Starship Asterisk*

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?

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?

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?

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.

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.

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.

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.

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

bystander wrote:

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

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

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?>>

https://en.wikipedia.org/wiki/Comet_tail#Tail_formation wrote:

[b][color=#0000FF]A comet's orbit showing the different directions of the gas and dust tails as the comet passes the Sun[/color][/b]

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<<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?>>

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.