I'm developing a simulator (it started as one of those "wouldn't it be intersting if..." projects
) which is in some ways similar to Celestia, although looking at the area from a different aspect (oorbital mechanics, rather than rendering and display). Not a good opening post, but stick with me 
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Anyway, my project started off as a numerical analysis of gravitaional systems in Matlab. However, the outputted co-ordinates and velocities were very hard to interpret meaningfully. So, I'm in the process of writing a module to output a video to an .avi, as well as moving from Matlab to something like c++.
However, seeing the quality of the results from Celestia, I wanted to get peoples' opinions on OpenGL - I've no experience in coding for it, and I'm interested to hear what people think is possible with it.