The letters in this volume cover Poincaré's multifaceted career in astronomy in its entirety, extending from the time of his first publications in 1880 to the end of his life in 1912. At a tender age, Poincaré established his authority in questions of celestial mechanics, and his counsel was soon sought out on technical points by the leading astronomers and geodesists of the day, including C.V.L. Charlier, G.H. Darwin, F.R. Helmert, A. Lindstedt A.M. Lyapunov, Simon Newcomb, and Karl Schwarzschild. Poincaré and his correspondents take up topics ranging from the three-body problem and the theory of orbits to perturbation theory and the figure of rotating fluid masses. The volume also sheds light on Poincaré's three terms as president of the Bureau of Longitudes, where he guided French astronomy through ambitious projects, such as measuring an arc of meridian near Quito.