The best alternative to Celestia is Stellarium, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Celestia and four of them are so hopefully you can find a suitable replacement. Other interesting alternatives to Celestia are Solar System Scope, Gaia Sky and WinStars.
Stellarium is a free open source planetarium for your computer. It shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope.
Your stuck on earth, Celestia you can explore space
3D simulation of the Solar System and night sky - real time virtual orrery and observatory
3D visualization software for ESA's (European Space Agency) Gaia mission to chart about 1 billion stars of the Milky Way Galaxy.
WinStars uses the latest 3D technology to show the objects of our solar system in a realistic manner. Thus, it is possible to move from one planet to another, to follow a space probe on its long voyage or to observe a celestial event from a place to which you could never go.
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Stellarium is a planetarium, Celestia not