Solar Eclipse Observing--Shadow Bands
Introduction. For a few minutes on
either side of totality, when the sun is a thin crescent and the
sky takes on the characteristic yellowish tint of
eclipse–induced twilight, alert observers may notice eerie
bands of undulating shadow racing across the ground, along the
sides of buildings or across other light–colored surfaces.
This atmospheric phenomenon, first described by H. Goldschmidt
in 1820 but undoubtedly observed since antiquity, is believed
caused by an irregular bending or refraction of the crescent
Explanations. Attempts to explain
the shadow band phenomenon have focused upon the atmospheric
rather than the astronomical sciences, for the bands do not
reliably appear or exhibit similar behaviors at each eclipse,
even when eclipses have similar geometric circumstances.
In 1900, H. C. Wilson proposed a
diffraction–ring hypothesis for their origin. Writing in the
now–defunct magazine Popular Astronomy, Wilson maintained that
a pattern of concentric rings surrounded the moon’s shadow,
and the shadow band movements were caused by this pattern moving
across the earth.
In the November, 1925, issue of that same
magazine, U.S. Weather Bureau meteorologist W. J. Humphreys
reported that the bands paralleled the solar crescent and moved
normally to their length during an eclipse earlier that year. He
concluded that the bands were pseudo–total reflections, or
mirage effects produced by transition shells between warmer and
cooler adjacent masses of air in a state of thermal convection,
and that there was no relation between their direction of travel
and that of the winds aloft at any level.
In the June, 1963, issue of Sky &
Telescope magazine, Edgar Paulton of the Amateur Astronomers
Association of New York City suggested their movements in such
widely divergent directions, when viewed from different
locations, were an illusion brought on by their rapid passage.
Many observers hold strongly to the belief
that changes in shadow band direction are caused by variations
in atmospheric temperature, humidity, density and pressure,
contending, therefore, that it would be difficult at best to
predict their appearance and motions for any specific eclipse in
To obtain the fullest possible picture of
these enigmatic bands, observations of their presence (or
absence) and behaviors, along with relevant meteorological data,
are desired for all total and near–total annular eclipses.
Observations are also desired from all possible sites along the
length and width of each eclipse path, in order to obtain
comparative data for various geometries and weather conditions.
Only then will a definitive explanation for shadow bands and a
reliable means of predicting their likelihood of occurrence for
any given eclipse be possible.
Appearance. Shadow bands vary
considerably in both width and separation, but range most
frequently between 2 and 5 cm (0.75 and 2 in) in width and are
separated from one another by 5 to 25 cm (2 to 10 in).
Their direction of motion across the ground
seems to depend upon where an observer is located along the
eclipse path and whether the bands are observed before or after
totality. Their velocities vary most often between 1.5 and 3 m
(5 and 10 ft) per second.
Equipment. Casual observers of
shadow bands can detect them against any light–colored
surface. For serious studies of the bands, however, a
standardized observing procedure was developed by Paulton in
1959 to simplify comparisons of observations made from widely
Paulton’s approach calls for setting up
projection screens perpendicular to the axis of the shadow cone.
Each observer is then examining a tiny segment of a plane upon
which the moon’s shadow appears circular.
First, a portable wooden frame about 1.5 m
(5 ft) square is constructed and supported by diagonal struts
joined at the center. The finished frame and mount should
resemble an oversized artist’s easel.
Photograph 7-1 A simple shadow band
Next, the screen surface is selected. This
may be a white cloth, sheet or canvas which completely covers
the frame (eclipse–chaser Norm Sperling recommends sandpaper
for greater contrast). A large azimuth circle, marked off and
labeled in five–degree increments, is then drawn or painted
onto the screen’s face. This is best done by projecting a
slide or overhead transparency of an azimuth circle onto the
screen and tracing the image.
Finally, a rod is mounted in the center of
the screen, projecting about 30 cm (12 in) out of the middle of
the azimuth circle.
If made of light–weight materials and
designed for easy assembly and breakdown, this piece of
home–made equipment can be transported and set up with a
minimum of effort.
Photograph 7-2 Bill Riebsame, left,
inspects a shadow band screen prior to the 7 March 1970 total solar
eclipse while Karl Simmons and Murray Daw, right, look
on. Photograph taken by Karl Simmons.
Procedures. Paulton’s ideal shadow
band observing team requires three people. A few minutes before
totality, the screen is oriented so that the central rod casts
When the first bands appear, the first
observer stretches a length of string across the screen parallel
to the length of the bands. The string may be attached to the
screen with push–pins, thumb–tacks, or tape. Some have
designed a more elaborate frame with hooks attached to the
screen at each five–degree mark on the azimuth circle; the
string is then wound around opposing hooks.
If the bands do not change direction, no
further action is necessary. If they do, every 30 seconds
another string is attached. The strings should be prepared in
advance, labeled in numerical order with masking tape flags and
laid out in the order in which they are to be used.
The first observer must also notice the
direction of motion of the bands, which may or may not parallel
their length. A twelve–hour clock face system is recommended
to avoid confusion with the orientation angles on the screen.
The observer might, for instance, observe the orientation of the
bands as stretching between 225 degrees and 45 degrees, while
their direction of motion is from 11 o’clock to 5 o’clock.
The second observer, armed with a
stopwatch, alerts the third observer with a pre–arranged
countdown and starts the stopwatch the moment one of the bands
begins its journey across the screen. At that same moment, the
third observer begins counting the number of bands which cross
some predetermined position on the screen, usually the edge from
which the bands are leaving.
Meanwhile, the second observer has been
following that one shadow band across the screen. The moment the
selected band leaves the screen, the second observer
simultaneously stops the stopwatch and alerts observer three
with another verbal signal. Observer three, upon hearing
observer two’s outcry, stops counting the departing shadow
bands and records the number counted.
When finished, the first observer will have
recorded valuable data on the orientation and direction of
travel of the bands, the second observer the time of travel
across the screen, and the third observer the number of bands
present in the circle’s diameter at that time.
Paulton has recommended that measurements
be repeated at 30–second intervals for as long as shadow bands
are visible, and that the entire experiment be repeated
following totality. If frequent observations are planned,
tape–recording the various measurements, directions and
counts, preferably against a background of WWV or CHU broadcast
time signals, can minimize confusion later.
Paulton’s experiment is relatively simple
and well suited to clubs or school classes which could be formed
into several teams and located at different sites. Considerable
practice by prospective team members is required to guarantee
Observations from totality’s edge.
Observers located near the edges of the path of totality will
experience shadow bands for longer periods of time. However,
since there is no guarantee that a particular eclipse will
produce the bands, and since the duration of totality diminishes
rapidly as the northern or southern limit of an eclipse path is
approached, observers will sacrifice valuable minutes from the
most spectacular part of a total eclipse. Large groups planning
to send volunteer teams to these hinterlands should make certain
that the participants fully understand what they’ll be giving
up in totality time to obtain more information on this
Photography and Video Imaging. Dim
lighting, poor contrast and rapid movement make still
photography of shadow bands difficult at best. Low–light
camcorders offer a significant advantage, and can be centered on
the viewing screen to capture the motions of the bands for later
playback and data reduction.