July 30, 2019, in the Main Reading Room of the Linda Hall Library. The lecture: One of the treasures of the Linda Hall Library is a relatively thin, unassuming volume printed in Venice in 1496, entitled Epytoma in Almagestum Ptolemei. This book, a reworking of Ptolemy’s astronomical masterpiece, the Almagest, was written in the early 1460s by Georg Peurbach and Johannes Regiomontanus, two of the most important figures of fifteenth-century astronomy. While it built upon earlier medieval commentaries on the Almagest, the Epitome Almagesti (as it is usually called) is remarkable for its depth of comprehension of even the most technical aspects of Ptolemy’s astronomy, its clear explanations, and its incorporation of new discoveries made by its authors and by Arabic astronomers. This work, which had circulated in manuscript form for 35 years before it was printed, became the textbook by which students of astronomy learned the intricacies of Ptolemy’s geocentric astronomy. In his writings challenging the Ptolemaic system, Copernicus did not always use Ptolemy’s own work, the Almagest; instead, he often referred to this book by Peurbach and Regiomontanus. Indeed, the Epitome Almagesti did not just serve as the foil to Copernicus’s new theories; on the contrary, it contained proofs that were fundamental to his development of a heliocentric system. Dr. Zepeda will tell the drama-filled story of how and why this book was written, as well as discuss its contents, its sources, and its influence upon the astronomy of the 15th and 16th centuries. The speaker: Henry Zepeda is a historian of science with his specialization in the medieval mathematical sciences, especially astronomy. He is a Teaching Fellow at Wyoming Catholic College. He received his Ph.D. from the University of Oklahoma, and he worked in Munich for several years as a postdoctoral researcher in the Ptolemaeus Arabus et Latinus group at the Bavarian Academy of Sciences and Humanities. His first book, The First Latin Treatise on Ptolemy’s Astronomy: The Almagesti minor (c. 1200), provides an edition, translation, and study of an influential medieval summary of Ptolemy’s Almagest. At the Linda Hall Library, Henry has been working on a critical edition and analysis of the Epitome Almagesti. The Epitome Almagesti is a summary of Ptolemy’s Almagest, and it can arguably be considered the high point of the Ptolemaic astronomical tradition because of its depth of comprehension of even the most technical aspects of Ptolemy’s astronomy, its clear explanations, and its incorporation of new findings made by both of its authors and by Arabic astronomers. Better knowledge of this work will help scholars better understand the state of astronomy at the time when Ptolemy’s system was challenged by Copernicus, Kepler, Galileo, and others.

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