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en.wikibooks.org...
Celestia provides photo-realistic, real-time, three-dimensional viewing of the solar system, the galaxy and the universe. It is an easy to use, freely-distributed, multi-platform, open source, software package which has become a valuable tool for astronomy education. Used in homes, schools, museums and planetariums around the world, it also is used as a visualization tool by space mission designers. Versions are available for computers running Windows, Macintosh (Mac OS X) and Linux operating systems.
Although it is optimized for 3D astronomical visualization, Celestia can be used to display and explore other 3D environments, too.
And, for some reason, many people seem to find exploring with Celestia to be a lot of fun.
It's perfectly "safe", if that's your concern randyv.