Libration is the apparent oscillation of a heavenly body (usually the moon) because the edges are coming in and out of view. I imagine it occurs more frequently after the viewer has taken a libation.
Hands up those who thought it involved drinking at the library. Oddly I didn't have a picture of me doing that, so here's one of me with The Sphere. Apparently he oscillates on windy days.
updated 18/12 - we're just back from a cruise. The ship very conveniently had a cocktail bar close to the library. I *may* have indulged ... It was for research though, obviously. Any minute I might need to describe the exacy taste of a White Lady, a Singapore Sling or a Brandy Alexander.