Sky-watchers know that if you look at the crescent moon on a dark night, you see not only the brightly sunlit portion but also a ghostly glow over the rest of the moon's disk.
That glow is earthshine—sunlight reflected by the Earth—and some scientists say it could help us gauge global warming or find life on other planets.
The brightness of earthshine, said Philip Goode, director of Big Bear Solar Observatory in Big Bear City, California, can be used to make precise estimates of changes in the Earth's reflectivity, or albedo.
Such changes are the "undiscovered country" in our understanding of global climate change, he said today in Baltimore, Maryland, at a meeting of the American Geophysical Union.
That's because even minor changes in the amount of sunlight that the Earth reflects into space could create major changes in our climate.
Changes in albedo could represent changes in the Earth's average cloudiness. More clouds mean more sunlight reflected into space—and less reaching the ground.
Scientists tracking earthshine, for example, have found that the amount of sunlight reflected by the Earth steadily declined from the mid-1980s until 2000.
Then the trend reversed: The amount of sunlight reaching the surface began declining.
Most likely, Goode says, this is part of a natural cycle and not a side effect of human activities.
But he added, "We don't know why this natural cycle is occurring."
New Planets
Other astronomers are interested in using earthshine as a way of testing methods for detecing life signs on distant planets.
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