Pink Floyd probably didn't tell you that the dark side of the moon glows with "earthshine," sunlight that bounces off our planet and faintly illuminates the lunar night. As astronomers report today in Nature, the intensity and polarization of earthshine  (seen reflecting off the moon above) varies as our planet turns, and the effect depends on how many clouds and how much ocean and vegetation face the moon on Earth's sunlit side. Such biosignatures might also be present in light from extrasolar Earth-sized planets, thereby distinguishing vibrant worlds like our own from dead ones like the moon. Moreover, starlight is nearly unpolarized, so by observing polarized light, future space-based telescopes may be able to glimpse Earth's twin directly despite its star's glare. And then everything under the sun will be in tune.
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