (Let us to seek the knowledge of the true things, either from our senses or from what is known to us from reliable history; of true things, I say, not conjectural ones, of which there is not knowledge, for in this way we shall not go astray, as long as we proceed in the right order. Translation by D. Sullivan, in CDR Atlas edition, Octavo, Oakland, 2000) Abstract In the 1574 protrait of Mercator by Frans Hogenberg, Mercator aged, sixty two, is portrayed as “the man of magnetic declination”. He underlines in this way his continued, life-long interest in the subject. Magnetism was, in fact, not only a scientific and technical argument useful for navigation; it also symbolized Mercator’s cosmological principle of harmonia mundi. This work underlines Franciscus Monachus’s probable role in training young Mercator in ermetic studies, then shared with John Dee, which influenced Mercator’s cosmology (that he considered “scientifically Christian”). In this cosmology, the device of the pythagoric Y is central. It represents the profound structure of his world, as in the “typus universitatis” sent to Vivianus in 1573, but it is also connected, I propose, to magnetic declination and the correct, new representation of the “rhumb lines” for sailing, featured in Sixteenth-century nautical maps. It was also the model for drawing the other letters of the alphabet in Mercator’s 1540 monograph about italic humanist writing. Pythagoric Y was, in fact, a symbolic and cabalistic device, also employed by Dee in his astrological and alchemic works, which finally represented moral choice; human free will in the providential mechanisms of Creation.