An orrery is a mechanical device that models the motion of the moon and planets. This being the age of the Internet, we no longer need complicated gears, levers and cranks to simulate such motion: we can use Flash and simulate it on the Web:
(Visit this page for a larger version; I don't know who created it.) The planets and their orbits aren't to scale, of course (and neither are they for mechanical orreries), but it gives a great view of the motions of the Solar System and where the planets fall in relation to each other.
The best part is the Tychonian view; click on the button in the bottom-right to make the Earth the centre of attention. This isn't the ancient Ptolemaic system where the sun and planets revolve around the Earth; it's just that Earth is fixed in the middle of the frame. It shows how an observer on Earth sees the motion of the planets. Use the slider at the top to speed up time, and watch how a Mars flies distant from Earth only to approach closer than the Sun, move backwards for a few days, and then fling out into space again. (Use the "trace planet" control in the upper-right to see the path it and the other planets take with respect to the Earth.) All of the planets — to varying degrees — trace such erratic paths through our skies: that's why the Greeks dubbed them "wanderers". Just goes to show why the Earth-centric view of the Universe couldn't stand — how can you explain these erratic motions within a perfect circular arrangement? Of course, it took us a couple of millenia and the odd inquisition to get us to the modern view.