The lunar limb is the edge of the visible surface of the Moon as viewed from Earth.[1] 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.[2] 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.[3] 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.