J. Brit. Astron. Assoc., 108, 3, 1998, p.127

Shadow bands recorded at February 26 eclipse

by Eric Strach

By extracting a frame from his video record, Eric Strach and Gerard Gilligan produced this image of the shadow bands seen before totality at Knip Beach, Curacao on 1998 February 26. E. H. Strach

Shadow bands are a curious phenomenon occurring a few minutes before and after a total eclipse of the Sun. They have been seen at many eclipses in the past to a varying intensity but they are notoriously difficult to photograph. These ripples of light and dark bands move very fast and attempts have been made to capture them on cine or video. I first saw them at the eclipse of 1976 in Zanzibar on a white screen constructed by Graham Broadbent, a former member of the Liverpool Astronomical Society. After he left, I inherited the screen and took it on many subsequent eclipse expeditions. At the 1991 eclipse in Baja California, I attempted to record these elusive bands on video without great success.

I tried again at the recent eclipse of 26th February in Curacao, using a 'Hi8 Sony Handycam' with a polaroid and UV filter. As the Sun's altitude was 60 degrees, I attached the camcorder to a fitting below the apex of the tripod, pointing it down towards the shadow band screen lying on the ground. A clear line was carefully oriented from E to W and was surrounded by a circle of 53cm diameter for subsequent measurements. I also used a one-second time generator with the pips changing pitch every half minute. This may help me to work out the speed of the movements of the bands.

I started the camcorder 4 minutes before second contact; the recording continued automatically and I was free to savour the spectacle and take a few photographs. I resisted the temptation of playing back my recording whilst abroad. Back home I was all the more surprised to see the bands played back on the TV screen. They were clearly seen for 32 seconds before second contact and a little fainter for 27 seconds after totality. They moved rapidly across the screen from E to W before totality and from NNE to SSW after 3rd contact. Slow motion studies of the video show occasional merging of the bands and at times they seem to move in opposite directions – probably a stroboscopic effect.

With the help of Gerard Gilligan I had access to a frame grabber at Liverpool University; the resulting individual frames were image-processed, resulting in the attached picture. The photograph facilitated measurements of the width of the bands, which varied between 2.36 and 6.63cm. Further experiments will be done at future eclipses when several observers will use different methods and – hopefully – an optimal setup may be determined.

Sketch of the shadow bands as seen at Knip Beach, Curacao. The arrows mark the direction of motion of the bands. Above: before totality; below: after totality. E. H. Strach

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