Introduction. As totality or very
deep annularity arrives, the pearly–white solar atmosphere
flashes into view surrounding the new moon’s darkened disk.
Known by such colorful names as “halo” and “glory” from
earliest times, this is the corona, a tenuous region of free
electrons and interplanetary dust extending millions of
kilometers into space and shining in the eclipse sky with the
intensity of the full moon.
Figure 11-1 Totality presents a
wonderful opportunity for artists to represent the event,
often in more detail than film or video. Artwork by
Mention of the corona appears in literature
dating back to a description of the 20 March 71 solar eclipse in
Plutarch’s dialogue On the Face in the Orb of the Moon. It was
not until the eclipses of 8 July 1842 and 28 July 1851 that the
corona was positively associated with the sun instead of the
The inner or K–corona, composed largely
of electrons, displays a continuous spectrum and provides a
backdrop for viewing prominences. The outer dust or F–corona
has an absorption spectrum, extends for several solar diameters,
varies in shape from eclipse to eclipse but is generally
irregular, and features spectacular radial streamers.
Procedures for observing the corona should
focus on its shape and extent, intensity variations and
Shape and extent. The sun’s corona
varies from a generally uniform and circular appearance around
times of sunspot maximum to a highly irregular one near sunspot
Measuring the shape and extent of the
sun’s corona is a project well–suited for the observer
without special optical equipment. This may be done
descriptively with the aid of a tape recorder (and hand–held
reticle for estimating position angle), or artistically using
rough sketches or “mapping” techniques.
Many science, engineering and surveying
supply houses sell inexpensive glass reticles etched with
bull’s–eyes, linear scales and azimuth circles. An excellent
home–made combination bull’s–eye or linear scale and
azimuth circle may be drawn, then photographed or photocopied
onto transparency film. When mounted in a cardboard viewing
tube, it is a simple matter to sight on the eclipsed sun and
orient the azimuth circle’s “zero degree” position with
the sun’s north pole. The corona’s extent may then be called
out and tape recorded in terms of solar radii at chosen position
Experienced sketch artists may accomplish
the same thing by pre–sketching a landscape feature close to
the eclipsed sun, then using the feature for scale and
orientation when roughing in the corona’s shape and extent
Those having less artistic skill may wish
to attempt a rough diagramming technique known as mapping, where
a circle to represent the eclipsed sun is drawn in the center of
a sheet of graph paper, and the surrounding squares are shaded
in during totality to match the appearance of the corona.
Intensity variations. The corona
does not exhibit the same brightness intensity throughout. These
subtle differences, which occur both radially and
circumferentially with respect to the sun, are caused by
variations in the density of the coronal gases and tend to
“wash out” in photographs. However, using techniques similar
to those employed for shape and extent observations, the uneven
texture of the corona’s brightness may be readily described or
Descriptions may be recorded in terms of
position angle and extent using a five–step linear scale,
where 0 is the darkest (the appearance of the moon’s center at
mid–eclipse, or of the surrounding sky) and 5 is the most
brilliant part of the corona.
Sketch artists will want to employ a
heavier stroke to emphasize brighter regions, while mappers
might use the same 0–5 intensity scale to outline changes
within the squares on their graph paper in a “paint by
Photograph 11-2 The inner corona of the
26 February 1979 total solar eclipse. Photograph taken
by Jay Anderson.
Streamers. These long finger–like
strands near equatorial regions and short fan–shaped
appendages near the poles of the sun align themselves with the
sun’s magnetic field. They also vary significantly in number,
structure and distribution throughout a sunspot cycle.
Estimates of their number, along with
descriptions of the shapes, position angles and extents of the
more prominent streamers, might be undertaken.
Sketch artists, especially those equipped
with binoculars or low–power telescopes, might want to make
detailed renderings of the more spectacular streamers.
Photography. The corona—in fact,
totality itself—was photographed successfully for the first
time by Berkowski at the eclipse of 28 July 1851; using the 15.9
cm (6.25 in) Königsberg heliometer, he required an exposure of
24 seconds. Fortunately, modern photographic emulsions offer
much greater versatility.
Photograph 11-3a, b These photographs of
the 11 July 1991 corona were taken normally (left) and with a
vignetting disk (right). Note the greater detail in the
inner corona visible with the vignetter, while the extent of the
streamers is almost the same in both images. the
defocusing of the vignetted image was anticipated for this
experimental photograph but is not the fault of the vignetter
disk; proper focus is achieved by simply focusing with the gals
vignetter-support in the optical system. Photographs ©
by Stephen J. Edberg.
With any diffuse subject like the corona,
its appearance on film is a function of the exposure time and
f/–stop used. There is wide latitude and ample time to shoot a
sequence at different settings, from the shortest exposure
likely to produce an image to time exposures as long as 15
seconds for moderately fast films (around ISO 200).
Shorter exposures will capture greater
structural detail in the inner corona, enhancing the streamers,
while failing to capture the impressive extent of the outer
corona; longer exposures do justice to the outer corona, while
washing out the other details. Skilled darkroom or computer
technicians can combine individual exposures to create composite
images which capture the best of both extremes.
Video photography should be attempted with
automatic focus and exposure features disengaged, since they
prove highly unreliable under low lighting conditions.
Experiment in advance with the full moon, and settle on a range
of f/–stops and focal lengths which show detail on lunar
surface features. This should provide an adequate mid–range
for coronal photography with most camcorders.
dangers. During totality there is no danger to the eyesight
from solar radiation. It is safe to view the corona and coronal
streamers without special filters, even through optical devices.
However, it is crucial that care be taken to discontinue any
direct visual viewing before third contact, when the sun’s
disk is again exposed to view. A countdown tape with built–in
several–second margin of error and attention–getting signal,
or a timekeeper serving the same purpose, will provide a
reliable warning system.