There was one a while back that was really well written and done (not a generic quizspot one but a semi-professional one) with the one bad thing that some of the code was messed up so the percentages were off and the creator didn't want to go back and fix it.
It's the libration. The icon was created using Astronomy Picture of the Day (http://antwrp.gsfc.nasa.gov/apod/astropix.html) and the entry for this picture explained the rolling motion.
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Here's the original entry:
http://antwrp.gsfc.nasa.gov/apod/ap040829.html