NASA and ESA in their educational and outreach program.
We really thanks that this software is free to download. Chris Laurel is the author of this software, released in 2001.
After downloading, check first the menu "Help" and "Run Demo" and don't forget to read "User Guide". Here is the Demo I captured, I made it in GIF Format.
Website to download : http://shatters.net/celestia/download.html