APOD: Quadruple Lunar Halo Over Winter Road (2022 Jan 02)

[APOD Robot]

Quadruple Lunar Halo Over Winter Road

Explanation: Sometimes falling ice crystals make the atmosphere into a giant lens causing arcs and halos to appear around the Sun or Moon. One Saturday night in 2012 was just such a time near Madrid, Spain, where a winter sky displayed not only a bright Moon but four rare lunar halos. The brightest object, near the top of the featured image, is the Moon. Light from the Moon refracts through tumbling hexagonal ice crystals into a somewhat rare 22-degree halo seen surrounding the Moon. Elongating the 22-degree arc horizontally is a more rare circumscribed halo caused by column ice crystals. Even more rare, some moonlight refracts through more distant tumbling ice crystals to form a (third) rainbow-like arc 46 degrees from the Moon and appearing here just above a picturesque winter landscape. Furthermore, part of a whole 46-degree circular halo is also visible, so that an extremely rare -- especially for the Moon -- quadruple halo was captured. Far in the background is a famous winter skyscape that includes Sirius, the belt of Orion, and Betelgeuse -- visible between the inner and outer arcs. Halos and arcs typically last for minutes to hours, so if you do see one there should be time to invite family, friends or neighbors to share your unusual lensed vista of the sky.

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[Ann]

Lovely image. The night sky is so stark and amazing. Note that the moonlit sky is blue, for the same reason that the clear daytime sky is blue: Shortwave (blue and violet) light from the Sun (at night, shortwave light from the Sun reflected by the Moon) scatter in the atmosphere through Rayleigh scattering.

Blue spherical planetary nebula Abell 39. Photo: Adam Block.

---

Note the circular blue splotch in the middle of Orion. You'd almost think it was a planetary nebula on the lam! But I guess it's just reflections in the camera of the brilliant moonlight!

Ann

[JohnD]

I can only see a circular halo all around the Moon, and a section of a more distant halo (46 degree?) between that and the horizon, so two (remarkable enough!) not four. Is the reproduction on the PC screen less than it might be in the original?
John

[Locutus76]

I really don’t see the distinction between the 4 phenomena either, aside from the 22 and 46 degree rings. The explanation seems to be pointing at different parts of the same circles and giving them different names. Also the streaks on the right of the 22 degree circle might just be thin clouds.

[Ann]

This is what I can see, and I'm not sure about the "fluff" I think I see to the right...

Photo: Dani Caxete

---

What do the rest of you think?

Ann

[johngauvreau]

The question of where the extra arcs or rings are is exactly why I came on the discussion board; I was hoping someone here explained it better than the usually excellent caption provided.
I just see two and some cloud. In fact, on the flicker page of the photographer even he lists it as a double halo. And he has another image in the sequence that shows the same two and shows the cloud has moved, making it clear it is not part of the halo complex.
Regardless, a lovely image and a confusing caption in no way diminishes the quality of this very fine image.

[RichC]

Click on “quadruple halo” link for an annotates version of the image.

[orin stepanek]

I can't see that the infralateral arc isn't part or the 46degree arc! Is
a nice photo!

Kitty wants to know; "what was That?"

[futurejohn]

Click on “quadruple halo” link for an annotates version of the image.

Thanks, I didn't realize that they included an annotated version.

It looks like the 22° and 46° halos are the only ones there. Why is there some distinction made for the circumscribed halo and infralateral arc?

[Chris Peterson]

Click on “quadruple halo” link for an annotates version of the image.

Thanks, I didn't realize that they included an annotated version.

It looks like the 22° and 46° halos are the only ones there. Why is there some distinction made for the circumscribed halo and infralateral arc?

The infralateral arc is revealed by its colors. I'd say it is unambiguous. I'm a bit skeptical of the circumscribed halo being real, as opposed to just being scatter from what appears to be a thin cloud band passing through that area of the 22° halo.

[geoffrey.landis]

Yeah, I'm having a lot of problems seeing four halos in this image. The text "a (third) rainbow-like arc 46 degrees from the Moon" followed by "furthermore, part of a whole 46-degree circular halo is also visible" seems to count the 46 degree halo twice, is it implying that there are two halos exactly in the same location, 46 degrees from the moon?
And I don't see a circumscribed halo.

[JohnD]

I'm glad it's not just me! To whom it seems that a "circumscribed halo" is merely a most intense, 22 degree halo. Likewise the 46 degree and infralateral arc. So intense that we can see the colours, as in a rainbow.
That we don't see colours in such 'bows' from moonlight must be due to the much lesser intensity of the light. Even under a full Moon (of whatever romantic name!) Earthly objects lose all their colour, because our retinal cones are not sensitive enough.

So this shows two halos, but so unusually intense that the colours are seen? .

John

[Chris Peterson]

I'm glad it's not just me! To whom it seems that a "circumscribed halo" is merely a most intense, 22 degree halo. Likewise the 46 degree and infralateral arc. So intense that we can see the colours, as in a rainbow.
That we don't see colours in such 'bows' from moonlight must be due to the much lesser intensity of the light. Even under a full Moon (of whatever romantic name!) Earthly objects lose all their colour, because our retinal cones are not sensitive enough.

So this shows two halos, but so unusually intense that the colours are seen? .

We don't see color in the halos because it largely isn't there. Not because of our insensitivity to color in low light. They aren't present in the identical daytime halos, either. The Moon is plenty bright enough to stimulate our color vision when it comes to coronas and other ice structures that actually produce color.

The infralateral arc is tangent to the 46° and has a different radius (which is not fixed). It is not a more intense part of the 46° arc. Its presence here is revealed by intense color at the tangent point.

[johnnydeep]

I'm glad it's not just me! To whom it seems that a "circumscribed halo" is merely a most intense, 22 degree halo. Likewise the 46 degree and infralateral arc. So intense that we can see the colours, as in a rainbow.
That we don't see colours in such 'bows' from moonlight must be due to the much lesser intensity of the light. Even under a full Moon (of whatever romantic name!) Earthly objects lose all their colour, because our retinal cones are not sensitive enough.

So this shows two halos, but so unusually intense that the colours are seen? .

We don't see color in the halos because it largely isn't there. Not because of our insensitivity to color in low light. They aren't present in the identical daytime halos, either. The Moon is plenty bright enough to stimulate our color vision when it comes to coronas and other ice structures that actually produce color.

The infralateral arc is tangent to the 46° and has a different radius (which is not fixed). It is not a more intense part of the 46° arc. Its presence here is revealed by intense color at the tangent point.

Like others here, I'm having a hard time distinguishing the infralateral arc from the 46° halo. Is the infralateral arc flatter where it meets the 46° arc at the bottom? And, yes, it does look more rainbow-like at the bottom.

And is the visual difference between the innermost 22° halo and the circumscribed halo evident in this APOD by the former being sharper and rainbow-like and the latter being more diffuse and almost monochrome white?

[JohnD]

Chris,
I beg to differ, from personal experience and with the agreement of NASA!
See: https://www.nasa.gov/vision/universe/wa ... light.html

They cite Said & Patternaik, J. Vision http://citeseerx.ist.psu.edu/viewdoc/do ... 1&type=pdf "The rod cells are highly sensitive to light. Rods work best in dim light less than 10-1cd/m2. They provide achromatic vision in dark conditions known as scotopic vision. The cone cells on the other hand are less sensitive than the rod cells. They are active in dim to bright light (10-1 to 106 cd/m2)."

Trying to match that with known light intensities, I read that bright moonlight provides less than1 lux of light intensity (compare 120,000 lux in bright sunlight). Converting that to the "candelas per square meter" used by Said & Patternaik escapes me!

Hoever, haloes are produced by internal reflection in the ice crystals, thus producing the typical 22 degree ring. Refraction will also occur, at the air/ice boundary, seperating the colours in sunlight, as in rain drops. So with respect, Chris, to say that "colour isn't there" in moonlight is wrong, and we need an explanation for seeing colourless moon haloes. This site ascribes a diferent mechanism to the appearance of 'coronas', and the Met Office should know! https://www.metoffice.gov.uk/weather/le ... nd-coronas

I also note that the index of refraction for water ice (1.31) is slightly less than for liquid water ( 1.33) I am quite unable to put that into maths, but presume that ice will refract sunlight slightly less, so producing a lesser seperation of colours.

John

[Ann]

Click on “quadruple halo” link for an annotates version of the image.

Thanks, I didn't realize that they included an annotated version.

It looks like the 22° and 46° halos are the only ones there. Why is there some distinction made for the circumscribed halo and infralateral arc?

The infralateral arc is revealed by its colors. I'd say it is unambiguous. I'm a bit skeptical of the circumscribed halo being real, as opposed to just being scatter from what appears to be a thin cloud band passing through that area of the 22° halo.

I'm not sure I get that, Chris, but I do know that I myself have seen very colorful lunar coronas and halos (not quite sure what the difference is between a corona and a halo). Of course, I have never seen anything as colorful and complex as the lunar corona and halo seen over Manitoba, Canada, in February 2020:

Complex and colorful lunar halo. Photo: Brent Mckean.

---

I apologize for quoting the full NASA caption:

NASA Science wrote:

Yes, but could you get to work on time if the Moon looked like this? As the photographer was preparing to drive to work, refraction, reflection, and even diffraction of moonlight from millions of falling ice crystals turned the familiar icon of our Moon into a menagerie of other-worldly halos and arcs. The featured scene was captured with three combined exposures two weeks ago on a cold winter morning in Manitoba, Canada. The colorful rings are a corona caused by quantum diffraction by small drops of water or ice near the direction of the Moon. Outside of that, a 22-degree halo was created by moonlight refracting through six-sided cylindrical ice crystals. To the sides are moon dogs, caused by light refracting through thin, flat, six-sided ice platelets as they flittered toward the ground. Visible at the top and bottom of the 22-degree halo are upper and lower tangent arcs, created by moonlight refracting through nearly horizontal hexagonal ice cylinders. A few minutes later, from a field just off the road to work, the halo and arcs had disappeared, the sky had returned to normal -- with the exception of a single faint moon dog.

Image Credit & Copyright: Brent Mckean

Ann

[Chris Peterson]

Chris,
I beg to differ, from personal experience and with the agreement of NASA!
See: https://www.nasa.gov/vision/universe/wa ... light.html

They cite Said & Patternaik, J. Vision http://citeseerx.ist.psu.edu/viewdoc/do ... 1&type=pdf "The rod cells are highly sensitive to light. Rods work best in dim light less than 10-1cd/m2. They provide achromatic vision in dark conditions known as scotopic vision. The cone cells on the other hand are less sensitive than the rod cells. They are active in dim to bright light (10-1 to 106 cd/m2)."

Trying to match that with known light intensities, I read that bright moonlight provides less than1 lux of light intensity (compare 120,000 lux in bright sunlight). Converting that to the "candelas per square meter" used by Said & Patternaik escapes me!

Hoever, haloes are produced by internal reflection in the ice crystals, thus producing the typical 22 degree ring. Refraction will also occur, at the air/ice boundary, seperating the colours in sunlight, as in rain drops. So with respect, Chris, to say that "colour isn't there" in moonlight is wrong, and we need an explanation for seeing colourless moon haloes. This site ascribes a diferent mechanism to the appearance of 'coronas', and the Met Office should know! https://www.metoffice.gov.uk/weather/le ... nd-coronas

I also note that the index of refraction for water ice (1.31) is slightly less than for liquid water ( 1.33) I am quite unable to put that into maths, but presume that ice will refract sunlight slightly less, so producing a lesser seperation of colours.

John

Do not confuse the ability to see color by moonlight with the ability to see color in ice structures around the Moon. They are very different things. We easily see color in coronas and various ice halos, and even in moonlit rainbows.

My point was that neither the 22° or 46° halos have much color. They are pale white, both in moonlight and in sunlight. So the color is not there. It is there, however, in the infralateral arc... which is how we know that's what we're seeing here.

Not sure about your comparison between water and ice. Whether illuminated by the Sun or the Moon, all of these features are caused by ice crystals, not water droplets. (Except for coronas, which are caused by water, day or night.)

Nothing in the Met Office information contradicts what I said.

[Chris Peterson]

Thanks, I didn't realize that they included an annotated version.

It looks like the 22° and 46° halos are the only ones there. Why is there some distinction made for the circumscribed halo and infralateral arc?

The infralateral arc is revealed by its colors. I'd say it is unambiguous. I'm a bit skeptical of the circumscribed halo being real, as opposed to just being scatter from what appears to be a thin cloud band passing through that area of the 22° halo.

I'm not sure I get that, Chris, but I do know that I myself have seen very colorful lunar coronas and halos (not quite sure what the difference is between a corona and a halo).

Coronas are caused by water droplets, and show considerably more color than the usual 22° ice halo.

[neufer]

Click on “quadruple halo” link for an annotates version of the image.

Thanks, I didn't realize that they included an annotated version.

It looks like the 22° and 46° halos are the only ones there.

Why is there some distinction made for the circumscribed halo and infralateral arc?

The infralateral arc is revealed by its colors. I'd say it is unambiguous. I'm a bit skeptical of the circumscribed halo being real, as opposed to just being scatter from what appears to be a thin cloud band passing through that area of the 22° halo.

The Madrid winter solstice moon should be ~64° high and is a good match for this 63° high solar version:

• https://www.atoptics.co.uk/halo/supim3.htm

It looks to me like the 22° & corresponding oval circumscribed halos are there along with the parabolic infralateral arc.

(Less clear about a rare circular 46° halo.)

[JohnD]

Ann's exxtraordinary photograph demonstrates that coronas may be coloured. Coronas are produced by diffraction around water droplets, which unlike crystals may be of various sizes and so diffract light to a varied extent, leading to the multiple bands seen in her pic.

Moreover, you are, of course, correct that moonlight rainbows are coloured. So, I abandon my theory and must agree with you that it is not human vision that makes a halo colourless. But why is that so? The tiny difference in refractive index between water and ice would (I think, correct me again, someone!) make ice seperate wavelengths less, but is that enough so that we see haloes as white?

[Chris Peterson]

Ann's exxtraordinary photograph demonstrates that coronas may be coloured. Coronas are produced by diffraction around water droplets, which unlike crystals may be of various sizes and so diffract light to a varied extent, leading to the multiple bands seen in her pic.

Moreover, you are, of course, correct that moonlight rainbows are coloured. So, I abandon my theory and must agree with you that it is not human vision that makes a halo colourless. But why is that so? The tiny difference in refractive index between water and ice would (I think, correct me again, someone!) make ice seperate wavelengths less, but is that enough so that we see haloes as white?

Most (maybe all?) halos are caused by either refraction or reflection from ice crystals. Neither of those phenomena, by themselves, results in color. They simply change the angle of an incoming beam of light. For refractive effects, what matters is dispersion. Dispersion is a measure of how the index of refraction changes with wavelength. Red light is refracted through a different angle than violet light. So we get a separation of colors. But unless the ice crystals are all aligned exactly the same way, those separated colors will all point in slightly different directions... essentially recombining again to form white light. So there is little or no color... except possibly at the edges. (This doesn't happen with refractive water effects, like rainbows, because water droplets are spherical, so their orientation doesn't matter- quite unlike ice crystals.)

Color produced by diffraction, where particle sizes and spacing is on the order of the wavelength of light, are much more complex.

[neufer]

Most (maybe all?) halos are caused by either refraction or reflection from ice crystals. Neither of those phenomena, by themselves, results in color. They simply change the angle of an incoming beam of light. For refractive effects, what matters is dispersion. Dispersion is a measure of how the index of refraction changes with wavelength. Red light is refracted through a different angle than violet light. So we get a separation of colors.

But unless the ice crystals are all aligned exactly the same way, those separated colors will all point in slightly different directions... essentially recombining again to form white light. So there is little or no color... except possibly at the edges. (This doesn't happen with refractive water effects, like rainbows, because water droplets are spherical, so their orientation doesn't matter- quite unlike ice crystals.)

One only sees colors due to those ice crystal (or water droplet) refractions that direct those sunlight/moonlight colors into ones eyes.

It is rather the lack of a proper orientation (or a proper type of ice crystal) that prevents colors from appearing (assuming it is bright enough and the colors well enough dispersed in the first place).

Alternatively, any refraction colors could be swamped by simple reflections as in the case of sun/moon pillars or parhelic circles.