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A: The same process that brings colorful sunsets – when the Earth’s atmosphere filters out the blue end of the sun’s visible light – causes the moon to appear red or orange when near the horizon. The most breathtaking red moons occur during a total lunar eclipse, when the moon passes through the Earth’s shadow. Again, the scattering of the blue end of the visible light spectrum allows the red end of the spectrum to illuminate the eclipsing moon.
Without the Earth’s atmosphere, the moon would look much different to us during a lunar eclipse. While some sunlight would still reach the moon, it would be much dimmer and it wouldn’t appear red or orange.
(Answered by Bob Swanson, USA Today's assistant weather editor, July 4, 2005)