Shadow bands are a phenomenon occurring 1 to 2 minutes before and after totality during a total solar eclipse. Over a white surface, you can see a succession of moving light and dark bands.
The origin of shadows bands is still not very well understood. Currently, the most satisfactory explanation describes this phenomenon as an effect of the "turbulent" spread of light through the Earth’s atmosphere. These more or less dark bands are produced by cells of different air densities and refractive indices, which act as lenses. These cells are either focussing or scattering the light that will be redistributed to areas forming bands that are more or less bright.
The key to understanding the shadows bands lies in the answer to another question: "Why do the planets twinkle much less than the stars?". The twinkling of the stars is caused by multiple and random refractions of light through the turbulent layers of the Earth’s atmosphere. If the stars twinkle while planets do not, it is because the stars are sufficiently far away to appear only as point sources. On the other hand planets appear as disks. Thus the variations of light, caused by the Earth’s atmosphere, will compensate for the various points of the planetary disks. The image of the planets is then more stable.
This difference in behavior is therefore a "smoothing" of the scintillation of different sources that make up an apparent planetary disk. An extended light source, like a planet, may twinkle if the atmospheric turbulence is close to the observer: the apparent disk of the planet is then affected and can’t be as "smooth".
During a total solar eclipse, the Sun appears as a thin crescent of changing size, just before or just after totality. It is therefore a light source of variable size, whose "scintillation" will be governed by turbulent atmospheric layers at different altitudes. The closer to totality, the more effect will the higher layers produce. Shortly before totality the thin solar crescent is similar to an infinitely thin line and its "scintillation" creates those more or les dark bands, whose movements are determined by the winds prevailing in the different atmospheric layers involved in the "scintillation" process.
There are a few particular conditions that can help enhance the visibility of shadow bands. First solar eclipses with a larger magnitude are usually better because the remaining thin solar crescent is less extended and not too semicircular. Then when the seeing is too good there are less atmospheric turbulences and as a consequence less chances to view clear shadow bands. Finally a Sun low over the horizon, for example 10 to 20 degrees, is to be preferred.
To film or photograph shadow bands, you’ll need the following equipment:
- Video camera on a tripod
- Digital camera on a tripod
- 2 white bed sheet
For the video, place a white sheet on a horizontal surface and the other on a vertical surface. Then, set the video camera on a tripod, so that it films the two sheets. Start shooting 5 minutes before totality and stop the recording 5 minutes after. Usually shadow bands will show up during about one minute around totality.
For still photography, take numerous pictures of the white sheets in program mode (or 1/125th of a second at 400 ASA with a wide open aperture). Don’t forget to take photos at a regular time interval.
An analysis of the data should then show the direction, orientation, spacing and speed of the shadow bands.
During the total solar eclipse of 2010 July 11, from at least two atolls in French Polynesia, shadow bands were observed for the first time as projected onto a screen formed by thin low clouds in the seconds leading to second contact.
Being myself on the atoll of Tatakoto, precisely on the Tahunatara motu, I could clearly see them in the sky, but also take pictures (see on the left). The alternation of light and dark bands is clearly visible as striations running tangentially to the last arc of photosphere.
The observation to the naked eye was very impressive, but fleeting, and seemed to highlight a certain three-dimensional structure similar to waves on the ocean.
Shadow bands projected onto a white low thin cloud
2010 July 11 Total Solar Eclipse