APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

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APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by APOD Robot » Sat Dec 12, 2015 5:06 am

Image Comet Meets Moon and Morning Star

Explanation: A crescent Moon and brilliant Venus met in predawn skies on December 7, a beautiful conjunction of planet Earth's two brightest celestial beacons after the Sun. Harder to see but also on the scene was Comet Catalina (C/2013 US10). The fainter comet clearly sporting two tails, lunar night side, bright sunlit lunar crescent, and brilliant morning star, are all recorded here by combining short and long exposures of the same field of view. Pointing down and right, Catalina's dust tail tends to trail behind the comet's orbit. Its ion tail, angled toward the top left of the frame, is blowing away from the Sun. Discovered in 2013, the new visitor from the Oort cloud was closest to the Sun on November 15 and is now outbound, headed for its closest approach to Earth in mid-January.

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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by Ann » Sat Dec 12, 2015 6:01 am

That's a great image! I instantly liked it when I saw it in the Recent Submissions thread in another forum here at Starship Asterisk*. Note the incredible brightness difference between Venus, the Crescent Moon and Comet Catalina. Venus is blasting forth like a searchlight in the morning, elegantly eight-spiked in this picture in a way that looks almost impossible (but that's because of reflections inside the telescope, I know.)
The crescent Moon is not as bright as Venus, but the Earthshine-illuminated Lady in the Moon is looking down on the comet and smiling. Can't spot the Lady? The picture at left shows you the full Moon and overlaid contours of the Rabbit, the Man and the plain-as-day Lady in the Moon.

Comet Catalina looks very small and faint in this image, but her ballerina split is as graceful as ever.

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Last edited by Ann on Sat Dec 12, 2015 9:28 am, edited 1 time in total.
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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by Guest » Sat Dec 12, 2015 6:04 am

The 'rays' projecting from Venus and the Moon. Are they added by photoshop? or are they a result of stuff like ice crystals high in the atmosphere?

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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by Chris Peterson » Sat Dec 12, 2015 7:37 am

Guest wrote:The 'rays' projecting from Venus and the Moon. Are they added by photoshop? or are they a result of stuff like ice crystals high in the atmosphere?
Diffraction due to an octagonal aperture (the lens iris). The way it is spread out (as opposed to clean spikes) may be caused by scatter in the optics. You can see the same pattern around the illuminated portion of the Moon.
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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by Boomer12k » Sat Dec 12, 2015 10:59 am

Awesome....I always see..."the Energizer Rabbit" in the Moon now....Tilt your head to the right, and look at the first and second picture...Ears to the right, then to the left, the head, then Body and BASS DRUM, and feet, blurred in motion, it appears... :shock:

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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by heehaw » Sat Dec 12, 2015 12:02 pm

"New visitor from the Oort cloud." The Oort cloud has millions of these little chappies. So just how does Mother Nature decide which one visits the inner solar system next? I'd really like to know! Could we suddenly get a burst of a few hundred of them? Why not?

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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by paulslittlebit » Sat Dec 12, 2015 1:11 pm

heehaw wrote:"New visitor from the Oort cloud." The Oort cloud has millions of these little chappies. So just how does Mother Nature decide which one visits the inner solar system next? I'd really like to know! Could we suddenly get a burst of a few hundred of them? Why not?
the short answer is we do not know. Some will have an elliptical orbit that brings them in from the Oort cloud, there ma be a large cold body that disrupts the orbits of other smaller body, although there is no proof of this.

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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by Chris Peterson » Sat Dec 12, 2015 3:33 pm

heehaw wrote:"New visitor from the Oort cloud." The Oort cloud has millions of these little chappies. So just how does Mother Nature decide which one visits the inner solar system next? I'd really like to know! Could we suddenly get a burst of a few hundred of them? Why not?
Assuming the Oort cloud is as modeled (which seems likely), a star passing within a light year or so of our system (which happens regularly over 10-million-year time scales) will perturb the cloud, resulting in thousands (or even millions) of bodies being ejected from the Solar System, and a similar number being directed towards the Sun. So yes, it's likely that the inner solar system has experienced many periods of intense comet activity over the ages.
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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by neufer » Sat Dec 12, 2015 6:33 pm

Chris Peterson wrote:
Assuming the Oort cloud is as modeled (which seems likely), a star passing within a light year or so of our system (which happens regularly over 10-million-year time scales) will perturb the cloud, resulting in thousands (or even millions) of bodies being ejected from the Solar System, and a similar number being directed towards the Sun. So yes, it's likely that the inner solar system has experienced many periods of intense comet activity over the ages.
I agree with the "thousands (or even millions) of bodies being ejected from the Solar System"; all that is really required is for a passing star to temporarily cancel the gravitational attraction of the Sun.

However, in order to direct an Oort body at ~36,000 AU into an inner solar system at ~40 AU requires a reduction of the angular momentum by a factor of 30 [= sqrt(36,000/40)]. This would seem to be a much rarer event (by a factor of at least 167,000 [=3*(30 sqrt(2eπ))3/(4eπ)] than simply ejecting the body.
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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by greghogan » Sun Dec 13, 2015 1:33 am

Chris thanks so much for explaining this so well. I used an 85mm lens at f2.8 its a prime lens and is amazingly sharp and creates some cool diffractions.
Very excited this was picked up by APOD. I had been planing this shot for months.

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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by bergen » Sun Dec 13, 2015 3:45 am

I am confused by the phase of the moon presented in this image. The moon was a waning crescent on 12/07. The image presented appears a waxing crescent already well past new, a point we have not yet even reached.

from USNO website:

"Phase of the Moon on December 12, 2015: Waxing Crescent with 2% of the Moon's visible disk illuminated."
by way of comparison
"Phase of the Moon on December 7, 2015: Waning Crescent with 13% of the Moon's visible disk illuminated."

http://aa.usno.navy.mil/rstt/onedaytabl ... lace=perry

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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by BMAONE23 » Sun Dec 13, 2015 4:44 am

Looks like a mirrored image

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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by Chris Peterson » Sun Dec 13, 2015 5:41 am

bergen wrote:I am confused by the phase of the moon presented in this image. The moon was a waning crescent on 12/07. The image presented appears a waxing crescent already well past new, a point we have not yet even reached.
What leads you to believe from this image that the Moon was waxing? The illuminated crescent is along the eastern limb, which occurs during a waning moon. The orientation of the objects is precisely as they were on the morning sky on December 7. The only thing that might be a little confusing is that the image frame isn't parallel to the horizon (unless the image was made near the equator). But a little image rotation doesn't change the basic geometry.
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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by DavidLeodis » Sun Dec 13, 2015 12:58 pm

In the information brought up through the 'Oort cloud' link it states "The Oort Cloud is believed to be a thick bubble of icy debris that surrounds our solar system. This distant cloud may extend a third of the way from our sun to the next start -- between 5,000 and 100,000 astronomical units. Earth is about one astronomical unit from the sun (roughly 93 million miles or 150 million kilometers). Consider this: At its current speed of about a million miles a day, NASA's Voyager 1 spacecraft won't reach the Oort Cloud for about 300 years. And it will take about 30,000 years to reach the other side." Wow that's a long time so I hope Voyager 1 makes it (and Voyager 2 also)!

PS. I wonder if the "next start" should be "next star" :?:

Edit done about 19:00 UT. Having just needed to look at the 'Oort cloud' link I noticed that the "next start" now states "next star" that I thought it should! The earlier quote I did was a direct copy and paste so I did not read it wrong so (coincidentally or not) the text has been amended since my post! :o
Last edited by DavidLeodis on Sun Dec 13, 2015 7:00 pm, edited 1 time in total.

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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by bergen » Sun Dec 13, 2015 3:08 pm

This is what I would have expected to see on 12/07.

http://api.usno.navy.mil/imagery/moon.p ... time=17:00

I understand a waning moon to be illuminated on the western limb. I agree the image for APOD 12/12 is a moon illuminated up the eastern limb, but I have learned to believe that to be a waxing moon. The moon was not not waxing on 12/07. Perhaps the image was rotated. I believe it was taken from Georgia in the USA.

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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by Chris Peterson » Sun Dec 13, 2015 3:20 pm

bergen wrote:This is what I would have expected to see on 12/07.

http://api.usno.navy.mil/imagery/moon.p ... time=17:00

I understand a waning moon to be illuminated on the western limb. I agree the image for APOD 12/12 is a moon illuminated up the eastern limb, but I have learned to believe that to be a waxing moon. The moon was not not waxing on 12/07. Perhaps the image was rotated. I believe it was taken from Georgia in the USA.
If taken from Georgia (which is also my assumption) it is slightly rotated. But that doesn't change the geometry, or which edge appears illuminated. It is clearly a waning moon in the image. You can look at other images of this event put up by people in this forum. They all show the same thing. And the simulation you put up also shows the same thing as the APOD.
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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by BMAONE23 » Sun Dec 13, 2015 6:08 pm

Rotate the image 90deg clockwise and all will appear proper
or simply tilt your head to the left till horizontal

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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by bergen » Sun Dec 13, 2015 6:58 pm

The simulation I shared by website form USNO has a waning crescent moon as it should have looked 12/07. There, the limbs reached a maximum in their arc from 5 to 11 on the western, left side of the moon. The limbs of the 12/12 APOD image of the moon on 12/07 stretch from 2 to about 8 on the right side of the moon. This to my eye is a significant difference for the same day. I was unaware that such a large angular rotation (45 degrees) in what we can see of the moon on a given day was possible. That some people see what appears to be a waxing moon and others a waning moon at the same time.

I would love to see the other images you refer to but don't see any on the discussion page. Here is a simulation of 12/07 from Sky and Telescope that fits my expectations. If I am told I am looking at a remarkable image of three wonderful celestial objects I'd like to feel I am looking at something that makes sense to me. APOD 12/12/2015 does not make sense to me. But I thank you for taking the time to reassure me and suggesting the problem is mine.

http://www.skyandtelescope.com/observin ... 220150212/

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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by Chris Peterson » Sun Dec 13, 2015 7:05 pm

bergen wrote:The simulation I shared by website form USNO has a waning crescent moon as it should have looked 12/07. There, the limbs reached a maximum in their arc from 5 to 11 on the western, left side of the moon. The limbs of the 12/12 APOD image of the moon on 12/07 stretch from 2 to about 8 on the right side of the moon. This to my eye is a significant difference for the same day. I was unaware that such a large angular rotation (45 degrees) in what we can see of the moon on a given day was possible. That some people see what appears to be a waxing moon and others a waning moon at the same time.

I would love to see the other images you refer to but don't see any on the discussion page. Here is a simulation of 12/07 from Sky and Telescope that fits my expectations. If I am told I am looking at a remarkable image of three wonderful celestial objects I'd like to feel I am looking at something that makes sense to me. APOD 12/12/2015 does not make sense to me. But I thank you for taking the time to reassure me and suggesting the problem is mine.
I think the problem is with your assumption that the lower edge of the image is parallel to the horizon. It isn't.
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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by bystander » Sun Dec 13, 2015 7:55 pm


Rotate the S&T image about 120° CCW and it aligns well with the APOD.
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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by neufer » Sun Dec 13, 2015 10:30 pm

bystander wrote:

Rotate the S&T image about 120° CCW and it aligns well with the APOD.
https://en.wikipedia.org/wiki/CCW wrote:
CCW may refer to:
  • Celebrity Championship Wrestling
    Carrying a Concealed Weapon,
    Catholic college wodonga
    Countryside Council for Wales
    The Center on Conscience & War
    The Chinese Civil War
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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by Beyond » Mon Dec 14, 2015 12:00 am

I understand the second one through personal experience, but "Catholic college wodonga"?? I'll have to Bing it and see what the return is.

Ah, it's an educational management company for the Catholic church, that is part of the CEW. It is located in Victoria Australia.
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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by JohnD » Mon Dec 14, 2015 10:32 am

The image shows especially well the diverging ion and dust tails of a comet.
The Ion tail is driven straight back from the Sun by the Solar Wind.

And the dust tail is composed of particles ejected by the core as it is is heated by the Sun. Those will leave the core at a random velocity, almost unaffected by the core's minute gravity, into free fall in the same orbit as the core. So why does a tail form, instead of a cloud around the core that gets bigger and denser until it moves away from the Sun?
The particles are mostly tiny so may be affected by light pressure, in which case they would move from the orbit away from the Sun, forming a tail at an angle to the orbit.
Is the dust tail along the core's orbit, or at an angle to it?

A rider Q is about the meteors that are associated with many comets. They are said to be spread out along the comet's orbit, which makes me think that the answer to my last is that the dust cloud is deposited along the orbit, not spread out away from it by light pressure. Is there an orbital mechanics explanation? It can't be tidal can it, in the way that Shoemaker's broke up and "spread out" along its orbit?
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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by neufer » Mon Dec 14, 2015 2:58 pm

JohnD wrote:
<<And the dust tail is composed of particles ejected by the core as it is is heated by the Sun. Those will leave the core at a random velocity, almost unaffected by the core's minute gravity, into free fall in the same orbit as the core. So why does a tail form, instead of a cloud around the core that gets bigger and denser until it moves away from the Sun?
The particles are mostly tiny so may be affected by light pressure, in which case they would move from the orbit away from the Sun, forming a tail at an angle to the orbit.

Is the dust tail along the core's orbit, or at an angle to it?>>
The dust tail is pushed away from the Sun by radiation pressure
just at a much slower rate than the ion tail (pushed by the ~400 km/s solar wind).
https://en.wikipedia.org/wiki/Comet_tail#Tail_formation wrote: <<As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun.

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the antitail, only when it seems that it is directed towards the Sun. At the same time, the ion tail, made of gases, always points along the streamlines of the solar wind as it is strongly affected by the magnetic field of the plasma of the solar wind. The ion tail follows the magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.>>
JohnD wrote:
<<A rider Q is about the meteors that are associated with many comets. They are said to be spread out along the comet's orbit, which makes me think that the answer to my last is that the dust cloud is deposited along the orbit, not spread out away from it by light pressure. Is there an orbital mechanics explanation? It can't be tidal can it, in the way that Shoemaker's broke up and "spread out" along its orbit?>>
The primary effect of radiation pressure is to spread the cometary material into a more circular orbit with a similar period as the comet's elliptical orbit. This is because the force is mostly radial and therefore is not changing the angular momentum. One should end up with more or less a circular ring feature in the plane of the comet's orbit.
Art Neuendorffer

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Re: APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

Post by Chris Peterson » Mon Dec 14, 2015 3:34 pm

JohnD wrote:And the dust tail is composed of particles ejected by the core as it is is heated by the Sun. Those will leave the core at a random velocity, almost unaffected by the core's minute gravity, into free fall in the same orbit as the core.
Art has answered your questions, but let me add that while the direction the particles leave the comet is essentially random (because of the rotation of the nucleus), but the speed is not. The ejection speed is determined in part by the composition of the outer layer of the nucleus, and from there by the particle mass. This predictable distribution is the reason that we can model comet debris trails so accurately now.
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