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Reading this article about the forthcoming eclipse on December 10th, I noticed mention that the moon passes through the red shadow of Earth; I'd always held the impression that a shadow is universally dark/gray.

I'm guessing there is some kind of prism effect coming into play over there particularly with this configuration of the celestial bodies involved. Why is the shadow of earth red?

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ur article itself provides the answer to your question.. "Earth's stratosphere is the key: "During a lunar eclipse, most of the light illuminating the moon passes through the stratosphere where it is reddened by scattering," he explains. "If the stratosphere is loaded with dust from volcanic eruptions, the eclipse will be dark; a clear stratosphere, on the other hand, produces a brighter eclipse. At the moment, the stratosphere is mostly clear with little input from recent volcanoes." " – Vineet Menon Dec 9 '11 at 5:24
@VineetMenon: Talk of a faux-pas ... Thanks /+: – Everyone Dec 9 '11 at 6:19

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When a total lunar eclipse occurs, the only irradiance of the moon is due to refracted light travelling through Earth's atmosphere. Blue light is scattered in the process by the Tyndall effect, leaving more red light. The Moon is red during a total eclipse for the same reason the sky is blue.

It's worth noting that the shadow has NO COLOR, as it is truly the absence of light. When in the Earth's penumbra, red light is more prevalent, giving this hue. In the umbra, the same effect can be overpowered by more direct solar irradiance.

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