The Baily's beads effect is a feature of total solar eclipses. 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. The name is in honor of Francis Baily who first provided an exact explanation of the phenomenon in 1836. The diamond ring effect is seen when only one bead is left; a shining diamond set in a bright ring around the lunar silhouette.
Lunar topography has considerable relief because of the presence of mountains, craters, valleys and other topographical features. 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.
It is not safe to view Baily's beads or the diamond ring effect without proper eye protection because in both cases the photosphere is still visible.
The Baily's beads phenomenon is seen during the credit opening sequence of the NBC TV show Heroes, while the Diamond Ring effect is seen during the credit opening sequence of Star Trek: Voyager, albeit from a fictitious extrasolar body, seen from space.
- Baily, Francis. "On a remarkable phenomenon that occurs in total and annular eclipses of the sun". Monthly Notices of the Royal Astronoy Society, Vol. 4, p.15. Bibcode:1836MNRAS...4...15B.
- Littmann, Mark; Willcox, Ken; Espenak, Fred (1999). Totality - Eclipses of the Sun. Oxford University Press. pp. 65–66. ISBN 0-19-513179-7.
- O. Staiger. "The Experience of Totality".
- "A Solar Eclipse Painting from the 1700s". Astronomy Picture of the Day. 2008-01-28.