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Moonshine & Earthshine

May 28, 2009
Harriet Witt

Have you ever looked at the slender crescent moon and wondered why the night side of it isn’t completely dark? The answer is zanily simple: In the same way that the moon lights up our nights, we light up the moon’s nights!

If you stood on the moon and gazed up, you’d see a big, round Earth gracing your sky. Planet Earth reflects the sun’s light, just as the moon does. However, since Earth is four times bigger than the moon, anybody standing on the moon would enjoy quite a sight in the moon’s sky!

Also, since the Earth is more reflective than the moon (mostly water), it reflects more sunlight than the moon does. So, if you were standing on the moon, you’d be drenched with Earthshine when the Earth is full. If you were on the moon, you’d see the Earth going through phases, just as we on Earth see the moon going through phases.

Years ago, I taught astronomy at an environmental education center in Southern Appalachia, where the night sky is so dark that you can barely see your hand in front of your face.

Southern Appalachia is “hillbilly” country. The Scotch-Irish folks who settled this mountainous area generations ago learned a difficult lesson: By the time you’ve transported your crops along the treacherous dirt roads into market places at lower elevations, your crops are bruised, wilted, rotted or bug-eaten. If you want to make money, you need a crop that can survive the long journey to market—you need to convert your corn into corn liquor. However, legalizing your distillery cost so much that you would have to make your liquor illegally in a remote spot in the woods. Since you can’t risk being seen, you work at night and avoid lighting up your operation. Thus, you make your liquor by the light of the moon, which is how “moonshine” earned its name.

One spring evening, my students and I were enjoying the slender, crescent moon low in the evening sky. “This faint light that we’re seeing on the moon’s night side is sunlight that’s reflecting off our Earth,” I explained. “It’s called Earthshine.”

“Could this mean,” asked one of the kids, “that hillbillies on the moon are brewing illegal booze at night by the light of the Earth overhead? Do you suppose those folks call their liquor ‘Earthshine?’”

Yes, imagining yourself standing on the moon is an entertaining mental workout. It helps you see the Earth up there. Of course, imagining yourself standing on Mercury, Venus or Mars also allows you to see the Earth up there as well. Actually, standing anyplace other than Earth allows you to see that the Earth is a heavenly body. May we treat her as such!

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