Lunar limb

The lunar limb is the edge of the visible surface (disc) of the Moon as viewed from Earth.[1] Libration of the Moon, with its irregular surface, leads to small changes in its profile; 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, which are collimated rays of sunlight that shine through in some places while not in others during a solar eclipse.

Two contrasting lightnesses of the lunar disc at crescent phase, overexposed to show earthshine

The contrast of the lunar disc, brightly illuminated by direct sunlight, against a black night sky makes it a popular target when testing telescope (including binocular) optics.

References
  1. "List of basic lunar features". Archived from the original on 2006-02-03.
  2. O'Byrne, Chris. "Lunar Limb Corrections". Archived from the original on 2008-01-13.
  3. Watts, C. B. (1963). The Marginal Zone of the Moon. Astronomical Papers. 17. Nautical Almanac Office. p. 951. Bibcode:1963USNAO..17....1W.
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