Wiktionary
librations
n. (plural of libration English)
Usage examples of "librations".
During the gloaming time when he had only one eye with which to observe the heavens or peruse his earlier notes and drawings, he wrote a final brief treatise on how best to gauge the diameters of stars and the distances between celestial bodies, and also made his last astronomical discovery, regarding the librations, or rocking, of the Moon.