This article presents four detailed case studies of sixteenth-century vernacular translations of Sacrobosco's De sphaera. Previous scholarship has highlighted the important role of Sacrobosco's Sphere in medieval and early modern universities, where it served as an introductory astronomy text. We argue that the Sphere was more than a university teaching text. It was translated many times and was accessible to a wide range of people. The popularity of the Sphere suggests widespread interest in cosmological questions. We suggest that the text was a profitable one for early modern printers, who strove to identify books that would be reliable sellers. We also argue that the Sphere was not a static text. Rather, translators and editors added commentaries and other supplemental material that corrected and updated Sacrobosco's original text and

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