Baily's beads

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The effect of Baily beads , or the diamond ring effect, is a feature of a total and annular solar eclipse. When the Moon covers the Sun during a solar eclipse, the rough topography of the lunar member allows the grains of sunlight to shine in some places while not elsewhere. This effect is named after Francis Baily, which explains the phenomenon in 1836. The effect of a diamond ring is seen when only one bead is left, appearing as a "diamond" that shines in a bright ring around the moon's silhouette.

Moon topography has been a great relief due to the presence of mountains, craters, valleys, and other topographic features. The irregularities of the lunar member profiles (the "edge" of the Moon, as seen from a distance) are known accurately from the observations of the malignant occultations of the stars. Therefore, astronomers have a pretty good idea that mountains and valleys will cause the beads to appear before the eclipse. While Baily's beads look briefly for a few seconds in the middle of an eclipse path, their duration is maximized near the curb of the umbra road, lasting 1-2 minutes.

After the effect of the diamond ring is reduced, the effect of the next Baily bead and the totality phase is safe to be seen without the sun filter being used during the partial phase. At that time, less than 0.001% of the Sun's photosphere is visible.

Observers on the path of the totality of the first solar eclipse saw the Sun cover gradually by the silhouette of the moon for over an hour, followed by the effect of diamond rings (visible unfiltered) as the last part of the photosphere disappeared. As the bursts of light from the ring fade, Bailey's beads appear as the last part of the bright photosphere shining through the paralleled valley at the edge of the Moon. When the Baily beads disappear behind the rising edge of the moon (the beads also reappear at the end of totality), a thin red edge called the chromosphere (Greek chromos meaning "color") appears. Although reddish hydrogen radiation is most visible to the naked eye, the chromosphere also emits thousands of additional spectral lines.

Video Baily's beads

Observation history

Although Baily is often said to have discovered the cause of the feature that bears his name, Sir Edmond Halley made the observations of Baily's first record of Baily's beads during the May 3 solar eclipse of 1715. Halley illustrates and verifies correctly the cause of the effects in his book. "The ultimate solar eclipse observations [...]" in the Transactions of the Royal Society Philosophy :

Approximately two minutes before the total immersion, the remaining Sun portion is reduced to a very fine horn, the Extremeties seem to lose their sharpness, and become round like a star... The appearance can be continued from no other cause than the inequality of the Moon Surface, there are some elevated parts there near the South Pole of the Moon, where the Interposition section of the very fine Filament of Light is tapped.

Maps Baily's beads

In media

Cosmas Damian Asam is probably the earliest realistic painter to describe a total solar eclipse and diamond ring. His painting was completed in 1735.

The phenomenon of Baily beads is seen during the opening sequence of NBC's TV credits Heroes , while the Gem Gem effect is seen during the credit opening sequence Star Trek: Voyager , though from a fictitious extrasolar body, seen from outer space.

Baily's Beads, Total Solar Eclipse - Sky & Telescope

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References

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The Solar Eclipse - Bailey's beads and the Diamond Ring - YouTube