Astronomy is a rapidly developing science, and its terms of reference also evolve at great speed. This reference book cuts a path through the maze of complex technical language and offers concise definitions of over 2400 entries, encompassing all aspects of classical and modern astronomy. It provides the names of constellations, stars, galaxies, asteroids, comets, nebulae and observatories. Entries on telescopes and space missions show how astronomers have opened new windows on the universe, while exact definitions of terms used in modern astrophysics and cosmology allow access to the most up-to-date research.