What Causes the Moon to Appear Orange?

[Pengwuino]

So what causes the moon to be orange? I saw one such display about an hour ago going to get food and i was like "aww... big cheese wheel... I am going to eat the moon". So what cuases that?

 

[SpaceTiger]

So what causes the moon to be orange? I saw one such display about an hour ago going to get food and i was like "aww... big cheese wheel... I am going to eat the moon". So what cuases that?

Was it near the horizon? Usually that sort of thing is cause by preferential scattering of blue light in the atmosphere (same as with a sunset). I suppose that high-pollution areas might have differently colored moons as well.

 

[Pengwuino]

Yes, very near the horizon. Probably 3/4 of a diameter from touching the horizon. And woo, pollution :D you couldn't imagine... oh wait new jersey... nevermind ;)

 

[HallsofIvy]

If the moon is close to the horizon, its light has to go through more atmosphere.
Typically, dust in the atmosphere causes the orange shade.

(But I thought the moon was made of GREEN cheese!)

 

[Mariko]

the reason the moon looks like its orange or red is because in our atmosphere and even in space where there are "dust clouds" or smog and dust between you and the object your looking at it filters out the blue light and appears to look reddish//orange something like interstellar reddening

 

[chroot]

There are no appreciable dust clouds between the Earth and Moon, Mariko.

- Warren

 

[Mariko]

maybe not in the cloud form but there is dust in our atmosphere is there not? also do you know why the sky is blue? doesn't it have to do with the size of the dust particle and the wavelength of the blue color in the visible spectrum which makes it so??

 

[russ_watters]

The sky is blue because nitrogen scatters blue light.

 

[Mariko]

interesting because my Astronomy professor just explained to the class when we were studying the electromagnetic spectrum the reason the sky appeared blue was that the particles such as the ones in our environment on Earth are about the same size as the blue light and i could have sworn she said something about the dust being the reflector but i suppose i should have taken notes on that fact well sorry...

 

[hypatia]

http://home.hiwaay.net/~krcool/Astro/moon/moonorange/

 

[SpaceTiger]

I think pretty much everybody's right here. A lot of things scatter light, including normal atmospheric molecules, "dust clouds", and even pollution. It just depends on where you're looking from.

 

[Billy T]

I think pretty much everybody's right here. A lot of things scatter light, including normal atmospheric molecules, "dust clouds", and even pollution. It just depends on where you're looking from.

I agree with ST, except I know of no reason (other than it is more common than oxygen) why nitrogen should scatter blue light better.
The scattering visible light is by atmospheric gases is not by individual molecules, but by the cooperative interaction all molecules in a volume of space in which there is a significant statistical percentage variation in the number of molecules that happen to be present.

Roughly speaking the volume of this region increases with altitude and at ground level is smaller than the wavelength - does not scatter much. At higher altitude it first becomes comparable to shorter wavelengths (blue light) and the density of the atmosphere is greater here than higher up (more different from vacuum - a better scatter) but the most important factor is that small density variations of a few wavelength typical size scatter as inverse fourth power of the wavelength. Ie. blue light will scater at least 10 times more than red light will.

If the scattering object /density fluctuation is ccomparable or slightly smaller than the wavelength (called the Mie scattering region, if memory serves) there is some resonance or oscillatory dependence upon the wavelenth and the scatterer is more complex than simple inverse fourth power.

In the IR, the nature of the molecules may be important also as in this part of the spectrum their absorption is active.

 

[SpaceTiger]

I agree with ST, except I know of no reason (other than it is more common than oxygen) why nitrogen should scatter blue light better.
The scattering visible light is by atmospheric gases is not by individual molecules, but by the cooperative interaction all molecules in a volume of space in which there is a significant statistical percentage variation in the number of molecules that happen to be present.

I think that's only true in the classical picture of light. With QM, individual photons will interact with individual molecules with a probability that mimics a classical EM wave. I don't know the relative cross sections of nitrogen and oxygen, but I assume that russ focused on nitrogen simply because it was more common.

 

[Billy T]

I think that's only true in the classical picture of light. With QM, individual photons will interact with individual molecules with a probability that mimics a classical EM wave.

We both agree that the photons interact with molecules, not density fluctuations, but it is these fluctuations that cause the scattering.
For example, when photons pass thru glass, their electric field excites (not in the sense of change in quantum level, but still speaking in quantum mechanical terms) the bound electrons, giving them slightly different energy levels (Stark effect) that would actually show up if they happened to emit a photon, but as they are almost all in the ground state, they don't.
What happens is (and this is now in classical terms) that the photon's force on the electrons of the glass, causes an acceleration of them, which radiates. The phase of this radiation is 90 degrees from the driving electric field of the photon, but of the same frequency. When you add the new induced radiation to the original of the photon, it is as if there were one radiation field of the same frequency, but its phase has been shifted slightly by the presence of the glass electrons - that is how glass interacts with photons. (in rough terms).
Because the induced radiation from the glass electrons is coherently additive only in the forward direction, the "beam" of phase shifted photons continues onward only in the original direction - no scattering. If however, in the glass you put some small (a few wavelengths or only roughly one wave length) "bubbles" with less ability to interact with the photons, (like the density fluctuation in the air) the cooperative cancellation in all directions except the forward direction is no longer true. That is, the induced field that should have come from the bubble (density fluctuation) will be too weak to do its share of the cancellation in the non-forward direction - I.e. scattering will occur.

(In the forward direction, with "bubbles", it will still be in 90 degree phase relative to the photon field, but slightly weaker, so there will be slightly less phase shift of the beam passing thru the glass).

We do not disagree. I am just trying to show (despite the difficulty of mixing quantum and classical pictures) why it is important to understand that it is the "cooperative interaction" (my original terms) of density fluctuations that is really the the cause of the scattering. Without the fluctuation, only a mix of some absorption and some transmission occurs.

I don't know the relative cross sections of nitrogen and oxygen, but I assume that russ focused on nitrogen simply because it was more common.

I noted this fact inmy original post. I also do not know the relative cross sections, but based on my understanding (presented inpart above) I can make a guess: The atomic versions of N & O have respectively 7 and 8 outer shell electrons (only these are significantly "accelerated" by the photon's field to "reradiate" with 90 degree phase shift.) Thus I am reasonably confident that atomic nitrogen would have 7/8's of the effect of atomic oxygen on the phase shift of the beam propagating thru air, if the number of N and O atoms per cc were the same. I am less confident about the molecules, N2 and O2, but suspect that their relative effects is still in approximately in this same ratio. I.e. I am assuming that molecular N2 has 14 electrons "exposed" to the photon field and molecular O2 has 16 "exposed." Someone with access to a table of refractive indexes can check my 7/8 ratio guess (stripping out the initial 1 of course).

 

[SpaceTiger]

We do not disagree. I am just trying to show (despite the difficulty of mixing quantum and classical pictures) why it is important to understand that it is the "cooperative interaction" (my original terms) of density fluctuations that is really the the cause of the scattering. Without the fluctuation, only a mix of some absorption and some transmission occurs.

I think it makes more sense to look at it the other way around. The fact that the molecules are distributed randomly means that there will be no interference of individual photons. Only in an organized lattice do they interfere to give a monodirectional beam after passing through the medium. An individual photon scattered from an individual molecule can go in any direction, not just the direction of incidence.

 

[Labguy]

I think it makes more sense to look at it the other way around. The fact that the molecules are distributed randomly means that there will be no interference of individual photons. Only in an organized lattice do they interfere to give a monodirectional beam after passing through the medium. An individual photon scattered from an individual molecule can go in any direction, not just the direction of incidence.

SpaceTiger is totally correct in the quoted post above. Plus, and I am no chemist, but wouldn't it be that:

I.e. I am assuming that molecular N2 has 14 electrons "exposed" to the photon field and molecular O2 has 16 "exposed." Someone with access to a table of refractive indexes can check my 7/8 ratio guess (stripping out the initial 1 of course).

is not correct since N has only 5 electrons in the second energy level ("shell") and O has 6 electrons at that level with both elements having two electrons in the first level that would be unavailable unless ionized?..

http://www.chemicalelements.com/elements/n.html
http://www.chemicalelements.com/elements/o.html

Either way, geesh, we are only talking about an orange moon here..

 

[Billy T]

SpaceTiger is totally correct in the quoted post above. Plus, and I am no chemist, but wouldn't it be that: is not correct since N has only 5 electrons in the second energy level ("shell") and O has 6 electrons at that level with both elements having two electrons in the first level that would be unavailable unless ionized?..

http://www.chemicalelements.com/elements/n.html
http://www.chemicalelements.com/elements/o.html

Either way, geesh, we are only talking about an orange moon here..

Thanks for catching my error. I must have been tired or something - my comments on relative effects of N and O probably have the correct principle, but clearly the wrong numbers. I simply forgot that the first two electrons, one spin projection up and one down, to keep Pauli happy, go in the n=1 level.
The n = 2 level has both s & p states (two again go in the s states and of the 6 possible p states (l = -1,0,1 for the 3 angular momentum variable projections, which each can hold two spin projections - i.e. 3 doubled for spin = 6 total in p states) only 3 for N and 4 for O are populated.
(I hope I still have the notation correct. I have not been to your sites, yet, but even though my Ph.D. was in another area, I was at JHU while Dr. Dieke was head of the department and most physics Ph.Ds. were related to spectroscopy then. - I could not avoid the area entirely so once spoke there language fluently.)

I now say "probably correct principle" because, even the the two "s state" electrons of the n = 2 shell have stronger (not by a lot) binding (they have less mutual screening of the nuclear charge by others because they "pass thru the nucleus," to speak classically.) I doubt if this small difference of energy levels in the n = 2 state makes any difference for visible photons, but refraction of radio waves in the ionosphere, may notice it. (I don't know - just wanted to be more careful this time, and show that it was not lack of knowledge, just tiredness that lead to my error.)

I am not an expert, but think I still have a small disagreement with you and SpaceTiger. One does not need a regular crystal structure to "keep the beam" propagating only in the forward direction. What is required, I am almost sure, is a lot of atoms inside a volume of roughly of wavelength dimensions. - so many atoms that the statistical variation is nil compared to the total.

For example glass free of bubbles etc. will coherently add the induced reradiation only in the forward direction but the very definition of glass contains the noncrystalline nature of it. You can also see this from a Hyggen (Spelling his name wrong?) approach to adding up the wavelets.

Dense clean air is another example of "forward only" propagation. - We see sharp edges to distant objects not fuzzy blurs. A distant light may be more orange, and blue is scattered out. Looking down into deep clean water also illustrates, blue and green multiple scatter, so I can not claim that there is no scattering, but if temperature variation is controlled, the forward "scatter" out of the beam is very small, despite it not being at all crystalline in either case. I think the spread of the beam in forward direction (other than diffraction -even a laser beam going to the moon grows by this.) is mainly the small effects that still do come from the small statistical variations in density, not the lack of crystallinity.

Thus, I think you and SpaceTiger are placing too much emphases on the need for a crystalline regularity (instead of random) location of the atom's whose outer shell electrons respond to the electric field of the incident photon by reradiating their own (classical view, but we are dealing with large number of events so OK to speak this way rather than add up zillions of photon waves and get the same result.)

Considering what I have noted about dense air and glass - Do you still think the induced (re)radiation wave fronts only constructively sum in the forward direction, if the scattering (reradiating) atoms are in a crystalline array?

 

[SpaceTiger]

Considering what I have noted about dense air and glass - Do you still think the induced (re)radiation wave fronts only constructively sum in the forward direction, if the scattering (reradiating) atoms are in a crystalline array?

Yup. Light that is passed through glass is not absorbed and reradiated (as with scattering), it is simply transmitted; that is, not absorbed at all. These are fundamentally different processes.

 

[Mariko]

The sky is blue because nitrogen scatters blue light.

After my Astronomy class this evening i asked my professor to again tell me why the sky appears blue < I also asked her about the nitrogen scattering the blue light and here's what she said:

The reason the sky appears blue is that dust particles which are approximately 1 micron across most effectively scatter the blue light and if it was nitrogen that made the sky look blue it would be blue all the time even at sunset. thus its not the nitrogen

 

[Phobos]

After my Astronomy class this evening i asked my professor to again tell me why the sky appears blue < I also asked her about the nitrogen scattering the blue light and here's what she said:

The reason the sky appears blue is that dust particles which are approximately 1 micron across most effectively scatter the blue light and if it was nitrogen that made the sky look blue it would be blue all the time even at sunset. thus its not the nitrogen

At sunset, there is more atmosphere for the light to go through...so there is more scattering of light before reaching your eyes...so there is less blue light reaching your eyes...so sunsets/sunrises are more red.

Question - perhaps nitrogen + small particles both contribute, yes?