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28-May-2004

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American Association for the Advancement of Science

Scientists' moon bounce



Mean cloud cover map over area contributing to ES (earthshine) during about 4 hours of observation from Big Bear Solar Observatory each night. Image courtesy of Big Bear Solar Observatory in Big Bear Lake, CA.
Click here for a high resolution photograph.

The moon bounce is always popular at carnivals. In a new study, scientists report that a different kind of moon bounce -- one involving bouncing light, not bouncing feet -- may be important for scientists who study how Earth's climate works.

If you've ever looked at the moon when it's a skinny crescent shape, you can sometimes see a faint, ghostly glow coming from dark part of the moon. What you're seeing is sunlight that first reflects off the Earth and then bounces off the moon. The official name for this glowing moon bounce is "earthshine."

In contrast, virtually of the bright light that defines the moon's bright crescent shape comes directly from the sun.



Big Bear Solar Observatory in Big Bear Lake, CA. Image courtesy of Big Bear Solar Observatory in Big Bear Lake, CA.
Click here for a high resolution photograph.

Enric Pallé and colleagues from Big Bear Solar Observatory, which is in California but managed by the New Jersey Institute of Technology, took pictures of the moon at night. They compared the brightness of the crescent part of the moon to the pale part. Then they were able to estimate how much light is bouncing off the Earth.

Any changes in the amount of light leaving the Earth may be tied to clouds and climate change, so the scientists also analyzed satellite measurements of cloud properties on Earth. They found that the amount of light the Earth reflects back into space decreased steadily from 1984 to 2000.

The earthshine data indicated that this decline then reversed completely after 2000.

Natural variability probably explains this turnaround. If this trend continues over the next few years, it might play an important role in future climate change, the authors write.

These findings appear in the 28 May 2004 issue of the journal Science.

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