Real-time space simulation using OpenGL. Celestia is an open-source, real-time space simulator. It is written in cross-platform OpenGL, allowing it to run on a multitude of operating systems. Although the Win32 and MacOS X ports are very popular, the source code can be compiled under Unix-based operating systems for four different front-ends: Gkut, Gtk, Gnome, and KDE. Celestia is scriptable using the Lua library. There are many third-party add-ons available, making this not only an educational tool, but also a fun diversion.