Date: Sat Nov 1 00:03:06 EST 1997
Subject: A.Word.A.Day--libration
li.bra.tion n. A very slow oscillation, real or apparent, of a satellite as viewed from the larger celestial body around which it revolves.[Lat. libratio, oscillation [librare, to balance [libra, balance]

Ernan McMullin, Galileo Galilei (1564-1642)., Vol. 10, Colliers Encyclopedia CD-ROM, 02-28-1996.
"After its appearance, Galileo made his last observational discovery, the libration (a slight apparent rotational variation) of the moon."

An effect caused by the apparent wobble of the Moon as it orbits the Earth. The Moon always keeps the same side toward the Earth, but due to libration, 59% of the Moon's surface can be seen over a period of time.