The slow waltz of the celestial heavens.
LEGO Orrery by Simon Liu, on Flickr
And video of it in action:
Celestial Mechanics - LEGO Orrery by Simon Liu, on Flickr
... Definitely the most challenging and annoying build yet playing with Technic, but The coolest motion I could think of was the orbital mechanics.
Took me a few tries to figure out how to make one of these things, and keeping it to 101 pieces - in the end it was just under 12 Lunar orbits per Earth Orbit.
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Lego orrery breakdown by Simon Liu, on Flickr