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by Dr. Wolfgang Strickling
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 shadow bands
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 is often
still published lithography of the shadow bands of the eclipse
1870-12-22 from Gela / Sicily (former Terranova) by Demetrio
Emilio Diamilla Müller (see picture left, from: G.F. Chambers: The
Story of Eclipses, 1900, Thanks to Michael Zeiler, click onto
image for larger version).
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 light 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 respectively. 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 projected 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 centre 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 kilometres 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 centre 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 centre 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 unfavourable, since no
turbulence cells develop. ("bad seeing is good for shadow
bands ").
Therefore observation places at sea level are more favourable
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 occurring at sunrise or sunset behind mountain ridges or linear clouds. They share a similar geometry like solar eclipses. Good transparency and the presence of appropriate air turbulences 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 as 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 non eclipsed 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!), is 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 travellers 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. Good for this are collapsible reflector screens
from a photo store. Your camera clock should be carefully adjusted
to 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 and your observations are ready for
your analysis.
Watch my shadow bands videos on Youtube:
Eclipse 2001-06-21, 2nd contact, 3rd contact, link to my observations website
Eclipse 2006-03-29, 2nd contact, 25 seconds: on YouTube, contrast enhanced and not enhanced, link to my observations website
Eclipse 2016-03-09: 2nd contact enhanced, 3rd contact enhanced, 2nd contact not eenhanced, link to my observations website
Because usually the image of a projection screen is not smooth enough, especially if a cloth was used, it is often necessary to eliminate digitally structures of the projection screen.
For still pictures I extract the 25 frames of
one second video and average 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.However, I always recommend recording videos
and extracting still frames from them.
For processing a video clip, I get an averaged video sequence, by making a multiple superimposition of the same video clip 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 superimposition I create a video file,
I subtract the averaged video from the original one and add a
constant value to obtain a neutral grey. All these steps are
possible with the same video software and no other software is
required. A fine software for doing these jobs is Blender. For the
pixel arithmetic use Blender's node editor. Pay attention to save
intermediate videos in a lossless codec to avoid generation
compression artefacts!
Amateurs may also use photoelectric detectors. 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!
Codona,
J, L: The scintillation theory of eclipse shadow bands.
Astronomy and Astrophysics 164, 415 - 427 (1986).
Codona, J. L.: The Enigma of Shadow Bands, Sky and Telescope, 81: 482, (1991)
Jones, Barrie W.:Shadow bands during the
total solar eclipse of 26 February 1998,
Journal of Atmospheric and Solar-Terrestrical Physics 61,
965-974 (1999)
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
The URL of this page in the Internet is http://www.strickling.net/shadowbands.htm