Librate

The currently observed orbital resonances at the 2:1 mean motion commensurabilities involving Io-Europa and Europa-Ganymede are such that the resonance variables #92;theta#95;1#61;#92;lambda#95;1-2#92;lambda#95;2#43;#92;omega#95;1 and #92;theta#95;3#61;#92;lambda#95;2-2#92;lambda#95;3#43;#92;omega#95;2 librate about 0° and #92;theta#95;2#61;#92;lambda#95;1-2#92;lambda#95;2#43;#92;omega#95;2 librates about 180°, all with small amplitude.

Their parts all librate on too nice a beam.

Her playful Sea-horse […] His watery way with waving volutes wins, / Or listening librates on unmoving fins.