Solar Eclipse Observing--The Sky at Totality
before second contact, the sky darkens appreciably and the
brighter stars and planets become visible to the unaided eye.
With the onset of totality, stars fainter than the second
magnitude may be seen.
Searches for faint comets,
asteroids or infra—Mercurian planets have been carried out
during total eclipses for more than a century, but usually such
searches, to be fruitful, require sophisticated cameras which
produce images for later analyses.
Total eclipses even provided
the first experimental support for Einstein’s General Theory
of Relativity. The theory predicted that light from a distant
star would be bent by the gravitational field of the sun (due to
the curvature of space itself) and the star would appear
deflected slightly from its actual position in space. Just such
a deflection was detected in a star close to the sun’s limb
during the total eclipse of 29 May 1919, only four years after
Einstein presented his theory.
Observers who are able to
tear themselves away from the beauty of the eclipse itself, may
find some interesting and worthwhile targets among the celestial
objects which would otherwise be hidden from view by the sun for
another month or two.
magazines, handbooks, almanacs and computer planetarium programs
usually provide diagrams of the sky during totality, including
the brighter objects likely to be visible.
Some observers want to
dark—adapt their eyes well in advance of totality, and do so
by wearing dark glasses or a single eye patch during the partial
Planets. The inner
planets, Mercury and Venus, are rarely far from the sun, and are
usually observed close to the horizon through turbulent layers
of atmosphere and in twilight. During totality, however, both
planets may be viewed high in the sky. Planetary enthusiasts
might take a few minutes from a particularly long eclipse to
observe and sketch telescopically the dusky markings which
Mercury and Venus exhibit.
Outer planets Mars, Jupiter
and Saturn, when near solar conjunction, may offer
out—of—season targets for enterprising planetary astronomers
wishing to see what features they are exhibiting in advance of
their return to morning skies from the sun’s glare.
Variable star enthusiasts may wish to check on the brighter
irregular or eruptive stars on their programs. Certainly, any
novae discovered shortly before becoming lost in the sun’s
glare would warrant examination during totality.
of bright comets in close proximity to the sun might be
conducted. Others may wish to search for previously undiscovered
comets which would otherwise remain lost in the sun’s glare
for weeks more. Dividing the sky around the sun into sections
assigned to many observers would be more likely to achieve
success than would one observer attempting to sweep a large area
of sky during the brief period of totality. Use wide—field
telescopes, and avoid areas known to have a lot of bright
nebulae or galaxies.
Meteors and fireballs.
An alert observer who spots a bright meteor or fireball might
make an effort to record its time, magnitude, duration and
direction of flight. For very bright objects, plotting the path
on a star chart or "Sky at Totality" illustration
would be useful. Note any bursts (and associated sounds, if any)
and any lasting trains left behind by its passage.
The brighter satellites should be readily visible during
totality. They are inarguably of less relevance than planets,
variable stars or comets, which may be obscured by the sun’s
glare for many weeks around conjunction. Still, the widely
available computer software and modem—accessible elements
warrants running predictions for the eclipse site and an effort
to observe them. This task is ideal for people with few other
projects to conduct.
Faintest star. One
project which relates to the sky’s overall darkness during
totality is to determine what the faintest star visible to the
unaided eye is. This may also be attempted using binoculars and
telescopes of various apertures. The best approach is to select
a star which you know will be bright enough to locate easily and
which also is surrounded by fainter stars offering a broad range
of magnitudes. Different observers might focus on several
different star fields at increasing distances from the eclipsed
sun to determine how sky darkness varies as the shadow’s edge
is approached. Observers who have not dark—adapted at least
one eye in advance should not expect their faintest star
observations to have any value.
Zodiacal light. The
zodiacal light is a faint glow that travels along the ecliptic
preceding and following the sun by up to several hours, and
appears to be caused by sunlight reflecting off interplanetary
dust particles in the inner solar system. In the evening and
morning hours, it may appear as bright as the Milky Way at times
when it stands nearly vertically off the horizons. For the
western evening sky, this occurs in February and March for
mid—northern latitude3, while the eastern morning sky is
favored in September and October, providing there is no
moonlight to interfere.
The sun’s brightness makes
it difficult to study the size and density of these dust
particles closer than about 20 degrees (80 solar radii) from the
sun. However, some studies of the zodiacal light have been
conducted closer to the sun during eclipses using cameras
carried aloft by aircraft and balloons. University of Minnesota
scientist Dr. Edward P. Ney led several expeditions to areas
where the eclipsed sun would be ten degrees below the horizon,
thereby enhancing the zodiacal light.
The zodiacal light is
probably too faint to be observed visually during totality,
except from very high elevations. However, eclipse observers
with a very low sun angle, or who find themselves just outside
of totality for an eclipse occuring before sunrise or after
sunset at their locations, might wish to attempt observations of
"counterglow" phenomenon is related to the zodiacal
light but appears on the ecliptic directly opposite the sun. It
appears as a very faint elliptical patch of light. Observers who
find themselves directly opposite some point along the path of
totality might monitor the gegenschein for several nights on
either side of the eclipse date and several times centered on
the time of totality in their opposite hemisphere for any
changes in its appearance.
Still, video and CCD imagery of the brighter planets should
present no obstacles. Short exposures on moderately fast films
through wide—field lenses should capture the appearance of the
sky at totality with the eclipsed sun as the centerpiece. Bright
satellites and meteors should also be relatively easy targets,
provided exposures are kept short enough to prevent sky fogging;
practice on full—moon nights or at twilight with exposures
ranging from a few seconds to a minute. The obstacles presented
by most variable stars, novae, comets and the zodiacal light,
especially under the illumination and during the brief time
afforded by totality, are probably too formidible for those
without highly sophisticated imaging equipment.