The lunar limb is the edge of the visible surface of the Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 as viewed from Earth. Libration

Libration

In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar 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

Baily's beads

As the moon "grazes" by the Sun during a solar eclipse, the rugged lunar limb topography allows beads of sunlight to shine through in some places, and not in others. This effect is called Baily's beads in honor of Francis Baily who first provided an exact explanation of the phenomenon in 1836...

 — beads of sunlight that shine through in some places, and not in others, during a solar eclipse

Solar eclipse

As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially blocks the Sun as viewed from a location on Earth. This can happen only during a new moon, when the Sun and the Moon are in conjunction as seen from Earth. At least...

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The contrast of a brightly illuminated limb against a black sky makes it a popular target when testing telescope and binocular optics.