Carte du Ciel

Carte du Ciel ("Map of the Sky") was an international project to map the positions of millions of stars — that is to say, of all stars to the 11th or 12th magnitude. In English, the project was sometimes known as the Astrographic Chart or the Astrographic Catalogue.

It was begun in 1887 by Paris Observatory director Amédée Mouchez, who realized the potential of the new technology of photography to revolutionize the process of making maps of the stars. He conceived of a project that would take 22,000 photographic plates of the entire sky, each 2°×2°, and enlisted the aid of numerous observatories around the world, who were each assigned a separate section of the sky to work on.

The work involved two stages. In the first stage, astronomers determined precise positions of several thousand reference stars in various parts of the sky by timing their meridian transits. In the second stage, astronomers took photographic plates, and then the plates were turned over to a large number of semi-skilled female "computers" to determine the positions of the stars on each plate. (Before its modern meaning, the word "computer" meant a person who performs calculations). The "computers" would manually measure each star with respect to the dozen or so reference stars within that particular plate, and then perform calculations to determine the star's right ascension and declination.

Results

Decades of labour were expended internationally before the project was superseded by modern astronomical techniques. The project was never completed, although a catalogue was published in 1958.

One problem was that the work took much longer than expected. For instance, the Algiers Observatory, which was the most active in the project, did not finish its allotted work until 1919. As originally envisaged, the project was meant to have taken only 10 to 15 years.

A more serious problem was that while French astronomers were preoccupied with this project, which required steady, methodical labor rather than creativity, in other parts of the world like the United States astrophysics was becoming far more important than astrometry. As a result, French astronomy fell behind and lagged for decades.

The vast amount of work invested in the Astrographic Catalogue, taking plates, measuring, and publishing, looked for a long time as giving only little scientific profit. But today we are very much indebted to this great effort. The data were brought from the printed books onto machine readable form in the 1990ies. The data were then reduced anew with the reference stars from the Hipparcos astrometry satellite, especially with the 2.5 million stars in the Tycho-2 Catalogue. The proper motions of all these stars could then be derived especially thanks to the Astrographic Catalogue, but star positions from more than 140 other ground-based catalogues were also used. The resulting Tycho-2 Catalogue is the present best, most accurate and most complete, star catalogue of the brightest stars on the sky. It is the basis for deriving positions for all fainter stars on the sky.

Bibliography

* Turner, Herbert Hall. "The Great Star Map, Being a Brief General Account of the International Project Known as the Astrographic Chart" (London: John Murray, 1912).

External links

* [http://www.imcce.fr/fr/ephemerides/astronomie/Promenade/pages5/549.html Histoire de l'observatoire de Toulouse (with section on the Carte du Ciel)]