Editing Carte du Ciel (section)

==Combination of the Astrographic Catalogue with Hipparcos==
[[File:IAU_1887.jpg|thumb|1887 International Astrographic Congress]]
The vast amount of work invested in the Astrographic Catalogue, taking plates, measuring, and publishing, was looked at for a long time as giving only a marginal scientific profit. But today, astronomers are very much indebted to this great effort because of the possibility of combining these century-old star positions with the results from ESA's [[Hipparcos]] space astrometry satellite, allowing high accuracy proper motions to be derived for 2.5 million stars.  

Specifically, the Astrographic Catalogue positions were transferred from the decades-old printed catalogues into machine readable form (undertaken at the [[Sternberg Astronomical Institute]] in Moscow under the leadership of A. Kuzmin) between 1987 and 1994. The data was then reduced anew (at the [[US Naval Observatory]] in Washington under the leadership of Sean Urban), using the reference stars measured by the [[Hipparcos]] astrometry satellite.<ref name="urban">{{cite magazine
| last1      = Urban
| first1     = Sean
| last2      = Corbin
| first2     = Thomas
| date       = June 1998
| title      = The Astrographic Catalogue: A Century of Work Pays Off
| url        = https://archive.org/stream/Sky_and_Telescope_1998-06-pdf#page/n100/mode/1up
| magazine   = [[Sky & Telescope]]
| volume     = 95
| issue      = 6 
| pages      = 40–44
| location   = Cambridge, MA
| publisher  = Sky Publishing Corporation
| access-date= 2016-09-12
}}</ref>  

The stars from the [[Hipparcos Catalogue]] were used to establish a detailed reference framework at the various epochs of the Astrographic Catalogue plates, while the 2.5 million stars in the [[Tycho-2 Catalogue]] provided a dense reference framework to allow the plate distortions to be accurately calibrated and corrected. The proper motions of all the Tycho Catalogue stars could then be derived especially thanks to the Astrographic Catalogue, but additionally using star positions from more than 140 other ground-based catalogues.  

Aside from the 120,000 stars of the Hipparcos Catalogue itself, the resulting Tycho-2 Catalogue (compiled at the [[Copenhagen University Observatory]] under the leadership of Erik Høg) became the largest, most accurate and most complete, star catalogue of the brightest stars on the sky. It was the basis for deriving positions for all fainter stars on the sky, until the [[Gaia catalogues|Gaia 2 Catalog]] became available in 2017.<ref>{{Cite web |last=McCracken |first=H.J. |date=2022-09-13 |title=How an enormous project attempted to map the sky without computers |url=https://arstechnica.com/science/2022/09/how-an-enormous-project-attempted-to-map-the-sky-without-computers/ |access-date=2022-09-13 |website=Ars Technica |language=en-us}}</ref><ref>{{Cite journal |last=Vereshchagin Sergei |last2=Chupina Nataliya |last3=Postnikova Ekaterina |date=2018 |title=What Will Lead the Astrometry Data Accuracy Breakthrough in the Study of Star Clusters? |url=https://www.elibrary.ru/item.asp?id=41112927 |language=en |pages=170–174}}</ref> Sean Urban of the [[US Naval Observatory]] wrote in 1998:<ref name="urban" /> <blockquote>''The history of the Astrographic Catalogue endeavour is one of dedicated individuals devoting tedious decades of their careers to a single goal. Some believe it is also the story of how the best European observatories of the 19th century lost their leadership in astronomical research by committing so many resources to this one undertaking. Long portrayed as an object lesson in overambition, the Astrographic Catalogue has more recently turned into a lesson in the way that old data can find new uses.''</blockquote>