A difficult classification so we have decided to extend the classification category to include simulations, some might argue the applications are educational and your right or at least they can be, they are also applications that require skill, however although these games (simulations) require skill to use properly it is still relatively easy to use without fear of going wrong.
This free space simulation that lets you explore our universe in three dimensions. Unlike most planetarium software, Celestia doesn't confine you to the surface of the Earth. You can travel throughout the solar system, to any of over 100,000 stars, or even beyond the galaxy. All movement in Celestia is seamless; the exponential zoom feature lets you explore space across a huge range of scales, from galaxy clusters down to spacecraft only a few meters across. A 'point-and-goto' interface makes it simple to navigate through the universe to the object you want to visit. Celestia is expandable. Celestia comes with a large catalog of stars, galaxies, planets, moons, asteroids, comets, and spacecraft. If that's not enough, you can download dozens of easy to install add-ons with more objects.
http://www.shatters.net/celestia
The above is a basic package installation if you use the Synaptic-Package-Manager and search for "celestia" you will see a list of available Celestia related packages. You will need a suitable graphics card with this application with a slow graphics card the displayed images may work but you will experience chunks missing or flashing of the screen or viewport. You may also need to run this from the command line. Other packages you maybe able to install include the following.
celestia-glut - A real-time visual space simulation (GLUT frontend)
celestia-gnome - A real-time visual space simulation (Gnome frontend)
xorsa - tool for Celestial Mechanics investigations (This may have dependent libraries that will also need to be installed these should be selected automatically)
We have not taken the time to install passed the basic installation however further help is available along with additional addons possibly past what is available from the repository.
KStars should just about works on anything visually I think it fair to say is less impressive than Celestia but each have their merits. KStars is a Desktop Planetarium for KDE. It provides an accurate graphical simulation of the night sky, from any location on Earth, at any date and time. The display includes 130,000 stars, 13,000 deep-sky objects,all 8 planets, the Sun and Moon, and thousands of comets and asteroids.