Baily's beads
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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 honour of Francis Baily, who first noted the phenomenon in 1836.
Lunar topography has considerable relief because of the presence of mountains, craters, valleys, etc. The irregularities of the lunar limb profile (the "edge" of the Moon, as seen from a distance) are known accurately from observations of grazing occultations of stars. Astronomers thus have a fairly good idea which mountains and valleys will cause the beads to appear in advance of the eclipse. While Baily's beads are seen briefly for a few seconds at the center of the eclipse path, their duration is maximized near the edges of the path of the umbra, reaching 1–2 minutes.
