Orientation of Solar System

[freeztar]

In all honesty, I'm still trying to make sense of all this, but I thought a comment was in order.

It seems to me that the rotation is clockwise. In other words, the "galactic arms" should be trailing rather than leading.

[belovelife]

i wonder what happens when 2 galaxies arms dance and hold hands

[Turtle]

Originally Posted by modest

Yes, I believe so. Clockwise in both pics. And, I should probably clarify (or at least give credit) the diagrams were not made by me, but came from this amateur radio astronomy type web page. But, I think, yes, the way the spirals are drawn in the second diagram would mean rotation would have to be clockwise.

~modest

EDIT: Wops, you said "counter-clockwise". I guess then, I disagree. Or, one of us is confused

OK We have to resolve this. Neither of your two diagrams specify up or down view or which pole is which.

"Get a Straight Answer"

... As for the rotation of the Earth... you should first of all realize that "clockwise" and "counter clockwise" are not absolute properties, but depend on your point of view. Imagine a clock with a transparent face, with you watching it from the rear. The number 12 is still on top and 6 still on the bottom, but now 3 is on the left and 9 on the right. So when the clock hand moves from 12 to 3, it moves ... counterclockwise!

To define rotations with no ambiguity, we can stipulate they are always observed from the NORTH, from some point far above the north pole of the Earth. ...

The more scholarly articles I've been off hunting simply don't seem to mention the direction, but I just found this answer for counter-clockwise. I honestly thought everything rotated counter-clockwise because of conservation of angular momentum?

Google Answers: In what direction does the Sun travel relative to the calendar year?

Originally Posted by Anonymous Blogger

The Sun is indeed in motion. It orbits the center of our galaxy at
the speed of approx 155 miles per second in a counter-clockwise
direction. And of course we are moving right along with it, being
"dragged" as you put it.

It takes the Sun (and us) about 200 - 250 million years to orbit once
around the center of the Milky Way Galaxy. So yes, the Sun does have
a "year" as well - about 200 million times or so as long as an Earth
year. ...

[modest]

Originally Posted by Turtle

OK We have to resolve this. Neither of your two diagrams specify up or down view or which pole is which.

"Get a Straight Answer"

No. They do not. (and by the way, this conversation seems so oddly familiar )

The only things for certain the diagrams show is that rotation is going towards Cygnus from our point of view. That is to say: if you are looking at the constellation Cygnus then you are looking toward the direction the solar system is moving in terms of galactic rotation. It's the fact that Cygnus is in the northern hemisphere that settles the question of which pole leads. It is the north pole.

Originally Posted by Turtle

The more scholarly articles I've been off hunting simply don't seem to mention the direction, but I just found this answer for counter-clockwise. I honestly thought everything rotated counter-clockwise because of conservation of angular momentum?

Astrnomical systems do usually rotate counterclockwise as a matter of convention. The right hand rule is usually used. If you point your thumb up then that defines the north pole. Your fingers then (not pointed straight out, but curved a bit) show the direction of rotation. Point your thumb at your face and look at the direction your fingers are pointing and it is indeed counterclockwise.

The direction of rotation usually defines the pole in this way. Looking at things from the top then (from the north—which is also convention) will show counterclockwise rotation.

The diagrams I linked obviously don't follow this convention. But, the pertinent thing is that the galactic coordinate system has Cygnus at 90 degrees which is the direction of rotation (as viewed by us). So if we are on earth's (or the sun's) north pole and looking up, we will see the general direction that we are rotating toward.

Is this much more than gibberish?

~modest

[modest]

Originally Posted by freeztar

It seems to me that the rotation is clockwise. In other words, the "galactic arms" should be trailing rather than leading.

I agree. The diagrams both show clockwise rotation as I see it.

~modest

[Turtle]

So, summing up in more colloquial terms, the Solar System is rotating around the Galactic Center in a counter-clockwise direction when viewed from above the North Galactic Pole. Additionaly, when viewing the Solar System from the the side of the Milky Way with North up, the Solar System Plane in its elliptical orbit of the Galactic Center is inclined to the Galactic Plane, leading with its North Pole(s), and above the Galactic Plane moving North.

All good?

[modest]

Damn Turtle, I think you've given me a brain hemorrhage.

I think... perhaps... just maybe... you've got your up and down backwards. Or, perhaps the astronomical community has their galactic coordinate system backwards. Or, perhaps I'm turned around... something has gotten turned around

Ok, starting over...

I don't think the galaxy rotates counterclockwise as viewed from the north galactic pole. I think it rotates clockwise as viewed from north. I don't think it follows the right hand rule (at least, not in the galactic coordinate system). A paper, such as this:

The Sun's Distance Above the Galactic Plane

which uses the galactic coordinate system would seem to be saying that we are above the ecliptic as is defined by that coordinate system. And I'm pretty sure, that according to that system, viewing the galaxy from the north galactic pole would show clockwise rotation.


-source

The above pic would have the galaxy rotating clockwise as viewed from the north. A positive latitude (above the galactic plane) would then be above the disc (as viewed from the side) where the disc is rotating left.

I have a suspicion of why the GCS doesn't follow the normal right hand rule. If it did then earth's north pole would point to the south side of the galaxy. In order to have earth's north and the galaxy's north roughly the same direction, the left hand rule is used. But, that's just a suspicion—I don't know if that's the reason. Nevertheless, as the pic above shows it, the galaxy does rotate clockwise as viewed from the north. It rotates in the direction of 90 deg. longitude.

Yes / no / maybe???

~modest

[Turtle]

Originally Posted by modest

Damn Turtle, I think you've given me a brain hemorrhage.

I think... perhaps... just maybe... you've got your up and down backwards. Or, perhaps the astronomical community has their galactic coordinate system backwards. Or, perhaps I'm turned around... something has gotten turned around

Ok, starting over...

I don't think the galaxy rotates counterclockwise as viewed from the north galactic pole. I think it rotates clockwise as viewed from north. I don't think it follows the right hand rule (at least, not in the galactic coordinate system). A paper, such as this:

The Sun's Distance Above the Galactic Plane

which uses the galactic coordinate system would seem to be saying that we are above the ecliptic as is defined by that coordinate system. And I'm pretty sure, that according to that system, viewing the galaxy from the north galactic pole would show clockwise rotation.


-source


The above pic would have the galaxy rotating clockwise as viewed from the north. A positive latitude (above the galactic plane) would then be above the disc (as viewed from the side) where the disc is rotating left.

I have a suspicion of why the GCS doesn't follow the normal right hand rule. If it did then earth's north pole would point to the south side of the galaxy. In order to have earth's north and the galaxy's north roughly the same direction, the left hand rule is used. But, that's just a suspicion—I don't know if that's the reason. Nevertheless, as the pic above shows it, the galaxy does rotate clockwise as viewed from the north. It rotates in the direction of 90 deg. longitude.

Yes / no / maybe???

~modest

I'm afraid I still have not achieved my 'Ahah!' moment on this. I'm particularly vexed that none of these sources say outright the direction of rotation. It's a simple question and by damn I want a simple answer. Your answer appears simple, but then you resort to suspicion which at the least tells me that the references so far don't make it clear to even you. Can we keep looking for an unambiguous source? If ya see a turtle on a fence-post, you know it didn't get there by itself.

[Turtle]

Land ho! By jove, I think I've got it!

The Milky Way Galaxy

...Considering the sense of rotation, the Galaxy, at the Sun's position, is rotating toward the direction of Right Ascension 21:12.0, Declination +48:19. This shows that it rotates "backward" in the Galactic coordinate system, i.e. the Galactic North Pole is actually a physical South Pole with respect to galactic rotation (defined by the direction of the angular momentum vector). ...

and

Curious About Astronomy: How often does the Sun pass through a spiral arm in the Milky Way?

The solar motion on top of it's circular orbit about the centre of the Galaxy (which has a period of about 230 million years) can be described by how fast it is going in three different directions

U = 10 km/s (radially inwards)
V = 5 km/s (in the direction of Galactic rotation)
W = 7 km/s (northwards out of the plane of the Galaxy)

I had a link saying about that half the galaxies rotate one way and half the other, but I lost it in the shuffle. Damn Three-card-Monte!! Anyway, I think I'm off that fence-post finally. .....................

[modest]

Excellent Turtle

And one more log on the fire:

Here, the galactic plane, or galactic equator, is used as reference plane. This is the great circle of the celestial sphere which best approximates the visible Milky Way. For historical reasons, the direction from us to the Galactic Center has been selected as zero point for galactic longitude l, and this was counted toward the direction of our Sun's rotational motion which is therefore at l = 90 deg. This sense of rotation, however, is opposite to the sense of rotation of our Galaxy, as can be easily checked ! Therefore, the galactic north pole, defined by the galactic coordinate system, coincides with the rotational south pole of our Galaxy, and vice versa.

http://seds.org/~spider/spider/Schol....html#galactic

So, indeed, the galactic coordinate system is backwards (left handed as it were). Also, check out this software which is bloody brilliant:

Where is M13? - Home

You can look at Polaris (the north star) for example and see exactly where it is in the galaxy which gives a very good sense of things.

~modest