I just registered today and so this is my first post so, please be gentle.

I was re-watching an old Star Trek re-run today and Captain Kirk referenced the planet Charon as being "in the southern part of the galaxy." This got me thinking: does a galaxy (or universe) HAVE a north and south pole? If so, how do you determine it?

Thank you,
Rog

A simplified answer would be southern in relation to Earths north and south- Keptain Kirk was just being simplistic for the laymen.

The north (by convention) galactic pole is "in" Coma Berenices. The universe does not, as far as I know, have an axis of rotation.

You are right

But no offense, you are kind of complicating it.
By your argument- the Earths north and south poles have no meaning. We will still refer to them as such though.

Totally of topic: Neverfly, do you ever sleep? What time zone are you? It's amazing, you seem always to be here.

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Andre

"They did not know it was impossible, so they did it!"
Mark Twain

Harry F. The question is part of what concerned people about a Foucault pendulum. When released, it assumes a trajectory that is fixed relative to the distant stars. How does it know to do that? It involves the deeper thinking of Ernst Mach, who commented to the effect of..."since the pedulum can in no way know how to communicate instantaneously with the distant stars, to choose it's plane of motion, they somehow must have predetermined what that plane is for it". It is also intrinsically involved in the local space-time continuum deformation described in GR...which is why Einstein was Einstein. pete

There is a galactic magnetic field which shows up in the orientation of SN remnants, and Cepheid variables' axes of pulsation....and each galaxy seems to have one.
It is odd that the poles of the solar system's axis....planar orientation ....are aligned with the WMAP "axis of evil" though....that's going to be interesting to see how it's resolved....because it looks like the universe pays particular attention to us. Why should that be? pete

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A third rate theory forbids
A second rate theory explains after the fact
A first rate theory predicts...A. Lomonosov

There is plenty of evidence that the earth spins, quite independently of the observation of the sun and stars. The earth is oblate (which can be determined by accurate surveying), and surface gravity varies with latitude (which can be determined by a sensitive spring balance).

The magnetic field is not directly relevant.

The galaxy does have a mean plane of rotation, the average of all the individual orbits of its stars. So we can define a vector at right angles to that which we call the rotation axis, and call the direction in which it points "north".
How do we decide which end is north? Same as for planets and moons, there are two conventions.
1) Whichever end points into the same hemisphere of the sky as Earth's north pole is north.
2) Hover above the rotation axis, and look down on the object. If it appears to rotate anticlockwise, you're at the north pole; if clockwise, you're at the south pole.

For objects with the same general rotation as Earth, conventions 1) and 2) produce identical results, because Earth satisfies 2) when you look down on it from geographical north. Objects (like Venus) with rotations that are "opposite" to the Earth's, produce opposite results for 1) and 2).

The point in Coma Bernices mentioned by agingjb is the galaxy's north pole by convention 1). But if you were to look down on the galaxy from somewhere in that direction, it would appear to rotate clockwise. So by convention 2), the galaxy's rotational north pole is in the southern hemisphere of Earth's sky.

(This appears to be the resolution to an oddity from Niven's Ringworld novel. the Fleet of Worlds is said to be heading to the Magellanic Clouds, but also towards galactic north: presumably that is "north" according to convention 2), which is south in Earth's sky. I asked Niven about this once, but all he said was that the Fleet of Worlds was "taking the scenic route".)

Presumably Kirk was referring to an object that lay south of the rotation plane of the galaxy, or south of Earth, by one or other of these galactic coordinate systems.

Grant Hutchison

The magnetic field is not relevant. Although it does align fairly closely
to Earth's rotation axis -- for a reason -- it doesn't have to. It could
have multiple poles in multiple places. But overall it is fairly simple,
and is simply aligned with Earth's rotation axis because of Coriolis
effect on the motion of charged particles within the liquid part of the
planet's core, as those particles rise and fall in heat-driven convection.

Which pole of a magnet is called 'north' and which pole is called
'south' was determined by the Earth's magnetic field: The magnetic
pole close to Earth's north rotational pole was naturally assigned
the designation 'north magnetic pole'. Because Earth's overall
magnetic field is strong enough to easily make a well-balanced
magnetized needle align with it, it is possible to determine which
end of a dipole magnet is north and which end is south without any
fancy laboratory apparatus -- just an ordinary compass.

As trinitree88 said, a Foucault pendulum shows the rotation. That
is a pendulum which is free to rotate in any direction, on which the
absolute minimum possible unwanted forces are acting, and which
is started swinging in a plane. The pendulum will continue to swing
in the same plane as the planet rotates beneath it. A Foucault
pendulum at either of Earth's poles will rotate 360 degrees in one
sidereal day of 23 hours, 56 minutes, 4 seconds. A pendulum on
the equator fails to show rotation. Between the poles and equator
a pendulum's rotation period is longer than at the poles and depends
on latitude.

A Foucault pendulum is in the main lobby of the United Nations
General Assembly building. I saw it when I was there in 1967, and
I saw it described on a website a few years ago. There is also one
at the Griffith Observatory in Los Angeles.

Spacecraft in Earth orbit act similarly to Foucault pendulums.
They continue to move in the direction they are launched,
maintaining a constant orbital plane as the Earth rotates
below them.

-- Jeff, in Minneapolis

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http://www.FreeMars.org/jeff/

"The other planets?
Well, they just happen to be there, but the point of rockets is to explore them!"
-- Kai Yeves

Well ...
They would if the Earth were perfectly spherical, and if the Sun and Moon had no gravitational effect. But under the influence of these perturbations, the plane of a satellite's orbit precesses. Quite briskly, if it's in an inclined low Earth orbit.

Grant Hutchison

OT: After a while you will notice I come and go in cycles... As to my somewhat severe sleep deprivation... You can find clues to that scattered in threads all over the board.
Many people have commented that I would make a good robot.
Also, I don't go work eight hours and go home. I come and go as job calls come and go- day and night. So I'll be here- post- then leave for a couple hours- then I'm back... I usually just leave the webpage up and signed in. To confuse the mods you understand.

On Topic: We are discussing, in depth something Kirk said in the Original Series...

The galaxy doesn't have a north and south. Well, ok. It doesn't. But for the gallant space captain, it was quick easy words. That Chekov could understand. So he would know where to steer the wessle.

angular momentum could easily be an intrinsic property--so the universe could "rotate" without rotating within something (I know in General Relativity, there is something called "frame dragging" around a rotating body--perhaps the entire universe "frame" could be in rotation--i.e. space itself is rotating, or equivalently I think, spacetime is twisted like a DNA strand). I recall a proposal to measure universal rotation by looking at the distribution of spiral galaxies to see if they tend to align a certain way (either their north poles or south pols should show a preference for pointing toward a hypothetical center of rotation). This would likely fail--since we may only see a very tiny part of the existing universe, and if I remember right, for all we can tell, galactic allignments tend to be random (suggesting no, or nearly zero, universal rotation).

Of course, a galaxy has a magnetic field--and I'd imagine that means it has a magnetic north and south pole. This would likely differ from what we call the galactic north and sout h pole, defined according to its overall rotation. As someone mentioned, the north pole points in a direction parallel to the direction of the constellation Coma Berenices. (incidentally, that is why one of the Hubble Deep Field images was taken there--perpendicular to the plane of the Milky Way so fewer stars to get in the way of far away galaxies in that direction).

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Todd (Bowie, MD, US, North America, Earth, Sol System, Vega region, Local Bubble, Orion arm, Milky Way Galaxy, Local Group, Virgo A Cluster, Virgo supercluster, the universe in which spock is clean shaven)

Quidquid latine dictum sit, altum sonatur.

personal page: http://blog.astrosketches.info

Isn't it, now. Don't think solid (rigid) bodies, think particles at the atomic and molecular level, gathered into a sphere, and rotating.

Galactic coordinates have been defined by convention.

http://en.wikipedia.org/wiki/Galactic_coordinate_system

http://www.thinkastronomy.com/M13/Ma...ic_coords.html

John. I was surprised as a young scientist to see the time Einstein spent studying rigid rotating bodies....after SR and before he finished GR....but when you think about it, he had to.

__________________
A third rate theory forbids
A second rate theory explains after the fact
A first rate theory predicts...A. Lomonosov

We arbitrarily use the right hand rule. Curl the fingers of your right hand in the direction of rotation with thumb extended. Your thumb points north and defines the north pole. Earth rotates from west to east. So hold your right hand flat, palm up, fingers pointing west, and curl your fingers into a fist from west to east, and your thumb is pointing north.

Magnetic poles are defined in a similar fashion. Curl the fingers of your right hand in the same way as above. If a positive electric current flows in a circle, as your fingers move, then your thumb points in the direction of the north pole of the magnetic field generated by the flowing current. In terms of magnetic field lines, the magnetic field comes out of the north magnetic pole, and goes into the south magnetic pole. Earth's magnetic poles are "mislabled"; Earth's north magnetic pole is called "north" because it is close to Earth's north rotational pole, but it is a magnetic south pole, and vice-versa.

The north & south poles of the galaxy are defined in the same way. The universe as a whole is not known to have any net rotation, and therefore has no physical north & south pole that we know of, although general relativity certainly allows it (i.e, the Gödel metric).

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Don't try this at home - We're what you call "professionals" - MythBusters.

In the context of GR, a rotating universe would be one in which what is best described as a local co-moving inertial observer sees the matter content of the universe rotating around him. IOW, the cosmic mass distribution has proper rotation.

The Godel metric is a doosie. That little whipper-snapper Kurt Godel came up with that to show Einstein that his own field equations allowed a space-time where closed time-like curves were possible (frame dragging is the key to this), something "Uncle Al" (that was sort of the relationship of Einstein to Godel) thought most absurd.

This has nothing to do with coordinates. We can simply go to a rotating frame and see the cosmic mass rotating (that's what we see on this spinning rock we live on) and do the physics from there -- GR is fine with that, although we aren't, since it's much more complicated.

And if you happened to be orbiting deep in the gravity well of a Kerr black hole, the frame dragging would be causing your own local inertial axes to rotate, and you would locally say the cosmic mass was rotating around you.

But in both cases, you'd find the proper rotation of the cosmic mass was zero.

GR is not Machian. Frame dragging has Machian looking overtones, but it is not Machian. Many want GR to be Machian and go through hoops trying to make it so, and just end up redefining Mach more than anything else.

I am many things. But one thing I am not is a Machian.

-Richard