Shadow bands


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 minute 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. They are so far from Earth that they appear as point sources of light easily disturbed by Earth's atmospheric turbulence which acts like lenses and prisms diverting the light's path. Viewed toward the collimated light of a star, the shadows bands from atmospheric refraction pass over the eye.

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