Shadow bands

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Shadow bands are thin, wavy lines of alternating light and dark that can be seen moving and undulating in parallel on plain-coloured surfaces immediately before and after a total solar eclipse. They are caused by the refraction by Earth's atmospheric turbulence of the solar crescent as it thins to a narrow slit, which increasingly collimates the light reaching Earth in the minutes just before and after totality. The shadows' detailed structure is due to random patterns of fine air turbulence that refract the collimated sunlight arriving from the narrow eclipse crescent. The bands' rapid sliding motion is due to shifting air currents combined with the angular motion of the Sun projecting through higher altitudes. The degree of collimation in the light gradually increases as the crescent thins, until the solar disk is completely covered and the eclipse is total. Stars twinkle for the same reason. All light passing through the Earth's atmosphere encounters tiny disturbances in temperature, pressure, and humidity. These disturbances change the air's refractive index, so that the light essentially passes through innumerable tiny prisms. Thus, the entire sky essentially "dances" randomly at the scale of the disturbances. However, they are so small that even the visible disk of planets (Venus, Jupiter, etc) are larger than the "dancing" scale, and so their brightness appears to remain steady. Only stars are visibly affected, because they are so far away that they are essentially pinpoints of light. This is why astronomers sometimes use the phrase, "stars twinkle, planets don't". Similarly, shadow bands are essentially the "twinkling" of the Sun's thin crescent in the seconds surrounding totality.