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by Dr. Wolfgang Strickling
On 21.06.2001 i experienced a total solar eclipse under optimal conditions for my first time. As our video recording of the shadow bands succeeded unexpectedly well, i looked for more information about this interesting phenomenon afterwards .
Contrast enhanced and
processed screenshot from
our Hi8-Video, 18 seconds before the second contact.
Videotaped by Dr. Andreas Dahm. Time is UT +2 h
For a highly resolved picture click on the right picture!
On the left you see an animated GIF of the shadowbands
During the central 10 minutes of the eclipse a video camera filmed a
white
cloth 1.4 x 2.4 m . More details, short videos and images can be
obtained
on my 2006 observation
report
page and my 2006 observations
page
(more detailed).
Unfortunately
there exist only few photos of the shadow bands world-wide. So there
ist
often still published a rather bad lithography of the shadow bands
of
the eclipse 1870 in Sicily and South Italy (see picture left).
The best theory for the emergence of the shadow bands is published by Codona 1986 [3]. His theory meanwhile accepted by the most scientists. Codonas scintillation theory is able to explain very well also subtle photoelectric observations .
After Codona the shadow bands at ground level result from interference of light rays, taking a somewhat different way in the atmosphere when crossing its turbulences and density variations .
The best observation conditions for such interferences can be expected from point ligth sources. On the other hand, the more extended the source of light is, the more less will such interferences be perceptible. Nevertheless you may observe the so-called " heat waves " on very hot days on homogeneous structured surfaces. In general, they are nothing different than the shadow bands.
During a solar eclipse however the solar crescent becomes more and more the shape of a slot. While a point light source would produce a spotted interference pattern, the pattern produced by this slit-shaped solar crescent is smeared to bands.
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Above: Photos after image processing, 45 s, 25 s and 10 s before second contact of the 2006-03-29 eclipse
The
wavelength (band distance) of
the shadow bands is expected to decrease to 2nd resp. to increase after
3rd
contact, see images above, taken on 2006-03-29. My
observations of 2006 show the
relation
of shadow band distance to contact time very clearly (see right graph).
Left: The shadow bands orientate parallel to the projected picture
of
the solar crescent. Their direction of motion is percepted always in a
right
angle to their orientation and is resulted from the wind direction in
the
creating air layers.
(according to B. W. Jones)
Right: function of the shadow bands wavelength resp to time.
The orientation of the resulting interference bands is therefore parallel to a projected image of the solar crescent on the projection surface. So the shadow bands orientate directly before or after the totality parallel to the edge of the moon's shadow. In larger distance from the totality are they right-angled to the center line.
We should expect an orientation following the
equation:
Ab = As -90° + ArcTan (Tan (Pa) / Sin (e))
with
Ab the Azimut of shadow bands
As the Azimut of the sun
Pa the Position angle of mid of the crescent (appr. 2nd resp. 3rd contact) to Zenit and
e the elevation of the sun
The height, in which turbulence cells creating perceptible shadow bands may lie, depends on the angular dimension of the source of light. The above mentioned " heat waves " can be produced only by convection cells a few meters above the ground. Higher cells will average their effects away, since the sun is not a point light source. The more narrow the source of light is, the higher may the causing cells lie. So the convection cells, which are mainly responsible for the shadow bands, have heights between some hundred meters at the beginning of the visibility and up to a few kilometers directly before the second or immediately after the third contact.
The movement of the shadow bands is caused by winds in the different atmospheric levels. The direction of motion appears always perpendicular to the orientation of the shadow bands, since one cannot recognize parallel shifts of the bands with the human eye. The velocity of the shadow bands depends therefore on the wind velocity! With zero wind speed they will hardly move and therefore will not be noticeable. On the other hand, if the wind is blowing very fast, the movement is so rapid that the eye can not follow the low-contrast structures any longer and therefore an observer will not see the shadow bands, although they are well provable with fast photometers [4]. For good observations wind velocity should lie in the range of one to few meters per second.
Codonas scintillation theory explains also some of our observations:
The orientation of the shadow bands changed, because our camp did not exactly lie on the center line, but some kilometres south of it (coordinates 31° 01.45 ' east, 16° 24.90 ' south, 487 m above sea level). The solar crescent was situated at the second contact approximately parallel to the horizon, after the third contact it was inclined to the horizon of about 43°. The shadow bands lied therefore before the totality perpendicularly to the sun's direction, afterwards they were twisted about 50° against it. Shadow bands are parallel before and after the totality only if the observer is placed exactly on the center line!
The observed increase in contrast near totality is predicted by Codona's theory as well as the decreasing of the band distance. It was at the beginning of visibility appr. 30 cm and before the totality approx. 10 cm.
The theory explains also the conditions for optimal visibility of the shadow bands:
Simulation
of eclipses in the last five minutes before the totality b) long 7-minute eclipse:
the crescent is nearly a slot.
Good seeing is unfavorable, since no turbulence
cells develop. ("bad seeing is good for shadow bands ").
Therefore observation places at sea level are more favorable than
higher locations.
Moderate wind velocities in middle
atmospheric heights let the shadow bands become well visible.
Very strong wind results in fast movements so that the eye cannot
follow any longer. With zero wind speed the shadows are nearly without
motion and therefore hardly remarkable.
A small elevation of the sun over the horizon
will produce stronger contrasts than eclipses near then zenith.
The shadows bands are visible in the two minutes before and after the totality. However, most observers do not see the shadow bands for such a long time. They are best to be seen about 20 seconds distance to totality.
Some observers report shadow bands very similar to eclipse shadow bands occuring at sunrise or sunset behind mountain ridges or linear clouds. They share a similar geomtry like solar eclipses. Good transparency and the presence of appropriate air turbulencences seem to be conditions for the occurrence of such kinds of shadow bands.The distance to the ridge or the elevation above the horizon seem to be less important.
An explanation more easy to understand is possible by using a ray-optic explanation instead of Codonas wave-optic. Like a refractor, whose image may be described by light rays as well als by light interference.
As a result of atmospheric turbulence and density fluctuations, the solar light rays are refracted. So parts of the atmospheric turbulence cells may work like positive optic lenses producing a real image on the ground. Normally, the images of the noneclipsed sun are too large so that they average themselves and remain invisible. Only if the solar crescent is small enough and the images become as small as the atmospheric turbulence cells are, we can see the shadow bands. They result as a superimposition of multiple crescent images and orientate along the tangents of them. The diameter of the atmospheric cells is about 10 to 20 cm. So, if the crescent becomes narrower, the distance of the shadow bands will decrease and their contrast will increase.
From the size of the shadow bands it is possible to calculate the focal length of our atmospheric lenses. We get a range of some hundred meters to 2 km.
Other theories tried to explain the shadow bands by Fresnel diffraction at the lunar limb. Although such diffraction should be expected, it does not seem to play a role in the production of shadow bands. As the diffraction pattern will move with the lunar shadow (ca. 1 km/s!), ist is too fast to recognize. It should also be expected only a very short time around the 2nd and 3rd contact (ca. 1 to 2 sec). I suppose, the contrast will be very low, much less than the measured 2 .. 4 % of our shadow bands. Nevertheless the distance of the diffraction rings (1 cm to meters, depending on the width of the crescent) matches our observations, although it will increase in approach to totality...
The shadow bands can easily observed and recorded with amateur means. As those observations have scientific interest, eclipse travelers should take appropriate equipment with them [4]. In order to record also high frequency variations of the shadow bands, exposure times of max. 1/100 second should be used. Because of the reduced brightness around the totality photographies are difficult to be made. The brightness lies in the range of 10 to 100 Lux (that is 1/1000 to 1/10000 of noon brightness!), so high speed films and fast lenses are required.
Fast CCD-video or digital cameras will probably provide better results. Due to rapid change in brightness near the totality, you should switch on automatic exposure and switch off autofocus! Take a projection surface of 1 x 1 m size minimal, better a larger one and note its orientation, size and the geographic coordinates of your location. In case of videotaping, film a carefully adjusted clock or a GPS clock before and afterwards to have good time information later. You may start your camera several minutes before totality and then observe the eclipse visually. Some minutes after totality, stop your camera with your observations finished.
Meanwhile also good photoelectric observations are possible with amateur means. You should use registration frequencies of about 1 kHz in order to register high speed changes. For more details see the publications of B.W. Jones [2].
To study contrast an the development of the shadow bands in detail, it
is
possible to take an intensity profile, for instance with
LIMOVIE.
With
SPECTROGRAM
you can make a fourier power spectrum of the intensity plot after
converting
the intensity graph to a WAV-file (done with
csv2wav).
The upper graphs show the spectrum before second contact (marked as
C2),
the graph right the development after third contact (C3) of the eclipse
2006-29-03. It can be seen, that the shadow bands do not develop
continuously,
but that there may occur short periods of less activity. Longer
measurements
reveal that the shadow bands activity begins several minutes before
naked
eye can watch them on the video.
Generally: Prefer such devices, which you can start some minutes before the totality! So you will not not forget your measurements and can enjoy the eclipse visually.
The weather and wind conditions, especially wind speed, wind direction and cloud movement should to be noted as well as geographic coordinates and visual impressions. An interesting location for shadow bands observation is the zone of grazing eclipse at the border of the band of totality. There the shadow band swill be seen over a long time and they will rotate, as the solar crescent changes its orientation in the sky!
Download my MPEG-2 videofiles for the
from the 2006-03-29 eclipse a 10-seconds video
MPEG2 file is here for download
(sofi2006_shb2.mpg, 487 kB from
12:26:05 - 12:26:15 (UT + 2h)).
The first second has a grid of 50 x 50 cm added as size reference.
Video 2. Contact 29.03.2006 , 25 seconds: sofi2006_shb2_unproc.mpg (7 MB).
(If you get problems in playing MPEG-2 files, download VLC media player or Microsoft's newest newest media player. The standard one often does not play MPEG-2.)
For good and smooth projection screens it is easy to analyze the video or photos. If the projection screen is not smooth, but eg. a folded cloth, it is often necessary, to eliminate structures of the cloth digitally.
For still pictures i extract the 25 frames of one second video and averge them by adding them (the German GIOTTO freeware does this very well). The result will be inverted to get a negative image and superimposed with 50% transparency to one of the original frames.
For processing a video clip, i get an averaged video sequence, by making a multiple superimposition of the same videoclip to itself, each clip separated by one frame in time (1/25 sec at 25 fps). To get an one-second averaged clip (in PAL resolution at 25 fps (, NTSC at 24 fps IMHO)), i superimpose this clip 25 (NTSC 24) times. In my video software (Ulead Media Studio) the clip on track "V1" gets 0% Transparency, V2 gets 50%, v3 67%, and any other track Vi gets a transparency of 100% * (i-1)/i with i = 1 to 25. So track 25 gets 96% transparency.
From the superimosition i create a video file, invert it to get a negative and superimose it again with its original clip at 50% transparency to average nonchanging structures. All these steps are possible with the same video software and no other software is required. (But, on my pentium 800 i need 45 mins for a 1 minute clip to calculate the average film...). Another, faster method is to superimpose the original video and an inverted and by 1 to 3 frames shiftet copy of it with 50% transparency. But i expect some artefacts and uncontrolable errors, making an exact analysis impossible, so i prfer the first, more precise method.
The result is a smooth grey film, showing only differences between the actual frame and the average of the 12 frames before and 12 frames after. It is optimal for coontrast enhancing amd further video processing.
Jones, Barrie W.:Shadow bands during the total
solar eclipse of 26 Februar 1998,
Journal of Atmospheric and Solar-Terrestrical Physics 61, 965-974
(1999)
Codona, J, L: The scintillation theory of
eclipse shadow bands.
Astronomy and Astrophysics 164, 415 - 427 (1986).
Codonas 'original article', where he published his theory
Codona, J. L.: The Enigma of Shadow Bands, Sky and Telescope, 81: 482, (1991)
my observation report of the eclipse 2001 in Zimbabwe with videos, photos etc. for download
more detailed: my observation report of the eclipse 2006 in Libya with videos, photos etc. for download
Shadow Bands Bibliography in the SENL (solar eclipse Newsletter) Dec. 1998
More photos and videos of shadow bands:
© Dr. Wolfgang Strickling, Germany
back to my German homepage
my observation report of the eclipse 2001 in
Simbabwe with my shadow
bands
observations
my observation report of the eclipse 2006 in
Libya with my shadow bands
observations
My solar eclipse projects
Hotlinks to interesting web pages for sun
observers
also Links on the solar eclipse pages
Last update of this page was 2012-12-13
The URL of this page in the Internet is
http://www.strickling.net/shadowbands.htm