Lunar limb
edge of the visible surface of the Moon as viewed from Earth
The lunar limb is the edge of the visible surface (disc) of the Moon as viewed from Earth. 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. In the 17th century, the irregularity of the lunar limb was observed by Galileo Galilei using a telescope, and the discovery was mentioned in his 1610 treatise titled Sidereus Nuncius.
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.
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).
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Vocabulary and entity names are the principal evidence signals here, because they determine the precision of every later search. The source revision retrieved here is dated May 15, 2026. The linked authority identifier is Q6703814. The Library of Congress control number is sh85078859. 1 of 1 selected statements include explicit references; 0 carry qualifiers and 0 use preferred rank. The first chronological checks are 1610.
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This entry incorporates text from “Lunar limb” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.