Celestia is a real-time visual simulation of space. Choose a point within the Local Group of galaxies, and Celestia will show you an approximation of how it would appear to your eyes were you actually there. Some of what Celestia shows is necessarily hypothetical--the farther away from Earth you get, the less real data there is and the more guesswork is involved. Thus Celestia supplements observational data with good guesses based on models of stellar and planetary processes.
Celestia and the Openuniverse project have joined forces to concentrate on developing Celestia.
Celestia is currently packaged in the main Debian archive.
Celestia Homepage http://www.shatters.net/celestia/
Presentation in French : http://www.tice.ac-versailles.fr/logicielslibres/spip.php?article7