However, when I started using Nautilus, it broke, and I never bothered to find a solution.
Today I realised that xearth has been replaced by xplanet, and it’s available as an Ubuntu package. And it’s beautiful!
However, the Nautilus problem remains: The window manager controls the background, so a program cannot write straight to it. The solution is to render a file and then ask Nautilus to reload it at regular intervals.
Fortunately, somebody written a script that does this, and one can download it here. Just remember to change the PREFIX to a path that exists.