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
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.
dangers. These are still considered partial eclipse
phenomena and great care must be taken when observing the
diamond ring or Baily’s beads. Direct viewing, even through a
camera viewfinder, requires use of adequate solar filters safe
for visual use.