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Wednesday, December 22, 2010

The Blood Red Moon of December 20-21, 2010

The colors of the eclipsed moon yesterday were generally reported as "blood red", and Jewish tradition considers the color of an eclipsed moon to be a part of its "omen" value. What causes these colors which can range from very dark and barely visible to red or copper?

The earth's shadow is considerably larger than the diameter of the moon, so when the moon passes into that shadow (called the "umbra") becoming eclipsed, it can pass through the center of the shadow or on various paths above or below the center. Generally speaking, the more centrally the moon passes through the earth's shadow, the darker and less colorful the eclipse. The various paths the moon can take through the earth's shadow also account for variations in brightness seen across the disk of the moon.

Here is the path the moon took through the earth's shadow yesterday:

Path of the moon through the earth's shadow on December 20-21, 2010

The moon passed well north of the earth's umbra, the central and darkest part of its shadow, causing the moon's northern limb to appear much brighter than the southern. 

The moon at mid-eclipse. (Tony Sharfman, Cranford, NJ)

Once the moon is in the umbra, the earth's atmosphere acts as a giant lens, bending the light of the sun around the earth and focusing it on the moon. Just as at sunset the long, red rays of the sun become enhanced because the thicker atmosphere scatters all of the shorter wavelengths, most of the light striking the moon is reddened by this atmospheric scattering and lensing. This effect can be enhanced or diminished by global weather effects along the rim of the earth, so the actual colors and brightness of an eclipsed moon are impossible to predict.

And that is why we had a "blood red" eclipse yesterday.

Keep on looking up!

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