Carte du Ciel

The Carte du Ciel (literally, 'Map of the Sky') and the Astrographic Catalogue (or Astrographic Chart) were two distinct but connected components of a massive international astronomical project, initiated in the late 19th century, to catalogue and map the positions of millions of stars as faint as 11th or 12th magnitude. Twenty observatories from around the world participated in exposing and measuring more than 22,000 (glass) photographic plates in an enormous observing programme extending over several decades. Despite, or because of, its vast scale, the project was only ever partially successful – the Carte du Ciel component was never completed, and for almost half a century the Astrographic Catalogue part was largely ignored. However, the appearance of the Hipparcos Catalogue in 1997 has led to an important development in the use of this historical plate material. A vast and unprecedented international star-mapping project was initiated in 1887 by Paris Observatory director Amédée Mouchez, who realized the potential of the new dry plate photographic process to revolutionize the process of making maps of the stars. As a result of the Astrographic Congress of more than 50 astronomers held in Paris in April 1887, 20 observatories from around the world agreed to participate in the project, and two goals were established: For the first, the Astrographic Catalogue, the entire sky was to be photographed to 11 mag to provide a reference catalogue of star positions that would fill the magnitude gap between those previously observed by transit and meridian circle instrument observations down to 8 mag – this would provide the positions of a reasonably dense network of star positions which could in turn be used as a reference system for the fainter survey component (the Carte du Ciel). Different observatories around the world were charged with surveying specific declination zones (see table). The Astrographic Catalogue plates, of typically 6 minutes exposure, were in due course photographed, measured, and published in their entirety.