"On his deathbed in 1601, the Danish nobleman and greatest naked-eye astronomer, Tycho Brahe, begged his young colleague, Johannes Kepler, "Let me not seem to have lived in vain." For more than thirty years - mostly in his native Denmark and then in Prague under the patronage of the Holy Roman Emperor, Rudolph II - Tycho had meticulously observed the movements of the planets and the positions of the stars. From these observations he developed his Tychonic system of the universe - a highly original, if incorrect, scheme that attempted to reconcile the ancient belief that the Earth stood still with Nicolaus Copernicus's revolutionary rearrangement of the solar system some fifty years earlier. Tycho knew that Kepler, the brilliant young mathematician he had engaged to interpret his findings, believed in Copernicus's arrangement, in which all the planets circled the Sun; and he was afraid his system - the product of a lifetime of effort to explain how the universe worked - would be abandoned." In point of fact, it was. From his study of Tycho's observations came Kepler's stunning three Laws of Planetary Motion - ever since the cornerstone of cosmology and our understanding of the heavens. Yet, as Kitty Ferguson reveals, neither of these giant figures would have his reputation today without the other. The story of how their lives and talents were fatefully intertwined is one of the most memorable sagas in the long history of science.