Back to school special

After a long hiatus, I’m reviving this blog. My plan is to post something short once a week. These posts may be brief descriptions of individual editions of Sacrobosco’s Sphere, little snippets of book history, musings on astrology and navigation, or other things I’m thinking about as I work on the larger book project. Right now, as I’m preparing my courses for the upcoming fall semester and longing for some crisp fall weather, I’m thinking of all those medieval and Renaissance professors gearing up to give their lectures on Sacrobosco’s Sphere. In honor of the beginning of a new school year, the next few posts will be devoted to versions of the Sphere used in university classrooms across Europe.

Diego Pérez de Mesa (1563–ca. 1632) was a professor of astronomy and mathematics at the University of Seville in the late 16th century. He gave classes on arithmetic, algebra, geometry, astrology, and navigation. Because he taught both university students and men training to be pilots, his lectures were in Spanish rather than Latin.Sometime around 1596 he gave a set of lectures on Sacrobosco’s Sphere. Although he never published these lectures, he did leave a set of manuscript notes, the Comentarios de Sphera, currently in the Biblioteca Nacional of Spain.

In his lectures on the Sphere, Pérez de Mesa discussed Copernicus’ heliocentric system, devoting a section to arguments for and against a moving earth. He also incorporated new information gleaned from voyages of exploration into his philosophical discussions. A particularly striking example of this is in his discussion of the four elements. He begins, conventionally enough, by stating that “earth has the lowest place of all the elements.” (“La tierra tiene el lugar mas baxo de todos los Elementos”) Earth is the heaviest element and it naturally moves down to the center of the cosmos. Here he notes that this is an objection to the Pythagoreans, Aristarchus (310–230 BCE), and Copernicus, all of who would “put the earth among the stars” (“las quales la ponía entre las estrellas”) This is an argument that comes straight out of Aristotle and would have surprised no one. His discussion of the element water, however, is considerably less conventional. Water is the second heaviest element, according to Pérez de Mesa, and it covers the earth. But unlike earth, which moves in straight lines, water has a circular motion: “water moves in circles over the surface of the earth.” (“…el agua se mueve sircularmente sobre la superficie de la tierra….”) His evidence for this position, which he acknowledges is a controversial one, is “the experience and certain reports of all navigators.” (“…la espiriencia y siertas rrelaciones de todos los navegantes….”) Sailors, he writes, know that ocean currents move in circles. He gives multiple examples of this phenomenon, ranging from the Pacific Ocean, the Indian Ocean, the Atlantic Ocean, the North Sea, and the Mediterranean.

In addition to his discussion of natural philosophical questions, Pérez de Mesa includes practical uses of astronomy. For example, when he discusses the meridian, which Sacrobosco defined as “a circle passing through the poles of the world and through our zenith,” he explains how to calculate one’s longitude using a table of the height of the sun at midday. His commentary is full of mathematical demonstrations that lend greater rigor and precision to Sacrobosco’s text.

Pérez de Mesa’s lectures on the Sphere demonstrate how university professors could use Sacrobosco’s original text as a starting point for wide-ranging discussions of new discoveries and ideas in astronomy and cosmography, as well as practical applications of astronomical knowledge.

Further reading

Kathleen M. Crowther, “Sacrobosco’s Sphaera in Spain and Portugal” in Matteo Valleriani (ed.), De sphaera of Johannes de Sacrobosco in the Early Modern Period: The Authors of the Commentaries (Springer Open, 2020)

Víctor Navarro-Brotóns, “The cultivation of astronomy in Spanish universities in the latter half of the 16th century” in Mordechai Feingold and Víctor Navarro-Brotóns (eds.), Universities and science in the early modern period (Dordrecht: Springer, 2006), pp. 83-98.

María Portuondo, Secret Science: Spanish Cosmography and the New World (Chicago: University of Chicago Press, 2009).