Hey, that's a neat effect! It shows how observant people are during solar eclipses.
Shadow bands are not spooky at all. The "atmospheric irregularity"
explanation makes lots of sense. Here's a web site with a video of
observed shadow bands along with that explanation:
.
I can add something from my own experience durung a solar eclipse.
When the visible part of the sun was just a thin crescent, shadows on
the ground looked really very weird, and I couldn't figure out why
until I walked under a leafy tree. The small holes in the shade of the
leafy tree acted like little pinhole cameras and projected real images
of the sun on the ground underneath the tree. On non-eclipse days we
get them too, but don't think too much of the round images which blend
together.
Without the tree, however, the atmosphere itself acts like a really
bad imaging system. The swirling turbulence of the air makes little
eddies of alternating high and low density. You can see this effect by
looking over a road on a hot day at a faraway object, or looking at the
heat-induced swirls over a hot stove or behind a jet engine. The bits
of air of different densities will have different refractive indexes
and will act like little swirling lenses. These lenses may
preferentially focus light from a point source towards a spot on the
ground more than towards another spot on the ground, making one place
light and another dark.
You can reproduce this effect yourself by shining a flashlight
beam over a lit candle in the dark, projecting the flashlight beam on a
white wall or piece of paper far away. The farther away the flashlight
is (the more it appears to be a point source of light) the sharper the
image of the heat eddies will be.
If instead of a point source of light you have a narrow slit of
light, you will get lots of smeared slit-shaped images superimposed on
each other. This is what happens in an eclipse just before the moon
blocks all the light from the sun -- a tiny little sliver of the sun
will be visible on one edge of the moon, and the images of this focused
and defocused by the atmospheric turbulence will be seen as bands on
the ground.
The bands move with the speed of the wind, and are observed to
travel perpendicular to their lengths. This is because motion along the
bands is not easy to make out because the bands look pretty much the
same along their lengths.
On a planet with no atmosphere, no shadow bands are expected to be
visible. For different atmospheres, they should be visible as long as
there is turbulence which creates regions of different densities and
refractive indexes.
On your gravity and planetary alignment question, the answer is
no, there is no reason to expect any dire consequences of planetary
alignment. Planetary alignment, when it does happen, does not create
easily measurable effects on anything. The planets interact
gravitationally with each other, and the force of gravity between one
planet and another is extremely weak, and becomes weaker very rapidly
with increasing distance away.
The gravitational tugs of planets on each other do have some tiny
effects. For example, Neptune's tug on Uranus is sufficiently large to
perturb Uranus's orbit over the course of years away from its normal
path, while Neptune is close. This is the way Neptune's existance was
predicted, by making very precise measurements of Uranaus's postion
over the course of many years and discovering tiny but significant
discrepancies. Pluto's orbit is calculated to have a chaotic component
to it due to the changing gravitaitonal interactions over the course of
hundreds of millions of Pluto orbits. Earth's orbit is dominated by the
Sun's gravitational field because the sun is so much very much more
massive than anything else in the solar system, and is also rather
close to the earth, when compared to how far away some of the more
massive planets in the solar system are from earth.
Tom
(published on 10/22/2007)
