Hey, I'm "helping" my brother to do his master thesis on real time orbits calculation for satellites. What I want to do is a 3d visualization of this: show the earth with the satellites orbiting around. I want the earth to look as good as possible (like one sees it in celestia, or other apps alike). What CG techniques could make an opengl earth look like earth? Thanks

Your question is not GL specific.
There is no "GL earth". GL has its graphics pipeline defined in its specification. If you want to use it, you need to study it.
You could do something simple like a textured sphere that shows the continents and oceans. Draw another larger sphere with a transparent regions and cloudy regions for the sky.
You can go futher with bumpmaping and scatter lighting, glow effect, ...