|MadSci Network: Astronomy|
Melissa, Interesting question. Sometimes in the fall the moon really can look orange and yellow. If you observe closely, though, you'll find that the in the fall, when the moon is right overhead, it looks pretty white. And in the winter, when the moon is close to the horizon, it can look pretty orange. Here's what's going on. The first thing that you have to understand is that there are different kinds of light. Green light, for instance, is different than red light. Blue light is different than yellow light. The main difference is that different colors of lights have different wavelengths. Light travels in waves, and the distance from one peak of a wave to another is called the wavelength. Blue light has a very short wavelength, and red light has the longest. The second thing to understand is that the air in our atmosphere acts like a filter. It filters out short wavelength light, and scatters the short wavelengths around. The short wavelengths are the blue ones. The greens, yellows, and reds come through. That's why the sky looks blue. All the blue light is bouncing around the air before it comes down, so the sky looks blue. At night, when the moon is right overhead, it looks white. White light is made up of all different colors of light, including blue. When the moon is right overhead, it comes straight through the atmosphere, so not much of the blue light gets scattered. But when the moon gets close to the horizon, it isn't coming at you straight on. It's kind of slanting at us, and it's going through a lot more air. The more air the white moonlight has to go through, the more chance for the short wavelength (blue) light to get scattered out. The green light is pretty short too, so it gets filtered out too. What's left is the red and yellow light. And that's why you only see the red and yellow light when the moon gets close to the horizon, and that's why the white moon looks red and yellow whenever it is about to rise or set. Hope that helps a little bit. At least now you know why the sky is blue too! Tom
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