A unique dictionary of astronomy specifically written for practical amateur astronomers. In addition to definitions, it provides an invaluable reference source for terms, techniques, instruments, formulas and processes for practising observers, both amateur and professional. A special feature of this dictionary is extended definitions for many topics; they give sufficient information for many of the techniques and items of instrumentation to be used as well as understood. With over 200 illustrations and extensive appendices, this is an essential reference book for every astronomer.