In May, 1539, a young, German mathematician named Georg Joachim Rheticus traveled hundreds of miles across Europe in the hopes of meeting and spending a few days with the legendary astronomer, Nicolas Copernicus, in Frombork, Poland. Two and a half years later, Rheticus was still there, fascinated by what he was discovering, but largely engaged in trying to convince Copernicus to publish his masterwork-De revolutionibus (On the Revolutions of the Heavens), the first book to posit that the sun was the center of the universe. That he was finally able to do so just as Copernicus was dying became a turning point for science and civilization. That he then went on to a legendary career of his own-he founded the field of trigonometry, for example-will be one of the many surprises in this eye-opening book, which will restore Rheticus to his rightful place in the history of science.