What Earth’s atmosphere does to sunlight and moonlight?

We see the sun and moon through Earth’s atmosphere. And the sun looks yellow — while the moon looks white. What Earth’s atmosphere does to sunlight and moonlight?

A reader: “I hear the sun appears yellow because its blue light is scattered, causing the sky to appear blue. So why doesn’t that happen with the moon?”

You’re right, the sun’s light is scattered by Earth’s atmosphere. Light at the blue end of the spectrum is most easily scattered by molecules of air — and that’s why the sun looks yellow — and the sky looks blue.

Moonlight is something else again. Remember that the moon doesn’t really have any light of its own — it shines with reflected sunlight. So maybe you’ll assume that the sun and moon should look similar — but then remember that sunlight reflecting from the moon is really bouncing off lunar dust and rock. The moon’s color depends both on the color of sunlight — and the reflectivity of the dust and rock. So the moon looks white.

And, meanwhile, the moon’s light does scatter in the atmosphere, just as sunlight does. But moonlight by itself is too weak to make the sky look blue. On the other hand, when the sun and moon are close to the horizon, either one can look yellow or orange in color.

That’s because there’s more atmosphere in the direction of the horizon. More molecules of air remove more blue light — and we see an orange moon or sun.

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