So many ideas; so little time...
I hope someone will run with this... it could be a lot of fun.
1774 a man started building an orrery (model showing planets moving around the sun), with models of the planets hanging from the ceiling of his home, and moving around in step with the actual planets in space.
2013: A lot easier to make something like that, now that we have the Arduino. The mechanical elements remain a slight challenge, but the controls, the "clock" to drive the mechanical side a lot easier.
More on the above in the "Now THAT's COOL!" item at...http://blogik.wikidot.com/BlogRave
Did you know that in early April, the earth, the moon, and 5 out of the other 7 planets (pluto left out) were all in a line! See... if you'd had an orrery, you wouldn't have missed that!) (For those who haven't time to build a mechical one, there's a link to a nice online (or Windows or Mac screensaver) one at BlogRave.
If you know of an Arduino orrery driver, I hope you will post a link at the Playground (See link in menu bar at top of this page)