QUESTION:
I teach High School Astronomy.  While we were talking about the problems
in ground-based astronomy (due to the atmosphere) a student asked me:
If the blue from the sun is scattered by the atmosphere, leaving it
yellow, why isn't the blue from the moon scattered to the same degree,
leaving it yellow also?     Thanks!!!

ANSWER from Megan Donahue on April 3, 1996:
Hi,
The sun is not yellow because the the blue is scattered, leaving it
yellow, the sun is yellow because its photosphere (the layer of the
Sun that we can see) is 5700 degrees K, so it emits most of its
photons in the yellow part of the spectrum. The Sun emits light of
all colors, but the most dominant color is yellow. The atmosphere does
scatter some of the blue light (which is why the sky is blue) but that
does not significantly change the color of the Sun. If anything it makes
the Sun look more yellow and less white (which is a mix of all colors).

For example, because the gas in the photospheres of other stars can be
at temperatures different from the Sun's, other stars can have colors
different from yellow.

If you are at a dark site of very good quality (clear, calm skies), you
can see that stars have colors. Some stars are red (Betelgeuse and
Antares are famous and bright examples). The photospheres of these
stars are cooler than the Sun's, and so the most of the light from these
stars are emitted from a lower energy or redder part of the electromagnetic
spectrum. Some stars look bluish. These stars have hotter, and therefore
bluer, photospheres than the Sun.

Now, the question you asked is a bit different from the concept I
just addressed. Because the
Moon does not have its own energy source, it does not glow like stars do.
Because it has a temperature, albeit very cold, it does emit a very faint
and extremely red spectrum. (Every object with temperature greater than
absolute zero emits radiation. You could ask your class what sort of radiation
THEY emit.) However, this is not the radiation we see when we look at
the Moon at night. (You could ask your class what light they see when
they look at each other. It isn't the infrared light that their bodies are
giving off, unless they have those special night goggles. But since we
don't go to class with night goggles, where does the light come from?)

The answer is, of course, reflected light. The Sun emits light in all
colors, and that light is reflected in varying degrees by your students,
which is why some students have brown hair and blue eyes and others
have brown eyes and blue hair. (heh). Every material reflects and
absorbs light in its own way, which is why things apparently have different
colors. If there were no red in the Sun's light, a red fire engine would look
black to us, unless it were under artificial light.

We see the Moon because it reflects sunlight, just as other everyday objects
reflect sunlight. The Moon looks white or light grey because the rocks on
the Moon reflect light of all colors, just like
a piece of blank typing paper does. The Moon absorbs most of the light
it receives, but it doesn't preferentially absorb one color more than another.

If you look at the Moon when it is low in the sky (and therefore you are
seeing it through a lot of atmosphere thickness), or after a volcano burst
has scattered lots of dust in the upper atmosphere, it can appear to be
redder than usual. Some of the blue light from the Moon is indeed scattered,
making it appear redder than usual. As the Moon gets higher in the sky,
and its light is travelling to our eyes through less and less atmosphere
(why is that?) the Moon's appearance changes back into its familiar
grey-white self.

A "blue Moon" is the second full moon occurring in a calendar month, which
doesn't happen too often. Hence the phrase "once in a blue moon." But the
moon itself doesn't look blue on these occasions. I think I was in college
before I knew that, so I always interpreted a "blue Moon" to mean "never",
from context. So for some reasons "blue Moons" please me these days.