Baily's beads

The Baily's beads effect or diamond ring effect is a feature of total and annular solar eclipses. As the Moon covers the Sun during a solar eclipse, the rugged topography of the lunar limb allows beads of sunlight to shine through in some places while not in others. The effect is named after Francis Baily, who explained the phenomenon in 1836.[1][2] The diamond ring effect is seen when only one bead is left, appearing as a shining "diamond" set in a bright ring around the lunar silhouette.[3]

Baily's beads photographed 4 seconds before totality of the solar eclipse of August 21, 2017

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, lasting 1–2 minutes.

After the diamond ring effect has diminished, the subsequent Baily's beads effect and totality phase are safe to view without the solar filters used during the partial phases. By then, less than 0.001% of the Sun's photosphere is visible.

Observers in the path of totality of a solar eclipse see first a gradual covering of the Sun by the lunar silhouette for just a small duration of time from around one minute to four minutes, followed by the diamond ring effect (visible without filters) as the last bit of photosphere disappears. 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.[4] As the Baily's beads disappear behind the advancing lunar edge (the beads also reappear at the end of totality), a thin reddish edge called the chromosphere (the Greek chrōma meaning "color") appears. Though the reddish hydrogen radiation is most visible to the unaided eye, the chromosphere also emits thousands of additional spectral lines.[5]

Observational history

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.[6][Note 1] Halley described and correctly ascertained the cause of the effect[6] 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.[6]

In media

Cosmas Damian Asam was probably the earliest realistic painter to depict a total solar eclipse and diamond ring.[7] His painting was finished in 1735.

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.

  • Diamond ring effect visible during the total solar eclipse of August 21, 2017, in Ravenna, Nebraska. (The diffraction spikes emanating tangentially from the diamond are an artifact of the camera optics, not a natural phenomenon.)
  • The diamond ring effect as totality ends during the total solar eclipse of August 21, 2017, in central Wyoming. The rainbow coloration is a lens flare and not a natural phenomenon.
    Diamond ring effect at third contact, marking the end of totality. Some prominences can also be seen.

References

Notes
  1. Great Britain did not adopt the Gregorian calendar until 1752, so at the time of the eclipse, the date was recorded as 22 April 1715.

Citations
  1. Baily (1836). "I. On a remarkable phenomenon that occurs in total and annular eclipses of the sun". Monthly Notices of the Royal Astronomical Society. 4 (2): 15–19. doi:10.1093/mnras/4.2.15.
  2. Littmann, Mark; Willcox, Ken; Espenak, Fred (1999). Totality – Eclipses of the Sun. Oxford University Press. pp. 65–66. ISBN 978-0-19-513179-6.
  3. O. Staiger. "The Experience of Totality".
  4. Pasachoff, J. M. & Covington, M. The Cambridge Eclipse Photography Guide (Cambridge Univ. Press, 1993)
  5. Pasachoff, Jay M. (2009). "Solar eclipses as an astrophysical laboratory". Nature. 459 (7248): 789–795. Bibcode:2009Natur.459..789P. doi:10.1038/nature07987. PMID 19516332.
  6. Pasachoff, Jay M. (1999). "Halley and his maps of the Total Eclipses of 1715 and 1724". Journal of Astronomical History and Heritage. 2 (1): 39. Bibcode:1999JAHH....2...39P.
  7. Nemiroff, R.; Bonnell, J., eds. (28 January 2008). "A Solar Eclipse Painting from the 1700s". Astronomy Picture of the Day. NASA.
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