CLASSIFICATION IN ASTRONOMY Discovery and Classification in Astronomy: Controversy and Consensus. Steven J. Dick (Cambridge University Press, Cambridge, 2013). Pp. xvi + 458. £30. ISBN 978-1107-03361-0.Steven Dick has written a thought-provoking book on the relationship between astronomical discovery and classification - a carefully documented historical, sociological, and philosophical study with an abundance of useful references and notes on where to find them. His chronologically ordered classification of discoveries and loosely defined pre-discoveries ranges from Tycho Brahe's pre-discovery that comets are extra-terrestrial to the modem era and its fragmentary findings on protogalactic clouds.Although the author does not summarize it in just this way, classification has three purposes. It distinguishes one discovery from all others, it endows it with a name, and it attempts to fit the discovery into some organizational scheme.The first task of classification thus is the mapping of the parameters over which a discovery ranges, in order to differentiate it from all previous findings. Most scientists recognize that the many implications of a major discovery seldom are self-evident. Further work is needed to gain greater insight. Regardless of whether a discovery is observational or theoretical, astronomical or astrophysical, some sorting and sifting must take place, so that all concerned may understand, and agree on, what the discovery entails. This process seldom provides explanations; it only prepares the ground for explanations to emerge.The second aspect of classification, involving nomenclature, may appear more trivial; but this is where the word 'controversy' in the book's title surfaces. Dick's narrative repeatedly returns to the controversy that surrounded the reclassification of the planet Pluto to a newly minted class of dwarf planets. As far as astronomers and astrophysicists were concerned, this change had no effect on Pluto's well-defined physical characteristics, which had led to its original classification. The change arose solely because a number of new Solar System bodies resembling Pluto had been found at the turn of the millennium and many more were expected to emerge as the search for them intensified. Should a flood of these new objects now be permitted to add many more names to our long-standing list of planets? For half a century school books worldwide had declared that our Solar System has nine planets. …