Solar Eclipse Observing--The Diamond Ring and Baily's Beads

Introduction. Two spectacular events signal the boundaries of totality for seasoned eclipse observers, the appearances of the diamond ring effect and Baily’s beads. The beads are also an annular phenomenon.

Unlike Baily’s beads, the diamond ring, for all its spectacle, is not a true phenomenon of totality but a product of the final moments of the pre–totality partial phases and their post–totality resurgence.

Sir Edmund Halley is credited with making the first observations of Baily’s beads during the eclipse of 22 April 1715. They were also seen by Maclaurin from Edinburgh during the annular eclipse of 1 March 1737 and by Williams from Revolutionary War America on 27 October 1780 (see Chapter 1) from just outside the path of totality. But it was Francis Baily’s widely–disseminated description of the phenomenon during the annular eclipse of 15 May 1836 that led to their bearing his name thereafter.

Explanations. Shortly before second contact of a total eclipse, the opposing horns of the slender crescent sun begin to converge on one another. At the same time, the tenuous solar atmosphere becomes visible against the darkening sky, shining out around the edge of the moon where the sun has already been covered. The combination of this “ring” of light and the single brilliant “diamond” of sunlight where the horns are converging creates a most striking appearance, the diamond ring.

Photograph 9-1  Francis Baily.  Portrait photograph © The Royal Astronomical Society.

The effect lasts for a very short time. Soon, the horns of the solar crescent close completely, and the diamond begins to break up, to be replaced by an array of brilliant beads of sunlight caused by the sun shining through valleys and depressions on the moon’s leading limb.

Photograph 9-2  A diamond ring of the 11 July 1991 total solar eclipse from Cabo San Lucas, Mexico.  This exposure was taken through an 80 mm f/8 refractor at prime focus with a Nikon F3 camera body on Ektar 125 film.  Photograph taken by Doug Berger. 

Baily’s beads also quickly succumb to the encroaching moon, winking out one or two at a time until totality is fulfilled; the disappearance of the last bead marks the moment of second contact and the beginning of totality. Totality ends with third contact when the beads reappear at the opposite (trailing) limb. Shortly thereafter, another diamond ring appears and quickly absorbs the beads as the horns of the crescent sun diverge once more.

In an annular eclipse, Baily’s beads first appear along the sun’s trailing, rather than leading limb, pinpointing the moon’s deepest limb depressions first and marking second contact. The beads increase in number as the limb moves further onto the sun’s disk until no breaks in the sun’s rim are visible. When the moon’s leading limb reaches the opposite rim of the sun, the beads return, then diminish in number as they merge together until the last one vanishes when the moon’s limb appears unbroken, marking third contact.

Photograph 9-3  A photograph of a projected image, 30 May 1984 total-annular eclipse, Atlanta, Georgia, showing Baily's beads.  Photograph taken by Richard Sweetsir. 

Photograph 9-4a-c  A series of photographs taken at the northern limit of the 5 January 1992 annular eclipse, Truk Lagoon, showing the development of Baily's beads.  The first photo was taken at 21:22:00 UT, middle photo at 21:22:14 UT, and the right photo at 21:23:17 UT.  These 1/125 exposures were taken through a 1000 mm f/11 lens on Kodachrome 64.  Photographs taken by Derald D. Nye.

Procedures. Diamond ring and Baily’s beads observations are limited primarily to measurements of their position angles, which are measured from north through east around the moon’s limb. This may be accomplished either by direct observation with appropriate filters, by photography, or by eyepiece projection onto a screen.

In both cases, care must be taken to insure proper orientation of the sun’s image with respect to the compass points. The best approach here is to use either an illuminated eyepiece reticle marked off in degrees or a similar scale drawn on the projection screen. Proper orientation of the reticle or screen may be accomplished by careful monitoring of the westward solar drift during the partial phases.

The last, faint beads visible around second contact, and the first ones visible around third contact of a total eclipse, or first and last beads, respectively, of an annular eclipse, have the greatest scientific value and should be carefully timed, measured and photographed.

It is possible to prolong the diamond ring and Baily’s beads phenomena by setting up an observing station near the edge of the path of totality or annularity. Observers at such locations sacrifice totality or annularity time, which is significantly reduced near the northern and southern path limits, but carefully timed measurements of the beads made during such “grazing” eclipses have greater scientific value and can yield fairly precise measurements of the shapes and relative positions of the sun and moon. A leading proponent of this work has been Dr. David W. Dunham and the International Occultation Timing Association. Dunham has spear-headed related research through observations of grazing occultations of stars by the moon.

Photography. The diamond ring and Baily’s beads should be photographed without the dense solar filters used for visual observing, yet they are still partial eclipse phenomena and should be considered hazardous to one’s eyesight. The safest approach is to center the camera on the sun during the last seconds of the partial phases while the solar filter is still attached.

When the horns of the crescent converge to form the diamond, remove the solar filter and begin exposures without looking through the viewfinder again. Specific exposure recommendations may be found in Chapter 17, but you’ll want to bracket them somewhat in order to capture the full range of ring and bead appearance. But keep in mind that if you’re shooting around f/11 at 1/250 second (or its equivalent) with ISO 200 film to capture the ring and beads, you will not want to photograph totality at these settings! Remember to adjust your exposure settings accordingly once the beads are gone and totality begins (and again at the end of totality when you are shooting the beads and ring again).

Video photography may be attempted with camcorders in the automatic exposure mode and the solar filter removed for satisfactory results, but most photographers will prefer to set exposure parameters manually here too. Vary the f/–stop through a pre–determined range to accent beads of varying intensities. Unlike still and motion picture cameras, most video camcorders have viewfinders which are miniature television monitors and present no danger to the eyesight since you are not looking at the sun itself. Still, safe habits are good to develop, especially if you are using an assortment of visual and photographic equipment, and exposure adjustments are easy to make without looking through the viewfinder. If you have one of the earliest models of a home video camera, consult your manual carefully for warnings about “burn–out” before pointing it directly into the sun!

It may be interesting to note that Baily’s beads were first successfully photographed at the eclipse of 7 August 1869 by C. F. Hines and members of the Philadelphia Photographic Corps from a site in Ottumwa, Iowa.