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Q & A: Shadow Bands in solar eclipses

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Most recent answer: 10/22/2007
Q:
This is totally unrelated to my field... I saw something on the discovery channel once regarding shadow bands and total eclipses. That program said that scientists didn’t yet have a concrete explination for why these occured, only theories. I found a site online that attributed shadow bands to "irregularties in the earth’s atmosphere," but the bands looked too uniform for that. In new age belief/astrology and tv shows there’s always something dire predicted to happen when the planets align and gravity pulls on the earth. Is it possible that the suns layers are pulled by gravity (similar to tides) in such a manner as to create the shadow bands? I guess a similar question is: do shadow bands occur on planents with different/no atmospheres?
- Timothy (age 22)
Baylor, Waco, TX
A:
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:

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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)

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