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Scientists have revived and modernized a nearly forgotten technique for monitoring Earth's climate by carefully observing "earthshine," the ghostly glow of the dark side of the moon. Earthshine measurements are a useful complement to satellite observations for determining Earth's reflectance of sunlight, an important climate parameter. Long-term observations of earthshine thus monitor variations in cloud cover and atmospheric particles known as aerosols that play a role in climate change.

Earthshine is readily visible to the naked eye, most easily during a crescent moon. Leonardo da Vinci first explained the phenomenon, in which the moon acts as a giant mirror, showing the sunlight reflected from Earth. The brightness of the earthshine thus measures the reflectance of Earth.

In May 1 issue of the journal, Geophysical Research Letters, published by the American Geophysical Union, a team of scientists from the New Jersey Institute of Technology and the California Institute of Technology report that Earth's albedo, the fraction of sunlight it reflects, is currently 0.297, with a margin of error of 0.005.

"Earth's climate is driven by the net sunlight that it absorbs," says Philip R. Goode, leader of the New Jersey Institute of Technology team, Director of the Big Bear Solar Observatory, and a Distinguished Professor of Physics at NJIT. "We have found surprisingly large--up to 20 percent--seasonal variations in Earth's reflectance. Further, we have found a hint of a 2.5-percent decrease in Earth's albedo over the past five years." If Earth reflected even one percent less light, the effect would be significant enough to be a concern with regard to global warming.

In the early 20th century, the French astronomer André-Louis Danjon undertook the first quantitative observations of earthshine. But the method lay dormant for nearly 50 years, until Caltech professor Steven E. Koonin described its modern potential in a 1991 paper. The newly published data are the...
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