Johannes de Sacrobosco’s De sphaera mundi was the most popular astronomical text in Europe from the late thirteenth century to the late seventeenth, and a core component of the university curriculum. This essay is the first published study of a remarkable copy of De sphaera in a manuscript recently acquired by the University of Pennsylvania (MS Codex 1881), which includes an unedited commentary on De sphaera and a variety of diagrams. I begin by addressing the textual relationships between this codex and other fifteenth-century copies of the main text and commentary, including both manuscripts and incunables. I then evaluate its diagrams, which would have assisted readers in visualizing and memorizing topics introduced in the main text, and which range from simple geometrical volvelles to a compendious climata diagram. To conclude, I consider what MS Codex 1881 might offer twenty-first-century audiences, including my initial work on digital editions of its diagrams. As a useful case study for both research and teaching, this manuscript will likely benefit several areas of inquiry in medieval and early modern studies, including the history of science and the history of education.