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Shadow Bands

Often, when we walk in the sunshine, we can see the shadows
of the atmospheric turbulence on the ground around us, or on
our clothes. And the shadows move with the breezes. It's a
little like what we see on the bottom of a swimming pool in
the sun. The water waves on the surface of the pool tend to
focus the sunlight to a pattern of dancing lines on the
bottom. And the lines move with the waves. Similarly, since
the air around us is not all of the same density, it too
tends to focus the sunlight a little, just a little. And
although it is true that under ordinary circumstances the
shadows of the atmospheric turbulence on the ground are not
conspicuous, still there is one circumstance where they are.
At the time of a total eclipse of the Sun, just before and
just after totality, these shadows become fairly conspicuous.
 
Just before and just after totality, at the site of a solar
eclipse, the Sun is seen as a slender crescent of light.
Under these conditions, and only under these conditions, the
shadows of the atmospheric turbulence are seen with slit
illumination. It is this slit illumination that makes the
shadows appear band-like with the bands parallel to the solar
crescent. It is these shadows of the atmospheric turbulence,
made linear by slit illumination that are known to the
astronomers as "shadow bands."