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What is shadow band ?

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For a few minutes before totality, a lot of bands like ripples move fast on the ground. These bands are called shadow band. They are also seen for a few minutes after totality. Shadow bands start to be seen from three or four minutes before totality in the earliest case. Shadow bands give observers unusual feeling that he/she is on the bed of a river or a lake. If an observer wants to see shadow bands, he/she has to see ground near his/her foots, instead of seeing eclipse. Although shadow bands can see clearly by naked eyes, their contrast is not so clear that it is difficult to record by videos. There are only a few videos that have caught shadow bands clearly. In case that the eclipse is seen through thin clouds, shadow bands sometimes cannot be seen. The reason why shadow bands are seen has not been clear yet. The major opinion is as follows. The air in the high atmosphere has bands of density like ripples, and they refract sun's ray. Since the sun's ray comes from sun's disk usually, it cannot cast the bands of density on the ground. However, for a few minutes before totality and after the totality, sun's ray comes from a small point by the eclipse. Therefore, it can cast the band of density on the ground. This principle is the same as a pin hole camera.