The Baily's beads effect, or Diamond ring effect, is a feature of total and annular 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 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.
After the diamond ring effect has diminished, it is safe to view it and the subsequent Baily's beads effect without the solar filters used during the partial phases of the eclipse. By then less than .001 of 1 percent of the photosphere is visible.
Observers in the path of totality at a solar eclipse see first a gradual covering of the Sun by the lunar silhouette for over an hour, and then, as the last bit of photosphere disappears, with filters removed, the diamond ring effect is seen. As the burst of light from the ring fades, Bailey's Beads appear as the last bits of the bright photosphere shine through valleys aligned at the edge of the Moon. . As the Baily’s beads disappear behind the advancing lunar edge (the phenomena occur in reverse at the end of totality), a thin reddish edge, called the chromosphere (named from the Greek chromos, color), appears. Though the reddish hydrogen radiation is most visible to the unaided eye, the chromosphere also radiates thousands of additional spectral lines.
Although Baily is often said to have discovered the cause of the feature which bears his name, Sir Edmond Halley made the first recorded observations of Baily's beads during the Solar eclipse of 3 May 1715.[Note 1] Halley described and correctly ascertained the cause of the effect in his "Observations of the late Total Eclipse of the Sun[...]" in the Philosophical Transactions of the Royal Society:
About two Minutes before the Total Immersion, the remaining part of the Sun was reduced to a very fine Horn, whose Extremeties seemed to lose their Acuteness, and to become round like Stars ... which Appearance could proceed from no other Cause but the Inequalities of the Moon's Surface, there being some elevated parts thereof near the Moon's Southern Pole, by whose Interposition part of that exceedingly fine Filament of Light was intercepted.
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.
- Great Britain didn't adopt the Gregorian calendar until 1752, so at the time of the eclipse, the date was recorded as 22 April 1715.
- Baily, Francis. "On a remarkable phenomenon that occurs in total and annular eclipses of the sun". Monthly Notices of the Royal Astronomical Society. 4: 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".
- Pasachoff, J. M. & Covington, M. The Cambridge Eclipse Photography Guide (Cambridge Univ. Press, 1993)
- Pasachoff, Jay M. "Solar eclipses as an astrophysical laboratory". Nature. 459 (7248): 789–795. doi:10.1038/nature07987.
- Pasachoff, J. M. (1999) "Halley and his maps of the Total Eclipses of 1715 and 1724" Journal of Astronomical History and Heritage ISSN 1440-2807, Vol. 2, No. 1, pp. 39–54
- Nemiroff, R.; Bonnell, J., eds. (28 January 2008). "A Solar Eclipse Painting from the 1700s". Astronomy Picture of the Day. NASA.