The lunar limb is the edge of the visible surface of the Moon as viewed from Earth. Libration of the Moon, with its irregular surface, leads to small changes in its profile, and this complicates the task of precisely calculating eclipse times and durations. However, data from the mapping of the lunar surface allows astronomers to predict the lunar profile for any given time with a high degree of certainty. The irregularity of the lunar limb is the cause of Baily's beads—beads of sunlight that shine through in some places, and not in others, during a solar eclipse.
The contrast of a brightly illuminated limb against a black sky makes it a popular target when testing telescope and binocular optics.
References
Lunar limb Wikipedia(Text) CC BY-SA