The letters in this volume cover Poincare'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, Poincare established his authority in questions of celestial mechanics, and his views were soon sought after on a vast array of questions by the leading astronomers and geodesists of his time, including C.V.L. Charlier, G.H. Darwin, H. Faye, F.R. Helmert, G. W. Hill, A. Lindstedt, A.M. Liapunov, N. Lockyer, S. Newcomb, K. Schwarzschild, and F. Tisserand. Poincare and his correspondents take...