One of the most important books in the history of science, Kepler's long-overlooked Somnium made a significant contribution to the study of astronomy. Its unique twofold nature--combining a serious scientific treatise on lunar astronomy with a fictional narrative about a trip to the moon--puzzled seventeenth-century readers as well as succeeding generations, and the work lapsed into obscurity. This excellent English translation of the Somnium (originally written in Latin) offers plentiful notes to assist in following Kepler's thought, along with an introduction that traces the work's history and significance. This edition reconstructs the Somnium's checkered history in complete and fascinating detail, with scholarly commentary and extensive appendixes.