Carte du Ciel
Carte du Ciel ("Chart of the Sky") was an ambitious program to photograph every part of the sky, showing stars as faint as 14th magnitude, and to compile a catalogue, known as the Astrographic Catalogue, from this photographic atlas, listing the positions of stars down to 11th magnitude. It was conceived by the Scottish astronomer David Gill (1843-1914) and Admiral Mouchez, director of the Paris Observatory, who called an international conference in 1887 to initiate the project. Eighteen observatories agreed to cooperate and to adopt, as a standard design for a photographic telescope, the 13-inch refractor developed for the Paris Observatory by the brothers Paul and Prosper Henry. The sky was divided into zones and allocated to the various observatories. Although parts of the program were never completed, the plates obtained are of great value for comparison with re-photographed fields many years later.