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Reading the Cosmos: Leonardo's Astronomy

In the late fifteenth century, astronomy was a science in transition but not yet fully transformed. The Sphere of Sacro Bosco was still the most important textbook in the field. In order to chart planetary motions with some precision, Leonardo needed to familiarize himself with the technical content of this book, and likely relied on the assistance of someone better versed in mathematical astronomy and Latin than he was. He also gained access to Latin translations of medieval Arabic astronomy and a treatise on the quadrant by Jewish astronomer Jacob ben Machir ibn Tibbon.

A well-informed Renaissance reader interested in astronomy knew the importance of recent publications by German astronomers, especially Georg Peurbach (1423–1461) and Regiomontanus (1436–1476). Their jointly authored Epitome of Ptolemy’s Almagest, completed around 1462 and published in 1496, provided a critical appraisal of Ptolemy’s ancient astronomy. Leonardo acquired a copy of Regiomontanus’s Calendarium (1474), an astronomical calendar that had been printed fourteen times by 1500. His notebooks contain two pages written by Regiomontanus’s Florentine friend Paolo Toscanelli (1397–1482) with diagrams from Regiomontanus’s book on triangles. Leonardo was aware of Regiomontanus’s reputation as an innovative mathematician and exacting astronomer.

Leonardo does not seem to have owned works by Sacro Bosco or Regiomontanus until shortly before 1504. His interest in astronomy predates this period, starting with his contact with Toscanelli, who died the year Leonardo left Florence, and his subsequent observations of a solar eclipse in Milan on March 16, 1485. These books belong to the period when Leonardo began to work on a treatise known today as the Codex Leicester, containing some of his most interesting astronomical drawings and observations. This project probably inspired Leonardo’s decision to read more about astronomy.

[Paula Findlen]

Johannes de Sacro Bosco, 1195–1256

Sphaera mundi

Venice: Ottaviano Scotto and Boneto Locatelli, 1490

Rare Book Collection KA1490 .S2 CB

Gift of Charles Albert Browne

First written around 1220 but not printed until 1472, Sphaera mundi became a fundamental manual for students and practitioners of astronomy, such as Nicolaus Copernicus and Johannes Kepler, largely because its Theoricae novae planetarum drew from Greek and Arabic traditions as well as providing the most up-to-date knowledge of contemporary astronomy. This edition contains supplemental texts by Johannes Regiomontanus (1436–1476) and Georg von Peurbach (1423–1461). The contemporary German scholarly annotations, mostly in Latin, written in the margins of various pages of this book resemble Leonardo’s Italian annotations in his own notebooks. The astronomer who added these notes and diagrams throughout the years remains unidentified, but it is evident that he reported several first-hand stellar observations following the guidelines written in the book’s texts.

A book titled Sphaera mundi appears in Leonardo’s 1504 inventory of his library, making it one of the resources for his studies of astronomy, astrology, and geometry. Scholarly examinations of Leonardo’s views on different forms of visual representation have demonstrated how Leonardo embraced Alberti’s claims that the visual grammar of geometry and, ideally, proficiency in advanced mathematics were integral to painting. Leonardo himself said that he studied optics and mechanics in an attempt to imitate nature in his artistic studies. Astronomy was a complementary discipline; indeed, observing solar and lunar eclipses was the pinnacle of understanding light and shadow. It is no surprise that he may have used this book to improve his understanding of the cosmos.

[Meagan Wu]

Georg von Peurbach, 1423–1461, and Johannes Regiomontanus, 1436–1476

Epytoma in Almagestu[m] Ptolomei

Venice: Johannes Hamman, 1496

Barchas Collection KA1496 .R4 F

Gift of Samuel I. and Cecile M. Barchas

Peurbach and Regiomontanus’s Epitome of Ptolemy’s Almagest is one of the most influential works of late fifteenth-century astronomy. The Almagest was a thirteen-part astronomical manual written by Claudius Ptolemy in 150 CE, and it served as the primary astronomical guide in both the Islamic and European worlds for centuries. The Epytoma, begun by Peurbach and completed by Regiomontanus in Italy after his mentor’s death, condensed Ptolemy’s books into one comprehensive tome with commentary and illustrations of the associated astronomical concepts. Though it is written in Latin, Leonardo would have found a way to interact with this work, as did the Renaissance owner of this copy, who filled it with notes and diagrams.

In the Codex Leicester, Leonardo explored the concepts of light and shadow in relation to Earth and the cosmos, a theory he later developed as “Earthshine.” Regiomontanus’s studies of light could have proved useful in testing Leonardo’s idea. Leonardo also might have used the Epytoma to elaborate on his theories of the microcosm and macrocosm, exploring relations between body, Earth, and cosmos. In the Codex Atlanticus, Leonardo explored different lunar phases, drawing what he saw and envisioning himself as the “man in the moon.” The lunar observations and calculations in the Epytoma could have lent themselves to Leonardo’s exploration of this analogy. Leonardo probably gained additional access to Regiomontanus’s astronomy through the work of the famous Florentine polymath Paolo dal Pozzo Toscanelli (1397–1482), who met Regiomontanus in Italy.

[Emily Quinn]