For a few hours on the night of Dec. 20 to Dec. 21, the attention of tens of millions of people will be drawn skyward, where the mottled, coppery globe of our moon will hang completely immersed in the long, tapering cone of shadow cast out into space by our Earth. If the weather is clear, favorably placed skywatchers will have a view of one of nature's most beautiful spectacles: a total eclipse of the moon.