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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 a High-Energy Astronomer team
Questions on this topic are no longer responded to by the "Ask a
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