Baily's beads
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As the moon "grazes" by the Sun during the eclipse, the rugged lunar limb topography allows beads of sunlight to shine through. This effect is called Baily's Beads, named in honor of Francis Baily who first noted the phenomenon in 1836.
The lunar topography is non-smooth (mountains, craters, valleys, etc.). The lunar limb profile is also known accurately from grazing occultations of stars. So in advance of the eclipse we have a fairly good idea which mountains and valleys will cause the beads to appear. While Baily's Beads are briefly seen for a few seconds at the central path of the eclipse, their effect is maximized near the eclipse limits to 1-2 minutes.
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