Lunar limb

Two contrastinglightnessesof the lunar disc atcrescentphase,overexposedto showearthshine.Across the Moon, thelunar terminatoris visible, the borderline between the Moon's day and night; the lunar limb is the Moon's profile against the dark sky, top right in this picture.

Thelunar limbis the edge of the visible surface (disc) of theMoonas viewed fromEarth.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.

The lunar limb is not always the same: because of thelibrationof the Moon, i.e. small rotational movements as seen from Earth, its profile changes; this complicates the task of precisely calculatingeclipsetimes and durations.However, with elevation data from the mapping of thelunar 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 ofBaily's beads,which arecollimatedrays ofsunlightthat shine through in some places while not in others during asolar eclipse.

Thecontrastof the bright lunar disc against a blacknight skymakes the lunar limb a popular target when testingtelescopeoptics(includingbinoculars).