Scanning the Skies: Astronomy and Medieval Society

Bread and Milk for Children.”

So reads the subtitle that appears in four manuscripts of Geoffrey Chaucer’s Treatise on the Astrolabe, a text that compels us to recognize the fluid boundary between art and science in the Middle Ages. Chaucer is widely celebrated for his wry humour and memorable characters, but less often for his impressive understanding of astronomy; we examine the literary contexts of his work, but rarely the scientific ones. In doing so, we overlook a body of knowledge on which many medieval writers drew. Whether this subtitle represents Chaucer’s own view of a text addressed to his young son or the harsh judgment of an unimpressed scribe, it encapsulates how fundamental astronomy was to late medieval natural knowledge, whether one’s work lay in mathematics, wool trading, or poetry.

In university contexts, astronomy was a branch of the quadrivium, but astronomical principles also played roles in medieval agriculture, navigation, medicine, music, and religious practice. Each sign of the zodiac governed a part of the human body, as the “zodiac men” in many manuscripts (e.g. LJS 463, fol. 54v) demonstrate. Similarly, establishing the date of Easter required astronomers to predict when the first Sunday would fall after the first full moon on or following the vernal equinox. Yet while astronomy was central to medieval natural philosophy, becoming an expert in this field typically required years of effort and training. The manuscripts in this exhibit testify to the sophisticated mathematical calculations required by the geocentric model of the universe and the spherical orbits of celestial bodies.

Astronomical texts also exemplify the complex intercultural pathways through which knowledge was transmitted in the High and Late Middle Ages. During the twelfth and thirteenth centuries, many of the foundational texts and methods of astronomy migrated across Western Europe through parts of Spain formerly under Muslim rule. In the 1270s, King Alfonso X of Castile founded the Toledo School of Translators, an organization that translated Arabic versions of Greek texts into Latin and Castilian, and also produced an updated version of the Toledan Tables (created by Islamic astronomers c. 1080) that came to be known as the Alfonsine Tables. Fourteenth- and fifteenth-century European astronomers seem to have continued to consult Middle Eastern astronomy; both Regiomontanus and Copernicus refer to al-Battānī, and it is possible that they would have known the theories of al-Ṭūsī or Ibn al-Shāṭir. It is therefore unsurprising that astronomical manuscripts are a major component of the Lawrence J. Schoenberg Collection at Penn, a collection designed to demonstrate continuity and change in scientific knowledge over time and across modern national borders.

This exhibit attempts to reproduce some of these channels of communication, beginning with Ptolemy’s Almagest, the Classical treatise on planetary motion upon which much of medieval astronomy was established. In "The Scholarly Tradition," you will find three copies of this text in Hebrew and Arabic. One (LJS 392) is a fifteenth-century Persian copy of al-Ṭūsī’s thirteenth-century Almagest recension, along with al-Nīsābūrī’s commentary; another (LJS 268) was produced in fourteenth-century Spain for Qursunna Isrāʼīlī, astronomer to King Pedro IV of Aragon. The scholarly manuscripts also include four copies of Johannes de Sacrobosco’s enormously popular Tractatus de sphaera (LJS 26, LJS 216, LJS 494, MS Codex 1881), which recapitulates and simplifies Ptolemaic principles for students of astronomy. LJS 26 and LJS 216 are roughly contemporaneous with Sacrobosco, while MS Codex 1881 includes rare examples of volvelles, or rotatable diagrams.

The codices and fragments in this exhibit date from the tenth century to the sixteenth, and reveal astronomical knowledge in this period to be a diverse and evolving set of traditions. Along with technical volumes, including an early fifteenth-century set of Alfonsine Tables (LJS 174), you will find almanacs and astrological treatises, such as a Persian almanac (LJS 434) and a German miscellany of astronomy and medicine (LJS 463). Still others contain texts on the astrolabe, a tool used to measure the height of an object above the Earth’s horizon. Several manuscripts combine aesthetics and practical uses, including a striking copy of Regiomontanus’s Calendarium and Ephemerides (LJS 300) created decades after these texts were printed, suggesting that its first owner sought a manuscript that was as beautiful as it was functional.

The most recent manuscript in this exhibit is a collection of mobile illustrations (including volvelles) made to accompany Peuerbach’s Theoricae novae planetarum (LJS 64). Peuerbach’s text addresses Ptolemaic concepts such as epicycles (small circles along which celestial bodies move) and deferents (larger circles along which epicycle centers move around the Earth). It became a standard university text in the late fifteenth century, and remained one until it was supplanted by heliocentric astronomy in the seventeenth century. Indeed, all of these texts contain knowledge that is obsolete, their claims undermined by the Copernican model and its later developments. However, they also reveal a period characterized by intense scholarly engagement with the cosmos, and the active movement of ideas among cultures that shared an interest in mapping the skies.

Credits

Exhibit curation and online version by Aylin Malcolm, University of Pennsylvania, 2019.