MadSci Network: Astronomy

Re: In outer space, are shadows 'hard' hard edged or Soft edged (fuzzy)?

Date: Wed Oct 12 04:30:50 2005
Posted By: Tye Morancy, Staff, Medical Physicist/Dosimetrist, Beth Israel Deaconess Medical Center
Area of science: Astronomy
ID: 1125625231.As

Thank you, Angelo, for the question.

Well, my input on this is simply that there should be a "hard" shadow, using your terminology. It seems to me you are implying that, let's say, I have an eclipse and there is a shadow being cast by the intervening planet or moon. If I could look at the edge of this "in outer space," I should see a "hard" or well-defined edge, simply because there is a near vacuum in outer space and should only cause a negligible scattering of the light I am seeing at the edge.

I am wondering if the discussion is back and forth because some people might be answering it as I am while others answering it as if I am somewhere on the Earth, peering up at a phenomenon through an atmosphere? The atmosphere being in the way would certainly add a "fuzziness" to your clarity of a shadow's "edge."

Another approach I might take on this, using the same eclipse example above, is that the hard vs. fuzziness could be a shadowing phenomenon where I have two distinct regions of my shadow called the umbra and the penumbra (I am including a link to further explain). When the light source, here the Sun, is being eclipsed by the Moon as seen from Earth, a diffuse shadow is projected, with a smaller inner shadow that is totally obscured of direct light. Those in the inner shadow, the umbra, would see the sun completely blocked out and perhaps fit your "hard shadow" reference, while those in the outer shadow, the penumbra, would not see a total blockage. These individuals would see an incomplete shadow, a "fuzzy shadow" perhaps?

These are my two thoughts on your question. Let us know if we can be of further assistance. I hope this helped out...


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