Emulation of an Orrery Orrery is a clockwork model of the solar system. I am trying to emulate one in 2D. Now, to emulate, I need to know what goes on inside. Can someone please explain the basic principle behind the clockwork? Or direct me to a resource that will explain all the machinery inside a simple Orrery.
The principle is almost exactly the same as a watch or clock, but instead of three concentric axles, you need 9 for the planets.

Have a google for Orrery kit - there are loads available. It is really all simple maths - you just need to know relative orbital periods in order to calculate cog sizes.

(picture from curiousminds.co.uk)
For moons, you do add a little complexity in the form of a transmission to support the Earth, so that the difference in movement can move the Moon.