to remain poised or balanced.
to oscillate or move from side to side or between two points.
Watching them to the ground, the wings of a hawk, or of the brown owl, stretch out, are drawn against the current air by a string as a paper kite, and made to flutter and librate like a kestrel over the place where the woodlark has lodged …
John Leonard Knapp, Journal of a Naturalist, 1829
At this period the balance of tropic and pole librates, and the vast atmospheric tides pour their flood upon one hemisphere and their ebb upon another.
Victor Hugo, The Toilers of the Sea, translated by William Moy Thomas, 1866
The verb librate comes from Latin lībrātus, the past participle of lībrāre “to balance, make level,” a derivative of the noun lībra “a balance, a pound (weight).” The further etymology of lībra is difficult. It is related to Sicilian (Doric) Greek lī́tra “a silver coin, a pound (weight),” also a unit of volume, e.g., English litre (via French litre from Latin). Both lī́tra and lībra derive from Italic līthrā. Lībra becomes lira in Italian, libra in Spanish and Portuguese, French livre (both coinage and weight). The abbreviation for lībra (weight) is lb.; the symbol for lībra (the coinage, i.e., the pound sterling) is £. Librate entered English in the 17th century.