APOD: A Halo Around the Moon (2012 Nov 20)

Comments and questions about the APOD on the main view screen.
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APOD: A Halo Around the Moon (2012 Nov 20)

Post by APOD Robot » Tue Nov 20, 2012 5:06 am

Image A Halo Around the Moon

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. The setting of the above picture is Athens San Sebastian, Greece. The distant planet Jupiter appears by chance on the halo's left. Exactly how ice-crystals form in clouds remains under investigation.

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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by stowaway » Tue Nov 20, 2012 7:26 am

How many millions of ice crystals can you count? Don't be tellin' me this picture is fake.

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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by 2birds » Tue Nov 20, 2012 8:21 am

Thanks for the explanation! I think this picture is beautiful - how come you think that it's a fake? I've seen a couple of these before...

Steve Case

Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by Steve Case » Tue Nov 20, 2012 8:33 am

Couldn't our photographer aimed the camera up just a little bit?
I'm reminded of people who cut the heads off in their snapshots.

Phil P.

Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by Phil P. » Tue Nov 20, 2012 1:01 pm

I've seen the 22 degree halo around the sun accompanied by "sun dogs" or mock suns that appear on the halo. The optics of our atmosphere can be pretty interesting!

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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by BDanielMayfield » Tue Nov 20, 2012 1:04 pm

Don't be tellin' me this picture is fake.
how come you think that it's a fake?
No one is saying THIS apod is a fake 2birds. Stowaway is referring to yesterday’s flaming arrow attack on John Wayne and the cavalry.
Just as zero is not equal to infinity, everything coming from nothing is illogical.

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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by FloridaMike » Tue Nov 20, 2012 1:16 pm

Did the photographer have rain by noon?
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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by Ayiomamitis » Tue Nov 20, 2012 1:51 pm

Steve Case wrote:Couldn't our photographer aimed the camera up just a little bit?
I'm reminded of people who cut the heads off in their snapshots.
The moon was around 60 degrees altitude when the image was taken. Now also add lots more degrees for the halo itself and you are basically shooting overhead. To add insult to injury, one needs a multi-second exposure to capture the halo itself.

It took me about ten minutes to properly orient the tripod and camera so as to capture this image with the camera virtually pointing at zenith. The back two legs of the tripod where at the minimum possible length the tripod allows whereas the third leg pointing ahead was at its maximum extended position out. I also placed some pieces of rock for the front leg to sit upon so as to tilt the tripod overhead as much as possible. The Bogen tripod head was also inclined as far back as possible and at its maximum position until it came into contact with the tripod.

Once the proper angle has been finally acquired, I had to make sure the tripod with camera were absolutely still so that the six-second exposure could be taken without movement. After each attempted exposure, I had to tilt forward all of the gear so as to look at the result on the LCD screen to make sure both the framing and exposure were proper. I was not as much worried for the exposure itself since I have taken similar images before and had a good idea of a starting exposure (ex. http://www.perseus.gr/Astro-Lunar-Scenes-Halo.htm ).

Personally, I am very relieved I pulled this image off (vis a vis the above) given the extreme angle of the halo itself. :wink:

Anthony.
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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by Ayiomamitis » Tue Nov 20, 2012 1:52 pm

Phil P. wrote:I've seen the 22 degree halo around the sun accompanied by "sun dogs" or mock suns that appear on the halo. The optics of our atmosphere can be pretty interesting!
I have been after a halo around the Sun for some time and it has managed to elude me so far.

Patience as usual! :)

Anthony.
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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by Ayiomamitis » Tue Nov 20, 2012 2:15 pm

Ayiomamitis wrote:
Steve Case wrote:Couldn't our photographer aimed the camera up just a little bit?
I'm reminded of people who cut the heads off in their snapshots.
The moon was around 60 degrees altitude when the image was taken. Now also add lots more degrees for the halo itself and you are basically shooting overhead. To add insult to injury, one needs a multi-second exposure to capture the halo itself.
I just checked and the moon was at 69.7 degrees altitude .... add the halo itself and we have a target which is effectively overhead.

As noted earlier, taking this six-second exposure was much more difficult than people may realize.

Anthony.
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MikeO

Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by MikeO » Tue Nov 20, 2012 2:21 pm

I must be missing a parameter. I cannot calculate the diameter of the halo around the moon from just the diffraction of light in an ice crystal. Perhaps the missing parameter is the thickness of the layer of clouds or ice crystals. Perhaps it has something to do with the altitude of the clouds. The photo was taken from Athens. The same halo centered on the moon was probably also visible from Thessaloniki and from Heraklion. It was a great photo. I would like to understand a little more about why it appears the way it does. A link to a technical reference would be most appreciated.

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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by Chris Peterson » Tue Nov 20, 2012 3:22 pm

MikeO wrote:I must be missing a parameter. I cannot calculate the diameter of the halo around the moon from just the diffraction of light in an ice crystal. Perhaps the missing parameter is the thickness of the layer of clouds or ice crystals. Perhaps it has something to do with the altitude of the clouds. The photo was taken from Athens. The same halo centered on the moon was probably also visible from Thessaloniki and from Heraklion. It was a great photo. I would like to understand a little more about why it appears the way it does. A link to a technical reference would be most appreciated.
This type of halo is created by refraction of light in ice crystals, not by diffraction. The thickness of the ice layer affects the brightness of the halo, but not its geometry. In a sense, a halo is completely unique to each observer- move to another location and the light is traveling through a completely different set of ice crystals. The size of the area on the ground where a halo can be seen depends on the size of the patch of atmosphere containing ice crystals with the right density and orientation.

There are a couple of links to references in the APOD caption. Probably the best reference for all sorts of atmospheric phenomena is here.
Chris

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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by LocalColor » Tue Nov 20, 2012 3:32 pm

The most impressive halo around the moon that I was witness to occurred December 31, 1999 just before midnight. As the moon was not yet very high in the sky, it appeared to be a white "moon bow" arching up from the mountains. Unfortunately I did not have a tripod and photos did not turn out (film.)

Last month my better half photographed half of a sun dog. (The sun is off the right side of the photo.)

Image

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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by StarCuriousAero » Tue Nov 20, 2012 5:58 pm

For some reason I was expecting one of those 2 last links to say something about this...
http://discovermagazine.com/2012/apr/07 ... term=cloud bacteria
In any case, there it is.

Lovely photo, I've always enjoyed a moon halo, I've seen a few nice ones in my life so far. :-)

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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by retrogalaxy » Tue Nov 20, 2012 7:15 pm

I saw one, like that. A perfect circle around the full moon. The streets in Athens looks strange on this picture.

Karon Adams

Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by Karon Adams » Tue Nov 20, 2012 8:35 pm

So, I see Jupiter. I also see quite a few other stars and planets peaking out. VERY lovely! I live in the US South. We only see Rings around the Moon from about December to February. rarely any other time (and, of course, only at full Moons) What are REALLY rare around here and always a thrill to see are Sun Dogs! Sadly my kidlets have seen them SO rarely, they don't recognize them! When I point this stuff out, they roll their eyes "MO-OOOOOMMM"

Guest

Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by Guest » Tue Nov 20, 2012 8:43 pm

I live in northern Illinois and we had very dramatic halos around the sun and moon during the overcast days brought by "superstorm Sandy". One of these nights I stepped out to see the moon and there was a huge brilliant halo around the moon and to my astonishment another halo connected to it, forming a number "8". So, the two halos were not concentric. This second halo was not nearly as bright, slightly smaller, had no central light source, and lasted no more than 5 minutes. I got my wife, she witnessed it, then went to get my camera and it was gone. I have looked all for other witnesses or other examples of the phenomenon to no avail. Any ideas anyone?

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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by saturno2 » Tue Nov 20, 2012 11:14 pm

This image is interesting
Moon, Jupiter, and halo of the Moon
It is like a small constellation
Each element is different distance in the space

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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by Anthony Barreiro » Tue Nov 20, 2012 11:23 pm

Ayiomamitis wrote:
Ayiomamitis wrote:
Steve Case wrote:Couldn't our photographer aimed the camera up just a little bit?
I'm reminded of people who cut the heads off in their snapshots.
The moon was around 60 degrees altitude when the image was taken. Now also add lots more degrees for the halo itself and you are basically shooting overhead. To add insult to injury, one needs a multi-second exposure to capture the halo itself.
I just checked and the moon was at 69.7 degrees altitude .... add the halo itself and we have a target which is effectively overhead.

As noted earlier, taking this six-second exposure was much more difficult than people may realize.

Anthony.
Thanks Anthony for this lovely photo. Just like at a football match, the stands are full of experts who are smarter than the coach and athletes who could run circles around the players on the field. :wink:
May all beings be happy, peaceful, and free.

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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by Boomer12k » Wed Nov 21, 2012 12:55 am

We see them here in Oregon. Many times.

"Exactly how ice-crystals form in clouds remains under investigation. "----Um....Pretty much FREEZING up there I think....maybe it's just me....oh, well...Pretty Picture.

If you want to see Full Halo Sun Dogs, search Sun Dog Images...plenty of them...

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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by BillBixby » Wed Nov 21, 2012 3:24 am

Thank you StarCuriousAero. Only 44 posts? The link you sent is why I love the Starship Asterisk*. Opening my mind to so many new relevant things in the world through an ageless science and stepping out of the bounds of just one science. I hope you and other will continue to use this forum as a way to keep me thinking and growing. Your post is one which makes me believe there is a core grope to the SA, intent on expanding our horizons and melding our science boundaries. Wow!

Any who missed his link, it was;

http://discovermagazine.com/2012/apr/07 ... term=cloud

Again, Wow, and thank you.

Bill

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Re: APOD: A Halo Around the Moon (2012 Nov 20)

Post by 2birds » Wed Nov 21, 2012 8:51 am

BDanielMayfield wrote:
Don't be tellin' me this picture is fake.
how come you think that it's a fake?
No one is saying THIS apod is a fake 2birds. Stowaway is referring to yesterday’s flaming arrow attack on John Wayne and the cavalry.
Ah, okay...sorry then - I misunderstood...