The moon's ruddy color during the lunar eclipse tonight is caused by sunlight filtering through Earth's atmosphere. It's what astronomers look at when distant planets pass in front of their own stars.
It will be a late, languid event. For people living east of the Mississippi, the eclipse begins well after midnight. The lunar eclipse will last about three and a half hours, with the moon falling in the depths of Earth's shadow for about an hour and twelve minutes at the height of the event.
As Earth slips between the sun and moon, changing the tint of the lunar surface from white to orange to russet and back, you're seeing the effect Earth's atmosphere is having on the color of sunlight passing through it. But the atmosphere is doing something else. It's in effect tagging the sun's rays with the chemical fingerprints of gases in the atmosphere.
Over the past two years, two teams of astronomers have been using this effect to figure out what Earth might look like as a distant, extrasolar planet orbiting another star. By analyzing the light reflected off the moon during a lunar eclipse – light that has passed through Earth's atmosphere – they have detected gases in the atmosphere that indicate the presence of organic life on the planet.
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