A Real-time 3D space simulation
"Celestia is a real-time 3D space simulation featuring a database of over 100000 stars, nearly a hundred solar system, objects, and a complete catalog You can travel throughout the solar system, to any of the over 100,000 stars, or even beyond the galaxy. Celestia comes with a large catalog of stars, galaxies, planets, moons, asteroids, comets, and spacecraft."
(Requires a download - version for Windows, Mac and Linux - the Mac version will run from the desktop with no installation required.)
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