Celestia is a free software package that graphically simulates celestial objects ranging from the earth and our solar system to distant constellations and galaxies.
Simulating motions, light and shadow in three dimensions, the program generates amazing 3D images and allows the user to travel to any place in the universe and visit planets, comets, and other objects in the solar system.
The program is licensed under GPL and versions are available for Linux, Windows, and Mac OS X.
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Celestia Homepage .
Celestia Screenshots .