Starship Asterisk*

A Colorful Lunar Corona

Explanation: What are those colorful rings around the Moon? A corona. Rings like this will sometimes appear when the Moon is seen through thin clouds. The effect is created by the quantum mechanical diffraction of light around individual, similarly-sized water droplets in an intervening but mostly-transparent cloud. Since light of different colors has different wavelengths, each color diffracts differently. Lunar Coronae are one of the few quantum mechanical color effects that can be easily seen with the unaided eye. The featured lunar corona was captured around a Strawberry Moon on June 2 from La Plata, Argentina. Similar coronae that form around the Sun are typically harder to see because of the Sun's great brightness.

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Thanks for posting these pictures for us, bystander.

Today's APOD is a fine picture, as usual. Indeed, that is some lunar corona! And congratulations to Sergio Montufar, one of the stalwarts of Starship Asterisk*.

Ann

I've never seen the moon looking anything like this. Thanks for filling in for Otto while he's on vacation, bystander.

/snip

Beyond wrote:Thanks for filling in for Otto while he's on vacation, bystander.

Is this the reason that I haven't been able to view the image of the day since Friday? 'Otto' is on some sort of vacation/hiatus/MIA..?

What?
Red, orange, yellow, green, blue, purple - orange???
How does that happen?

JOhn

Hi, can someone please explain to me (or just post a link) why this is a quantum effect? I am used to thinking of diffraction of light as something that can be explained using classical wave physics in the Fraunhofer regime (i.e. Airy disks and all that) and so I don't see the quantum nature here. Any help would be appreciated, thank you

merryjman wrote:Hi, can someone please explain to me (or just post a link) why this is a quantum effect? I am used to thinking of diffraction of light as something that can be explained using classical wave physics in the Fraunhofer regime (i.e. Airy disks and all that) and so I don't see the quantum nature here. Any help would be appreciated, thank you

Exactly what I was wondering too! The links in the caption on diffraction lead to the usual classical explanations. The two links related to quantum mechanics deal with interference in a "two paths situation" and reflections by metal and I don't see how this relates to the lunar corona. Where is the link to the source where it's shown that the lunar corona can only be described by quantum mechanics?

Put differently, if this effect is described by quantum mechanics, can one measure Planck's constant from this image?

I always feel great joy whenever I see a lunar corona in the sky, it makes me feel glad to be alive!

JohnD wrote:What?
Red, orange, yellow, green, blue, purple - orange???
How does that happen?

JOhn

I can see how repeating color bands can occur. Just like multiple rainbows in the sky after a rainstorm. The same process? But when I looked at the high-res image, there appears to be a light colored ring around the moon. Is this an artifact of imaging, post processing of the photo, or an actual compressed ring of color visible right near the apparent surface?

merryjman wrote:Hi, can someone please explain to me (or just post a link) why this is a quantum effect? I am used to thinking of diffraction of light as something that can be explained using classical wave physics in the Fraunhofer regime (i.e. Airy disks and all that) and so I don't see the quantum nature here. Any help would be appreciated, thank you :ssmile:

All diffraction is a quantum effect. That is, while it can be approximately described by classical theory, in certain regimes it is best explained using QM. A rigorous analysis of Mie scattering and the color effects seen in a lunar corona is best handled with QM. I think it is slightly misleading to say that this effect is "purely" quantum mechanical, however.

Chris Peterson wrote:

merryjman wrote:Hi, can someone please explain to me (or just post a link) why this is a quantum effect? I am used to thinking of diffraction of light as something that can be explained using classical wave physics in the Fraunhofer regime (i.e. Airy disks and all that) and so I don't see the quantum nature here. Any help would be appreciated, thank you

All diffraction is a quantum effect. That is, while it can be approximately described by classical theory, in certain regimes it is best explained using QM. A rigorous analysis of Mie scattering and the color effects seen in a lunar corona is best handled with QM. I think it is slightly misleading to say that this effect is "purely" quantum mechanical, however.

I'm sorry, but saying "all diffraction os a quantum effect" is like saying "everything is a quantum effect." It's not false, but it's not helpful, either.

The fact that the lunar halo splits up the white-light color components and enhances the intensity of different colors needs some explanation. I'm familiar with the way conventional rainbow scattering splits up colors and enhances intensity at the rainbow angle-- but I don't see an obvious analogy here.

♪ I can see clearly now, the 404 is gone! ♪

I have seen this effect many times. I never knew what caused it.
Even around the Sun on particularly misty mornings. (it can get pretty misty in the Netherlands).

Oddly enough if you reduce the photo to 256 colors the effect becomes even stronger; the differentiation in color bands becomes more pronounced and easier to see. I can see 4 orange bands that way.

MattF wrote:

Chris Peterson wrote:All diffraction is a quantum effect. That is, while it can be approximately described by classical theory, in certain regimes it is best explained using QM. A rigorous analysis of Mie scattering and the color effects seen in a lunar corona is best handled with QM. I think it is slightly misleading to say that this effect is "purely" quantum mechanical, however.

I'm sorry, but saying "all diffraction os a quantum effect" is like saying "everything is a quantum effect." It's not false, but it's not helpful, either.

It's also not true. Not everything is a quantum effect. But diffraction is. Diffraction is only fully understood by utilizing QM. Classical theory is only an approximation. It's a very good approximation in many cases (but not so good in the case of diffraction by small particles), but an approximation all the same. This is analogous to the way that gravity can only be fully described using General Relativity, even though classical (Newtonian) theory is simpler and often serves analysis.

I have no complaint with the caption describing this effect as the product of quantum mechanical diffraction, only with the use of the word "purely", which is somewhat confusing.

Chris Peterson wrote:

MattF wrote:

Chris Peterson wrote:All diffraction is a quantum effect. That is, while it can be approximately described by classical theory, in certain regimes it is best explained using QM. A rigorous analysis of Mie scattering and the color effects seen in a lunar corona is best handled with QM. I think it is slightly misleading to say that this effect is "purely" quantum mechanical, however.

I'm sorry, but saying "all diffraction os a quantum effect" is like saying "everything is a quantum effect." It's not false, but it's not helpful, either.

It's also not true. Not everything is a quantum effect. But diffraction is. Diffraction is only fully understood by utilizing QM. Classical theory is only an approximation. It's a very good approximation in many cases (but not so good in the case of diffraction by small particles), but an approximation all the same. This is analogous to the way that gravity can only be fully described using General Relativity, even though classical (Newtonian) theory is simpler and often serves analysis.

I have no complaint with the caption describing this effect as the product of quantum mechanical diffraction, only with the use of the word "purely", which is somewhat confusing.

Maybe the issue is one of level of understanding.
In the sense that the effect can be somewhat described using Wave Mechanics, but only fully understood using Quantum Mechanics.
And since many of us....well, I...can't even pretend to understand Quantum Mechanics I tend to stick with Wave mechanics. It gets me by.

Chris Peterson wrote:

MattF wrote:

Chris Peterson wrote:All diffraction is a quantum effect. That is, while it can be approximately described by classical theory, in certain regimes it is best explained using QM. A rigorous analysis of Mie scattering and the color effects seen in a lunar corona is best handled with QM. I think it is slightly misleading to say that this effect is "purely" quantum mechanical, however.

I'm sorry, but saying "all diffraction os a quantum effect" is like saying "everything is a quantum effect." It's not false, but it's not helpful, either.

It's also not true. Not everything is a quantum effect. But diffraction is. Diffraction is only fully understood by utilizing QM. Classical theory is only an approximation. It's a very good approximation in many cases (but not so good in the case of diffraction by small particles), but an approximation all the same. This is analogous to the way that gravity can only be fully described using General Relativity, even though classical (Newtonian) theory is simpler and often serves analysis.

I have no complaint with the caption describing this effect as the product of quantum mechanical diffraction, only with the use of the word "purely", which is somewhat confusing.

Okay, let me try again (and thanks to everyone, especially Chris, who's replied so far):

What is observable in the image that can't be explained using classical wave ideas? That's what I really meant to ask but I guess it wasn't very clearly stated. The person who wrote the caption seemed to think it an important point to make. Chris, you mentioned color effects - which ones in particular? Thank you

Chris Peterson wrote: [...] Not everything is a quantum effect. But diffraction is. Diffraction is only fully understood by utilizing QM. Classical theory is only an approximation. It's a very good approximation in many cases (but not so good in the case of diffraction by small particles), but an approximation all the same. [...]
I have no complaint with the caption describing this effect as the product of quantum mechanical diffraction, only with the use of the word "purely", which is somewhat confusing.

Yes, the interaction with light and matter is, when you get down to it, of quantum mechanical nature. But I don't understand why you need to employ QM to describe diffraction. You need QM if you want to calculate material properties that are important to diffraction, such as the polarisability of the atoms. Once you take these properties as given, you can employ classical scattering theory.

Also, classical Rayleigh scattering applies when the scatterer is much smaller than the wavelength in question. This is why it successfully describes the blue sky, etc, where the wavelength of visible light is a few hundert nanometers compared to the size of the air molecules (~0.1 nm). Only when the wavelength becomes comparable to the atom radius (e.g. x-rays) do you need QM.

The works by Rayleigh were published in the 1870, well before QM, and those of Mie around 1910 (most likely still classical, but I didn't look up the references).

Chris Peterson wrote:All diffraction is a quantum effect. That is, while it can be approximately described by classical theory, in certain regimes it is best explained using QM. A rigorous analysis of Mie scattering and the color effects seen in a lunar corona is best handled with QM. I think it is slightly misleading to say that this effect is "purely" quantum mechanical, however.

I looked up Mie scattering on Wikipedia and in Hecht. In Mie scattering, the cross section is independent of the wavelength, while today's APOD shows an effect which is wavelength dependent. My guess is that it is related to Rayleigh scattering (which, of course, is a limiting case of Mie scattering). Do you know a reference that discusses the lunar corona with QM?

Chris Peterson wrote: I think it is slightly misleading to say that this effect is "purely" quantum mechanical, however.

OK. Thanks Chris. Yes. I have now removed "purely" from the APOD text.

- RJN

Markus Schwarz wrote:Do you know a reference that discusses the lunar corona with QM?

No. I'll mention, however, that in a previous life I designed nephelometers used for performing immunological assays- watching the rate characteristics of scattered light in solution as antibodies and antigens reacted. Some of these instruments monitored the scatter at two different wavelengths. I worked out the Mie scattering involved from QM first principles. Classical theory was inconsistent with observation except when a narrow wavelength source was used (and the instrument used a white light source).

Nice shot Sergio, thanks

Fascinating discussion of QM and so on, but none of the orange ring inside the purple one!

Double rainbows are allegedly due to double reflection inside the raindrops, with the secondary bow clearly separated by 9 degrees from the primary, with the colour order reversed.
http://www.atoptics.co.uk/rainbows/sec.htm
Red should face red, if the inner ring is part of some secondary effect, not purple face orange.

Can anyone explain this, or is it an artifact?
John

JohnD wrote:Fascinating discussion of QM and so on, but none of the orange ring inside the purple one!

Double rainbows are allegedly due to double reflection inside the raindrops, with the secondary bow clearly separated by 9 degrees from the primary, with the colour order reversed.

This is not a rainbow. It isn't caused by refraction, dispersion, and internal reflection, but by diffraction, a completely different process. So there's no reason to think the structure should be similar.

(If I have time later, and nobody jumps in first, I'll try to devise a simple explanation for how diffraction describes what we're seeing. It's not complicated, but it's not easy to put into words, either.)

I believe I can make out 3 sequences from bluish to reddish-orange as you work out from the center of the image. I wonder if with color enhancement of the image one can see any more. Also, incidentally, is that small white dot in the image (at an orientation of about 260 degrees using standard polar coordinates with the Moon at the origin) a star ? And I too, am wondering about the blur at the edge of the Moon in the image.

Wonderful and instructive picture, today! I'm looking forward to Chris' further description.

https://en.wikipedia.org/wiki/Airy_disk wrote:

[b][size=150]Airy pattern for circular obstruction[/size][/b]

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<<In optics, the Airy pattern is the diffraction pattern resulting from a uniformly-illuminated circular [obstruction].

The angle at which the first [Airy pattern] minimum occurs, measured from the direction of incoming light, is given by the approximate formula:



where θ is in radians, λ is the wavelength of the light
and d is the diameter of the [water droplets].>>

The APOD is essentially a superposition of different colored Airy patterns with the central peak 'Airy disk' (along with the direct moon image) optically (or digitally?) reduced in intensity.

With the first [green Airy pattern] minimum at ~ 2.2 lunar diameters
(i.e., the first [green Airy pattern] maximum at ~ 3 lunar diameters)
the water droplets are ~32 microns in diameter using the formula.

I would call this something else....because The Sun's Corona...is Different....not the Diffraction through thin clouds... This is a Lunar AURA....or Solar Aura....as apposed to the extension of the Sun's surrounding plasma....which is the Sun's "atmosphere"...as seen in Solar Eclipses...

This is an Earth Atmospheric Effect...and we need a different term... But Rainbow...does not quite work as it is not really from a RAIN...but is from the moisture of the clouds, (and Moisturebow, is too weird)... so it could fit..."Lunar Rainbow". BUT AS CHRIS POINTED OUT...NOT THE SAME PROCESS....THUS... We could call it a Lunar SPECTRUM....generally if it were out further, we would call it a HALO... a Lunar Halo...but in close we call it something else....sooooooo...any suggestions?

Personally "Lunar Aura" should be fine, as it is basically used in the description...and even the definition of "Corona"...

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