I went into town at lunchtime and was instantly drawn into a gift shop by what appeared to be a self turning globe on an acrylic base. It consists of a static outer shell containing a close fitting moving inner graphics shell suspended in liquid that moves under solar power. They range in price from £250 for 4" to £400 for 6". The efficient solar powered motor that drives the globe torque-reacts against an armature that is attracted to the earths magnetic field. At that price, I was not particularly tempted, but it did make me stop and ask questions. Neil
Years ago our glass-blower in the Physics dept used to take an old light bulb and replace the filament with something that looked like a black weather vane. When re-sealed the light bulb was fitted to a base and placed in the window at which point the black vane would start to rotate. VB might be familiar with these - I don't remember what they were called radiation lamps or something, it was very novel at the time and I never saw anything like it since.
I first saw them at Proops in Tottenham Court Road in the 1960s.
What's going on inside these is pretty complex (the Wiki article explains it correctly I believe). The basic principle seems to be that any explanation which is reasonably simple is also, sadly, wrong.