APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

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APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by APOD Robot » Mon Feb 01, 2021 5:13 am

Image Lunar Halo over Snowy Trees

Explanation: Have you ever seen a halo around the Moon? This fairly common sight occurs when high thin clouds containing millions of tiny ice crystals cover much of the sky. Each ice crystal acts like a miniature lens. Because most of the crystals have a similar elongated hexagonal shape, light entering one crystal face and exiting through the opposing face refracts 22 degrees, which corresponds to the radius of the Moon Halo. A similar Sun Halo may be visible during the day. Exactly how ice-crystals form in clouds remains a topic of research. In the featured image taken last week from Östersund, Sweden, a complete lunar halo was captured over snowy trees and rabbit tracks.

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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by Chris Peterson » Mon Feb 01, 2021 5:16 am

APOD Robot wrote:
Mon Feb 01, 2021 5:13 am
Each ice crystal acts like a miniature lens.
Lenses have curved surfaces. It would be more accurate to say that each ice crystal acts like a miniature prism.

(I'm surprised such a bright halo doesn't show any hint of moondogs.)
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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by hanche » Mon Feb 01, 2021 8:24 am

Lovely picture. But this being Sweden, I think the tracks are from a hare, not a rabbit. We don't have rabbits in the wild in Scandinavia (I live in Norway myself.)

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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by VictorBorun » Mon Feb 01, 2021 8:25 am

In the pic the hole in the halo ring is 12 times the disk. Therefore the radius of the ring is about 6°.
But they say the angle of deviation after entering a face of hexagon ice prism and exiting the next to the next face is 21.54° (red) to 22.37° (blue) at minimum and 50° at maximum.
I think there is a near halo that saturates the sensor's pixels so much that we see a large white disk.

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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by hanche » Mon Feb 01, 2021 8:34 am

Just a quick correction to my statement that we don't have rabbits in Scandinavia (I should have said the Scandinavian peninsula, i.e., Sweden and Norway): We do, but only in the far south, mostly in southern Sweden. But Östersund is about the same latitude as Trondheim, where I live. There definitely aren't any rabbits around here. But hares are plentiful.

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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by Ann » Mon Feb 01, 2021 9:05 am

hanche wrote:
Mon Feb 01, 2021 8:34 am
Just a quick correction to my statement that we don't have rabbits in Scandinavia (I should have said the Scandinavian peninsula, i.e., Sweden and Norway): We do, but only in the far south, mostly in southern Sweden. But Östersund is about the same latitude as Trondheim, where I live. There definitely aren't any rabbits around here. But hares are plentiful.
There are indeed rabbits in southern Sweden. I live in southernmost Sweden, and I see rabbits every day, often on the lawn outside our condominium.

17th century painting of Stockholm, a copy of the so called Vädersolstavlan, depicting a halo display event in 1535. Cleaned in 1998. The visible haloes are: 22 ° halo, at upper right (should be centered on the Sun) parhelic circle, large white circle (centered on the zenith: appears 'horizontal' in the sky) parhelia including 2 sundogs, 2 120° parhelia and the anthelion (dots on the parhelic circle, resp. nearest to farthest from the Sun) upper tangent arc and possible Parry arc (2 crossing arcs just left of the 22° halo (actually 'above' the 22°, in the sky); not realistically shown) circumzenithal arc, smaller crescent inside the parhelic circle (also centered on the zenith: appears 'horizontal', high in the sky) infralateral arc (bottom right) Note that the whole sky appears strongly tilted in the image: the upper right corner is actually down in the sky (when looking towards the Sun), the zenith is at the center of the circumzenithal arc and parhelic circle. This may result from the artist's choice to represent the display in a realistic orientation relative to the landscape: in this case the sun would have shone from 3/4 back to the right of an observer facing the city. The relative brightnesses of the haloes are quite accurate. Painting by Jacob Heinrich Elbfas


As for the lunar halo over Östersund, Sweden, it's the Stockholm people who decide what is trendy in in this country. The people in our capital have no time for a lunar halo in Östersund, but they have been agog recently over a solar halo in Stockholm.

And there was a really nice halo display over Stockholm in 1535, you've got to give them that.

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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by heehaw » Mon Feb 01, 2021 10:52 am

MUCH better than the wimpy Lunar Halo in today's ESPOD!

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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by De58te » Mon Feb 01, 2021 11:29 am

Hmm, I wonder why if those ice crystals all form the similar identical shape for this phenomenon to happen, then why do the closely related cousin, the snowflake, all form different shapes? It is said no two snowflakes are identical.

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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by Tszabeau » Mon Feb 01, 2021 11:56 am

Assuming it was a not a rabbit... I wonder what split that hare.

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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by Chris Peterson » Mon Feb 01, 2021 2:15 pm

De58te wrote:
Mon Feb 01, 2021 11:29 am
Hmm, I wonder why if those ice crystals all form the similar identical shape for this phenomenon to happen, then why do the closely related cousin, the snowflake, all form different shapes? It is said no two snowflakes are identical.
Snowflakes are not "pure" ice crystals. They are amalgamations of ice crystals, or crystals that have grown through processes like twinning where growth axes change during development.
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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by Chris Peterson » Mon Feb 01, 2021 2:19 pm

VictorBorun wrote:
Mon Feb 01, 2021 8:25 am
In the pic the hole in the halo ring is 12 times the disk. Therefore the radius of the ring is about 6°.
But they say the angle of deviation after entering a face of hexagon ice prism and exiting the next to the next face is 21.54° (red) to 22.37° (blue) at minimum and 50° at maximum.
I think there is a near halo that saturates the sensor's pixels so much that we see a large white disk.
The radius of the halo is 22°. The Moon is simply overexposed, such that diffraction and scatter blow it out on the sensor. That is, the lunar disc is not visible at all. This does not require any sort of inner halo. All single exposures that show dim structures like halos or stars will fail to show the Moon at its true size.
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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by neufer » Mon Feb 01, 2021 3:30 pm

hanche wrote:
Mon Feb 01, 2021 8:24 am

Lovely picture. But this being Sweden, I think the tracks are from a hare, not a rabbit.

We don't have rabbits in the wild in Scandinavia (I live in Norway myself.
The divergent tree shadows indicate that the lovely picture was faked on a sound studio in Hollywood.
https://www.spacecentre.nz/resources/faq/spaceflight/moon-hoax/shadows-parallel.html wrote:
Why aren't shadows parallel in the Moon photos?

<<Some photographs taken during the Moon landings show shadows that are converging (as highlighted here by red lines).

Conspiracy theorists argue that, since light rays from the sun are parallel, any shadows cast by the Sun should also be parallel. Shadows that are not parallel are said to be caused by studio lighting, either by a light source which is relatively close and/or by multiple light sources.>>
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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by orin stepanek » Mon Feb 01, 2021 3:38 pm

Wow! :D The halo is fantastic; I love it; so are the rabbit tracks (hare); the snow and the trees! Beautiful! 🌲 🌲 And the Bunny is the cutest! I looked up hare vs rabbit! we always called them Jack Rabbits! :D

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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by johnnydeep » Mon Feb 01, 2021 4:26 pm

My questions are in which direction were the two hares/rabbits going - to the trees or from them? And how close in time were their paths?

EDIT: actually I can now see that one path was from one of the trees and one path was toward the other one. And I guess it could even have been the same rabbit/hare!
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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by orin stepanek » Mon Feb 01, 2021 5:25 pm

johnnydeep wrote:
Mon Feb 01, 2021 4:26 pm
My questions are in which direction were the two hares/rabbits going - to the trees or from them? And how close in time were their paths?

EDIT: actually I can now see that one path was from one of the trees and one path was toward the other one. And I guess it could even have been the same rabbit/hare!
Big feet probably a Jackrabbit--Hare went to trees on right & came back from trees on Left! Or it could be different ones! :wink:
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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by neufer » Mon Feb 01, 2021 6:54 pm

orin stepanek wrote:
Mon Feb 01, 2021 5:25 pm
johnnydeep wrote:
Mon Feb 01, 2021 4:26 pm

My questions are in which direction were the two hares/rabbits going - to the trees or from them? And how close in time were their paths?

EDIT: actually I can now see that one path was from one of the trees and one path was toward the other one. And I guess it could even have been the same rabbit/hare!
Big feet probably a Jackrabbit--Hare went to trees on right & came back from trees on Left! Or it could be different ones! :wink:
https://en.wikipedia.org/wiki/Lotka%E2%80%93Volterra_equations wrote:
<<The Lotka–Volterra equations, also known as the predator–prey equations, are a pair of first-order nonlinear differential equations, frequently used to describe the dynamics of biological systems in which two species interact, one as a predator and the other as prey. The populations change through time according to the pair of equations:


where:
  • x is the number of prey (for example, rabbits);
    y is the number of some predator (for example, foxes);
    t represents time;
    α, β, γ, δ are positive real parameters describing the interaction of the two species.
The Lotka–Volterra system of equations is an example of a Kolmogorov model, which is a more general framework that can model the dynamics of ecological systems with predator–prey interactions, competition, disease, and mutualism. The Lotka–Volterra models have been used to explain the dynamics of natural populations of predators and prey, such as the lynx and snowshoe hare data of the Hudson's Bay Company and the moose and wolf populations in Isle Royale National Park.>>
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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by johnnydeep » Mon Feb 01, 2021 8:54 pm

neufer wrote:
Mon Feb 01, 2021 6:54 pm
orin stepanek wrote:
Mon Feb 01, 2021 5:25 pm
johnnydeep wrote:
Mon Feb 01, 2021 4:26 pm

My questions are in which direction were the two hares/rabbits going - to the trees or from them? And how close in time were their paths?

EDIT: actually I can now see that one path was from one of the trees and one path was toward the other one. And I guess it could even have been the same rabbit/hare!
Big feet probably a Jackrabbit--Hare went to trees on right & came back from trees on Left! Or it could be different ones! :wink:
https://en.wikipedia.org/wiki/Lotka%E2%80%93Volterra_equations wrote:
<<The Lotka–Volterra equations, also known as the predator–prey equations, are a pair of first-order nonlinear differential equations, frequently used to describe the dynamics of biological systems in which two species interact, one as a predator and the other as prey. The populations change through time according to the pair of equations:


where:
  • x is the number of prey (for example, rabbits);
    y is the number of some predator (for example, foxes);
    t represents time;
    α, β, γ, δ are positive real parameters describing the interaction of the two species.
The Lotka–Volterra system of equations is an example of a Kolmogorov model, which is a more general framework that can model the dynamics of ecological systems with predator–prey interactions, competition, disease, and mutualism. The Lotka–Volterra models have been used to explain the dynamics of natural populations of predators and prey, such as the lynx and snowshoe hare data of the Hudson's Bay Company and the moose and wolf populations in Isle Royale National Park.>>
Thanks, neufer. I also found this pic from https://www.wildernessawareness.org/art ... -and-sign/, which shows the direction of travel:
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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by drjhammond » Mon Feb 01, 2021 9:51 pm

"Sun Dogs" or parhelia are produced along with halos when the conditions produce ice crystals that orient themselves to the horizontal due to gravity. As usually seen, sun dogs are usually directly horizontal from the sun, when the sun is low in the sky. When they are observed higher in the sky, they are produced outside the 22 deg halo. Best place to learn about these phenomena is here: https://www.atoptics.co.uk/halosim.htm Another commenter mentioned the fact that the ~22 deg is the angle of minimum deviation that varies with wavelength thus producing color in the halo (not a "rainbow") and the difference in sky brightness between inside and outside the halo.

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Re: APOD: Lunar Halo over Snowy Trees (2021 Feb 01)

Post by orin stepanek » Mon Feb 01, 2021 11:16 pm

I think today's thread got a little hare---y! :mrgreen:
Orin

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