What Are Shadow Bands?

Shadow bands are among the most ephemeral phenomena that observers see during the few minutes before and after a total solar eclipse.Simply stated, they are the wavy lines of alternating ight and dark that can be seen moving and undulating in parallel.They appear as a multitude of faint bands that can be seen by placing a white sheet of paper several feet square on the ground. They look like ripples of sunshine at the bottom of a pool, and their visibility varies from eclipse to eclipse.The simplest explanation is that they arise from atmospheric turbulence or the thin slot-like solar crescent illuminating the earth’s atmosphere mere moments before and after totality, and that their movement is caused by winds.When light rays pass through eddies in the atmosphere, they are refracted. Unresolved distant sources simply “twinkle,” but for nearby large objects, the incoming light can be split into interfering bundles that recombine on the ground to give mottled patterns of light and dark bands, or portions of bands.Near totality, the image of the Sun is only a thin crescent a few arc seconds wide, which is about the same size as the atmospheric eddies as seen from the ground.Bands are produced because the Sun’s image is longer in one direction than another. The bands move, not at the rate you would expect for the eclipse, but at a speed determined by the motion of the atmospheric eddies.