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Lunar limb
Edge of the visible surface of the Moon as viewed from Earth / From Wikipedia, the free encyclopedia
The lunar limb is the edge of the visible surface (disc) of the Moon as viewed from Earth.[1] Seen from afar, it looks like a circular arc, but with higher resolution, it can be seen that the Moon’s surface is not flat and therefore its profile is irregular.
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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.[2] 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.[3] 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.
The contrast of the bright lunar disc against a black night sky makes the lunar limb a popular target when testing telescope optics (including binoculars).