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Which way does the galaxy rotate?



 
 
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  #1  
Old September 18th 03, 08:54 AM
PrisNo6
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Default Which way does the galaxy rotate?

Which way does the galaxy rotate from the perspective of the north galacti pole? Is it rotating clockwise - that is from Cygnus
toward Sagittarius - the direction that the arms appear to wind-up towards (from galactic longitude 90 deg to 0 deg)? Or does the
galaxy rotate the other way - counterclockwise - Cygnus to Orion (from galactic longitude 90 deg to 180 deg)? I've read some books
and articles that state that the appearance of the arms as winding up in a particular direction is a visual illusion - that the arms
are simply density waves in an otherwise uniform rotating disk.
- Kurt


  #2  
Old September 18th 03, 03:42 PM
Sam Wormley
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Default Which way does the galaxy rotate?

PrisNo6 wrote:

Which way does the galaxy rotate from the perspective of the north galacti pole?


Point your right thumb toward the south galactic pole (Sculptor)... then your
curled fingers show the direction of the Milky Way rotation.

The rotation curve of the Milky Way
http://nedwww.ipac.caltech.edu/level...ner/node9.html
  #3  
Old September 18th 03, 04:39 PM
Bill Foley
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Default Which way does the galaxy rotate?

With regard to what?? To give a direction, there must be a frame of reference.
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

  #4  
Old September 18th 03, 05:37 PM
Sam Wormley
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Default Which way does the galaxy rotate?

Sam Wormley wrote:

PrisNo6 wrote:

Which way does the galaxy rotate from the perspective of the north galacti pole?


Point your right thumb toward the south galactic pole (Sculptor)... then your
curled fingers show the direction of the Milky Way rotation.

The rotation curve of the Milky Way
http://nedwww.ipac.caltech.edu/level...ner/node9.html


Another way to visualize this --- in the late spring (in the northern
hemisphere around 40 degrees north latitude), when Coma Berenices is
high almost overhead and the Milky Way lies all around the horizon--its
rotation is clockwise around the horizon (not to be confuse with motions
of the sky) and seen from the rotating Earth!

Cygnus is flying backwards!
  #5  
Old September 18th 03, 06:23 PM
Brian Tung
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Default Which way does the galaxy rotate?

lal_truckee wrote:
So the galactic north is defined from what? the ecliptic north?


I'm not sure of the motivation for the definition, but the galactic
north pole happens to be the one with a positive declination.

Because for other "norths" rotation is defined by the right thumb toward
the north. Think of the rising sun. Actually north is defined by the
rotation, not the other way round.


North is defined by convention. For the planets, I guess it's defined
by the rotation, since Venus is said to have an obliquity (axial tilt)
of 177 degrees. Colloquially, we say that it rotates backward.

Pluto's obliquity is about 120 degrees; that of Uranus, about 98. The
other six all have, I believe, obliquities less than 30 degrees.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
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  #6  
Old September 18th 03, 06:30 PM
lal_truckee
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Default Which way does the galaxy rotate?

Sam Wormley wrote:

PrisNo6 wrote:

Which way does the galaxy rotate from the perspective of the north galacti pole?



Point your right thumb toward the south galactic pole (Sculptor)... then your
curled fingers show the direction of the Milky Way rotation.


So the galactic north is defined from what? the ecliptic north?

Because for other "norths" rotation is defined by the right thumb toward
the north. Think of the rising sun. Actually north is defined by the
rotation, not the other way round.

  #7  
Old September 18th 03, 11:18 PM
PrisNo6
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Default Which way does the galaxy rotate?

Sam Wormley wrote in message ...
Point your right thumb toward the south galactic pole (Sculptor)... then your
curled fingers show the direction of the Milky Way rotation.
The rotation curve of the Milky Way
http://nedwww.ipac.caltech.edu/level...ner/node9.html


Thanks Sam. Your "Cygnus is flying backwards!" is a good short hand, sorry to be dense on you, but I'm still confused. The source
of my confusion is that in rotation described in the cited reference, Cyngus is flying forward, not backward - or maybe I'm
misreading the reference and am switching around my galactic quadrants:

"The points of the circle with a Galactic Centre-Sun diameter are characterized by a radial velocity from the Sun equal to their
rotation velocity, and this velocity is determined by the fact that it corresponds to the largest redshift (in the first quadrant)
or the largest blueshift (in the fourth quadrant). The different values at the points of this circle give us the rotation curve."

E. Battaner & E. Florido. The rotation curve of spiral galaxies and its cosmological implications. Fund.Cosmic Phys. 21:1-154
(2000).
at
http://nedwww.ipac.caltech.edu/level...ner/node9.html

I read "first quadrant" to be galactic longitude 0-90 deg (Sag to Cyg) and "fourth quadrant" to be galactic longitude 270-360 deg
(Crux-Sag).

Thus, the answer, per your cited reference, appears to be that the galaxy rotates in the common sense direction that the arm spirals
appear to visually be winding up (although they are not actually winding-up). From the reference viewpoint of the north galactic
pole, that is clockwise. Per the reference, the evidence for the direction of rotation of the galatic disk, relatively to the
north galactic pole (looking down), appears to be based on redshift measurements of gas clouds.

Again, the source of my confusion is that in this clockwise rotation, Cygus is flying forward (pointing to Sag), not backward - or
maybe I'm misreading and am switching around my quardants - that is "first quadrant" should be galactic longitude 270-360 deg
(Crux-Sag) and "fourth quadrant" to be galactic longitude 0-90 deg (Sag to Cyg). In that case the galactic disk are rotating in
opposite direction from the apparent "winding up" of the galactic arms.

Again, sorry to be dense on this.

- Kurt

P.S. - Thanks for the recent and continuing solar flare updates. I looked at Salt Lake City (40° North) - saw nothing at 6 UTC.

The results of your hand gesture (thumb pointed to Sculptor and curled knuckles) depends on whether you extend your left or right
hand and which way you extend your hand (palm up or palm down).


  #8  
Old September 18th 03, 11:44 PM
PrisNo6
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Default Which way does the galaxy rotate?


PrisNo6 wrote in message .. .

This lay online diagram illustrates what I am talking about - Cygnus is flying forward:

http://ottawa.rasc.ca/observers/1999/an9912i8.gif

from

http://ottawa.rasc.ca/observers/1999/an9912p8.html





 




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