This encyclopedia is a landmark publication that summarises astronomers' knowledge of the universe and its contents in 1000 pages. There are 403 alphabetically arranged articles on topics from Accretion to Zodiacal Light. The articles are written by more than 100 professional astronomers, selected on a world wide basis by an advisory board of 24 distinguished scientists. The articles are carefully cross-referenced, and they are supported by 520 photographs and diagrams. An in-depth index makes it easy to retrieve information on any subject. The articles range over every kind of astronomical object and astrophysical concept. The rich density of information on current research topics can be appreciated by considering supernovae: there are five articles on supernovae, four on supernova remnants, and five on cosmic rays. All modern topics are treated with this degree of thoroughness. Accepted facts are always clearly separated from proposed explanations. The coverage includes quasars, the background radiation, every planet and the major moons in the solar system, solar system debris, stars and stellar evolution, and galaxies.