The lunar limb is not always the same: because of the Libration of the Moon, i.e. small rotational movements as seen from Earth, its profile changes; this complicates the task of precisely calculating eclipse times and durations. However, with elevation data from the mapping of the lunar surface, a three-dimensional model including mountains and valleys can be created and the lunar profile for any given time predicted with a high degree of accuracy. The irregularity of the lunar limb is the cause of Baily's beads, which are collimated rays of sunlight that shine through in some places while not in others during a solar eclipse.
