Pole Dancing Moons!

Southwind17

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Southwind17
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From the BBC News App today:
Hubble has revealed fascinating new details about Pluto's four smaller moons.

At a distance of five billion km, the telescope only sees the satellites as faint pinpricks of light, and yet it has been able to discern information on their size, colour, and rotational and orbital characteristics.

Hubble finds the little objects to be somewhat chaotic in their behaviour.

They are very likely wobbling end over end as they move through their orbits.

"If you can imagine what it would be like to live on [these moons], you would literally not know where the Sun was coming up tomorrow," said Mark Showalter from the Seti Institute, US.

"The Sun might rise in the west and set in the east. The Sun might rise in the west and set in the north for that matter.

"In fact, if you had real estate on the north pole… you might discover one day you’re on the south pole."
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One should expect that somebody speaking on behalf of the Seti Institute would at least understand that the concept of north and south poles derives from a body rotating about an axis, surely!
 
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Southwind17 said:
From the BBC News App today:

One should expect that somebody speaking on behalf of the Seti Institute would at least understand that the concept of north and south poles derives from a body rotating about an axis, surely!
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They are talking about the instantaneous axis of rotation. At any given moment, a moon is 'spinning' about some axis. Therefore, the north and south directions are well defined for an infinitesmal duration. However, the axis is constantly changing. So the north and south points are wandering over the surface of the planet.

As long as the angular momentum of that moon has a nonzero magnitude, the instantaneous axis is well defined. There are of course ininitesmal moments where the moon won't be spinning. The axis is not well defined in the infinitesmal intervals where the angular momentum is zero. However, they are not referring to those moments in time.

Surely a person who would challenge the wise and knowledgeable SETI would have taken a course in calculus!
 
Darwin123 said:
As long as the angular momentum of that moon has a nonzero magnitude, the instantaneous axis is well defined. There are of course ininitesmal moments where the moon won't be spinning. The axis is not well defined in the infinitesmal intervals where the angular momentum is zero. However, they are not referring to those moments in time.
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This is not correct: the angular momentum for the moons will not be zero at any point in time. It will say pretty much constant at all times (there may be some tidal drag on the rotation, but that's not relevant at these time scales).

What's happening is a classic unstable rotation. If you start a rigid body spinning about an axis with an intermediate moment of inertia, that rotation will be unstable, and the object will "tumble" over time. See here for a video demonstration:

 
Ziggurat said:
This is not correct: the angular momentum for the moons will not be zero at any point in time. It will say pretty much constant at all times (there may be some tidal drag on the rotation, but that's not relevant at these time scales).

What's happening is a classic unstable rotation. If you start a rigid body spinning about an axis with an intermediate moment of inertia, that rotation will be unstable, and the object will "tumble" over time. See here for a video demonstration:

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You are right. I made one small mistake. The angular momentum of any one moon will never be zero.

My main point is now even stronger. During ANY instantaneous interval of time, the direction of the angular momentum around the center of mass is well defined. That direction defines the instantaneous pole through that moon. Therefore, the instantaneous positions of the north and south of that moon is ALWAYS well defined. However, the positions of the north and south poles are drifting around the surface of the moon in a chaotic fashion.

The chaotic motion insures that every point on the surface will eventually be at both the instantaneous North and South poles, though not simultaneously. The point will be between these two positions for most of the time.


If OP is talking about finite observers and instantaneous directions, then the he is correct. If a finite observer is located at the north pole at any moment of time, and waits at that position on the surface, then he will be at the south pole in a finite amount of time.
 
I believe Mr Showalter was relating the movement of the moons to the north and south poles in the context that we generally understand them in relation to Earth. In which case it's not the best analogy, although I can see what he means.
 
Given the size and mass relationship between Pluto and Charon, it's more correct to call it a double-(dwarf) planet system. In which case I would think the gravitational interactions are rather complex. And given that the gravity influences obliquity (which is what seems to be the subject of the article in the opening post), that the small moons are exhibiting rapid change in the direction of their axial tilts ought not to be that surprising.

In any case, when New Horizons zips by the Pluto system next month there'll no doubt be much better data (and pictures!) to work with... :)
 
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Corsair 115 said:
Given the size and mass relationship between Pluto and Charon, it's more correct to call it a double-(dwarf) planet system. In which case I would think the gravitational interactions are rather complex. And given that the gravity influences obliquity (which is what seems to be the subject of the article in the opening post), that the small moons are exhibiting rapid change in the direction of their axial tilts ought not to be that surprising.

In any case, when New Horizons zips by the Pluto system next month there'll no doubt be much better data (and pictures!) to work with... :)
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I know, I've been excited about this mission since it was launched... you know, back when Pluto was still a regular planet... I almost can't believe how fast the near decade has gone by.
 
BazBear said:
I know, I've been excited about this mission since it was launched... you know, back when Pluto was still a regular planet... I almost can't believe how fast the near decade has gone by.
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Yeah, I'm looking forward to that too. Almost certainly it'll see things that will be surprising and there'll be new mysteries to solve. :)
 
Southwind17 said:
I believe Mr Showalter was relating the movement of the moons to the north and south poles in the context that we generally understand them in relation to Earth. In which case it's not the best analogy, although I can see what he means.
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About ten hours prior, you seemed to suggest he was speaking out of ignorance of basic facts, rather than presenting an analogy for laypersons.
 
I may well start one when it gets close to the flyby date. There'll be lots to talk about then! :)
 
theprestige said:
About ten hours prior, you seemed to suggest he was speaking out of ignorance of basic facts, rather than presenting an analogy for laypersons.
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I'm sure Mr Showalter understands the pertinent basic facts. My point is that he's referenced the poles out of context, as though they're arbitrary designated points on the moon's surface, that just happen to be diametrically opposite each other.

In the context of a body not rotating about a constant axis the concept of north and south, as we understand those words, becomes obsolete.
 
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