An Orrery of the Earth, Moon and Sun. It's reasonably accurate in terms of the rotation and orbital periods of the objects, but clearly it isn't to scale with respect to size/distances.
I'm using the new 60 tooth version of the large turntable. You can use the older 56 tooth version, but because it is slightly smaller in diameter you need to use a 12 tooth double bevel gear to drive it instead of the standard 8 tooth gear.
When assembling the various gears, it’s important to make sure everything rotates freely. That is, don’t push the gears onto the axles too tightly, otherwise there will be a lot of friction between the gears an the Technic liftarms beside them. This is especially important when connecting the earth.
The model can be motorized by replacing the crank with a gear and attaching a Power Functions motor to it. There are many ways to do that, but you can see how I did it in the video.