How an all-but-forgotten medieval astronomy book ushered in the modern age of science Few people today have heard of Johannes de Sacrobosco, yet his textbook was the most widely read astronomy book ever written. For five hundred years, from when it was first written in the Middle Ages to the dawn of the modern world, the Sphere introduced Europeans to the cosmos. The Book Everyone Read traces the unpredictable twists and turns of scientific knowledge and discovery through the stories of Sacrobosco's readers-and in doing so, tells a new story of the emergence of modern science. Kathleen Crowther shows how the secret to the Sphere's longevity lay with its readers themselves, who chose which aspects of Sacrobosco's original thirteenth-century text to accept, which to modify, and which to reject. Far from unchanging, editions of the book were accompanied by commentary, corrections, and details that challenged and revised the book's original worldview of a finite, Earth-centered cosmos. Crowther introduces us to the university professors who peppered their lectures on the Sphere with tales of the exciting new discoveries made with telescopes and musings on the new Sun-centered model proposed by Copernicus, the navigators who found their way across the Atlantic using information about the stars they found in the book, the missionaries who brought translations of it to the Americas and Asia, and many others. Through the surprising life of a medieval book of astronomy, The Book Everyone Read charts a scientific conversation that extended far beyond what we understood before.