Scanning the Medieval Skies: An Exhibit of Astronomy Manuscripts at the University of Pennsylvania

“Bread and Milk for Children.”

So reads the subtitle that appears in four manuscripts of Geoffrey Chaucer's Treatise on the Astrolabe. Chaucer is widely celebrated for his wry humour and memorable characters, but his impressive understanding of astronomy is less well-known. Yet 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 conveys how fundamental astronomy was in the late Middle Ages, whether one chose to pursue a career in academia or (like Chaucer) wool trading and poetry.

Astronomy was one of the seven disciplines in a medieval liberal arts education, and a prestigious discipline in academic contexts. Yet astronomical principles also played roles in medieval agriculture, navigation, medicine, music, and religion. Each sign of the zodiac governed part of the human body, as the prevalence of “zodiac men” in manuscripts (e.g. LJS 463) demonstrates. Perhaps most importantly, to determine the date of Easter Sunday, religious leaders had to predict when the first Sunday would fall after the first full moon following the vernal equinox. Medieval astronomy was therefore basic to natural philosophy, although far from simple. In fact, in order to preserve the geocentric model of the universe that is now discredited, medieval astronomers had to develop ingenious mathematical calculations.

Astronomical texts also exemplify the intercultural pathways through which knowledge was transmitted in the High and Late Middle Ages. During the 12th and 13th centuries, many of the foundational texts and methods of astronomy migrated across Western Europe through parts of Spain formerly under Muslim rule. In the 1270's, King Alfonso X of Castile founded the Toledo School of Translators, a group of Jewish, Christian, and Muslim intellectuals who translated Classical Arabic versions of Greek texts into Latin and Castilian. Fourteenth- and fifteenth-century European astronomers seem to have continued to respect Middle Eastern astronomy; both Regiomontanus and Copernicus cite al-Battānī, and it is possible that they would have known the theories of al-Tūsī or Ibn al-Shatir, if not these astronomers’ names. This exhibit attempts to reproduce some of these channels of communication, including a map that offers a visual guide to the transmission of scientific knowledge across modern national borders.

In "The Scholarly Tradition," you will find three copies of Ptolemy's Almagest, the Classical treatise on planetary motion upon which much of medieval astronomy was established. These include a fifteenth-century Persian copy of al-Tūsī's thirteenth-century Almagest recension, (LJS 392) and an Arabic copy produced in fourteenth-century Spain for Qursunna Isrāʼīlī, astronomer to King Pedro IV of Aragon (LJS 268). The scholarly tradition continues with Sacrobosco’s enormously popular Tractatus de sphaera (LJS 26, LJS 216, LJS 494), which recapitulated and simplified Ptolemaic principles for students.

The codices and fragments in this exhibit date from the twelfth 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), they include a Persian almanac (LJS 434) and a German compilation of astronomical and medical information (LJS 463), both with attractive illustrations designed to appeal to a broad audience. Other manuscripts combine aesthetics and practicality, including a striking copy of Regiomontanus’s Calendarium and Ephemerides (LJS 300) that was created decades after these texts were first 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 illustrations (including many volvelles) to 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 describe an obsolete form of science, their functions negated by the Copernican model and its later developments. However, they also exemplify a period in Western history characterized by intense scholarly engagement with the cosmos, and the active movement of ideas among cultures that shared an interest in mapping the skies.

This is an online version of an exhibit curated for the 2019 Annual Meeting of the Medieval Academy of America at the University of Pennsylvania. All manuscripts currently in this exhibit are from the Lawrence J. Schoenberg Collection. I welcome your feedback at malcolma[at]


Text and curation by Aylin Malcolm. Advised by Nicholas Herman (original exhibit) and Whitney Trettien (online version).