Editing Carte du Ciel (section)

==Origins and goals==

{| class="wikitable" align="right"
|+ Participating observatories and stars measured in the context of the Astrographic Catalogue<ref>Source: http://ad.usno.navy.mil/proj/AC {{Webarchive|url=https://web.archive.org/web/20071109160031/http://ad.usno.navy.mil/proj/AC/ |date=2007-11-09 }} and A.Kuzmin et al. 1999 Astronomy & Astrophysics Supplement, Vol.136, pp491-508</ref>
|-
! Observatory !!colspan="2"| [[Declination]] !! width="80" rowspan="2" |Epoch !! No. of 
|-
! (Zone) !! From !! To !! stars
|-
| [[Royal Observatory, Greenwich|Greenwich]] || +90° || +65° || align="center"| 1892&ndash;1905 || 179,000
|-
| [[Vatican Observatory|Vatican]] || +64° || +55° || align="center"| 1895&ndash;1922 || 256,000
|-
| [[Catania Astrophysical Observatory|Catania]], Sicily || +54° || +47° || align="center"| 1894&ndash;1932 || 163,000
|-
| [[Helsinki University Observatory|Helsinki]] || +46° || +40° || align="center"| 1892&ndash;1910 || 159,000
|-
| [[Leibniz Institute for Astrophysics Potsdam|Potsdam]] || +39° || +32° || align="center"| 1893&ndash;1900 || 108,000
|-
| Hyderabad north || +39° || +36° || align="center"| 1928&ndash;1938 || 149,000
|-
| [[Royal Observatory of Belgium|Uccle]], Belgium || +35° || +34° || align="center"| 1939&ndash;1950 || 117,000
|-
| Oxford 2 || +33° || +32° || align="center"| 1930&ndash;1936 || 117,000
|-
| Oxford 1 || +31° || +25° || align="center"| 1892&ndash;1910 || 277,000
|-
| [[Paris Observatory|Paris]] || +24° || +18° || align="center"| 1891&ndash;1927 || 253,000
|-
| [[Bordeaux Observatory|Bordeaux]] || +17° || +11° || align="center"| 1893&ndash;1925 || 224,000
|-
| [[Toulouse Observatory|Toulouse]] || +10° || +05° || align="center"| 1893&ndash;1935 || 270,000
|-
| [[Algiers Observatory|Algiers]] || +04° || &minus;02° || align="center"| 1891&ndash;1911 || 200,000
|-
| [[Real Instituto y Observatorio de la Armada|San Fernando]], Spain || &minus;03° || &minus;09° || align="center"| 1891&ndash;1917 || 225,000
|-
| Tacubaya, Mexico || &minus;10° || &minus;16° || align="center"| 1900&ndash;1939 || 312,000
|-
| Hyderabad south || &minus;17° || &minus;23° || align="center"| 1914&ndash;1929 || 293,000
|-
| [[Argentine National Observatory|Córdoba]], Argentina || &minus;24° || &minus;31° || align="center"| 1909&ndash;1914 || 309,000
|-
| [[Perth Observatory|Perth]], Australia || &minus;32° || &minus;37° || align="center"| 1902&ndash;1919 || 229,000
|-
| Perth/[[Royal Observatory, Edinburgh|Edinburgh]] || &minus;38° || &minus;40° || align="center"| 1903&ndash;1914 || 139,000
|-
| [[Royal Observatory, Cape of Good Hope|Cape Town]]|| &minus;41° || &minus;51° || align="center"| 1897&ndash;1912 || 540,000
|-
| [[Sydney Observatory|Sydney]] || &minus;52° || &minus;64° || align="center"| 1892&ndash;1948 || 430,000
|-
| [[Melbourne Observatory|Melbourne]] || &minus;65° || &minus;90° || align="center"| 1892&ndash;1940 || 218,000
|}

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|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 &ndash; 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. They yielded a catalogue of positions and magnitudes down to about 11.5 mag, and the programme was largely completed during the first quarter of the 20th century.

For the second goal, a second set of plates, with longer exposures but minimal overlap, was to photograph all stars to 14 mag. These plates were to be reproduced and distributed as a set of charts, the Carte du Ciel, in contrast to previous sky charts which had been constructed from the celestial coordinates of stars observed by transit instruments. Most of the Carte du Ciel plates used three exposures of 20 minutes duration, displaced to form an equilateral triangle with sides of 10 arcsec, making it easy to distinguish stars from plate flaws, and asteroids from stars.

A contemporary account of this vast international astronomical collaboration, published in 1912, was given by [[Herbert Hall Turner]], then [[Savilian Professor of Astronomy]] at Oxford University.<ref>H.H. Turner, 1912 ''The Great Star Map, Being a Brief General Account of the International Project Known as the Astrographic Chart'' (John Murray)</ref> Other aspects are covered in various papers in the Proceedings of IAU Symposium Number 133 held in 1988.<ref>Proceedings of IAU Symposium Number&nbsp;133, ''Mapping the Sky'', editor S. Debarbat, 1988</ref>