In a unique collaboration, Nature Publishing Group and Institute of Physics Publishing have published the most extensive and comprehensive reference work in astronomy and astrophysics. This unique resource covers the entire field of astronomy and astrophysics and this online version includes the full text of over 2,750 articles, plus sophisticated search and retrieval functionality and links to the primary literature. The Encyclopaedia's authority is assured by editorial and advisory boards drawn from the world's foremost astronomers and astrophysicists. This first class resource is an essential source of information for undergraduates, graduate students, researchers and seasoned professionals, as well as for committed amateurs, librarians and lay people wishing to consult the definitive astronomy and astrophysics reference work.

An introduction to practical astrometry, dealing with the determination of positions, motions, distances and dimensions of celestial bodies ranging from quasars to artificial satellites. For this 2nd edition, the release of the Hipparcos and Tycho catalogs, the rise in CCD astrometry and the adoption of a new celestial reference frame by the IAU led to a significant modification of the text. And, especially, the outlook for astrometry has been completely rewritten.