Book excerpt: Sextants at Greenwich consists of two main sections: The introductory chapters and the catalogue of navigating instruments of the National Maritime Museum. The first section gives a general overview of the history of celestial navigation with an emphasis on the instruments that were developed and used for that purpose, between about 1450 and the 1970s. The instruments in the catalogue form the main thread in these chapters. The catalogue consists of 347 entries of instruments for celestial navigation, the octants, sextants and related instruments preserved in the National Maritime Museum. Each entry includes the place of the object's origin, its maker, the object's date, inscriptions (by the maker and/or relating to an owner), the graduated scale, the instrument's dimensions and a general description that includes details such as used materials and detached parts. Finally the object's provenance (previous owners and/or users) and references to literature on its history and handling are given.