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The Question

(Submitted October 21, 1997)

I am three years old. Since I cannot type, I am having my Dad write you. I asked this question to my Dad, who is an astronomer, but he could not answer. I guess he is silly. My question is simple: 'Why does the Moon looks white in daytime while it is yellow at night?'.

The Answer

There are a couple of things which might cause this.

Color vision is not very well understood. It is known that the eye and brain try to adjust the colors you see to correct for the color of the light shining on it. That is why grass looks green even when you see it under the red light of sunset. When you look at the Moon during the day, your eye sees the blue background of the sky, and your brain thinks that the light is blue, and (incorrectly) figures out what color the Moon must actually be to look the way it does under blue light. When you look at it at night, the brain has more trouble since it has no way of guessing what color the light is. This might account for the apparent color difference.

Another possibility, which is certainly part of it, is that the color difference is due to skylight. When you look at the Moon during the day, you see the moonlight, plus all the blue sunlight which is scattered by the atmosphere between the Moon and you. At night, the atmosphere doesn't have any sunlight to scatter.

David Palmer
for Ask an Astrophysicist

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