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The Question

(Submitted March 14, 1997)

Why does the Moon sometimes come out during the day?

The Answer

You can see the Moon in the daytime because it is big and brightly lit by the Sun. The surface of the Moon is about as reflective as an asphalt road--rather dark but not totally black. When you look at the Moon, you are seeing the light which reflects off it. This is not nearly as bright as the Sun, but it is up to 100,000 times as bright as the brightest nighttime star.

During the day, the brightness of the sky washes out the light from the stars: a region of the sky including a bright star is only very slightly brighter than a region of the sky without a bright star, so your eye cannot notice the difference. However, the region of the sky containing the Moon is much brighter, so you can see it. You can also sometimes see Venus during the day if the conditions are right and you know exactly where to look, but anything dimmer is lost.

It might be useful to think of the Sun as a large light bulb, and the moon as a large mirror. There are situations where we can't see the light bulb, but we can see the light from the bulb reflected in the mirror. This is the situation when the moon is out at night. We can't see the Sun directly because the earth is blocking our view of it, but we can see its light reflected from the moon. However, there are also situations where we can see both the light bulb and the mirror, and this is what is happening when we see the moon during the day. You can explore this for yourself with a light and a hand mirror. Depending on which way you face (away from the light or sideways to the light) you can see either just the mirror, or both the light and the mirror.

I hope this helps!

David Palmer and Tim Kallman
for the Ask an Astrophysicist team

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