# In which direction in the sky are we headed?

dotancohen
I know the direction to the center of the galaxy is near Sagittarius. Therefore is it safe to conclude that the direction of our solar system's travel through the galaxy is 90 degrees from that point, along the strip of the Milky Way in the sky? As we are in the Orion arm, is the Orion constellation in the direction of our travel? Or perhaps in the direction from where we came? What are the exact points of our direction of travel and the reverse direction?

I tried to calculate them in Stellarium but I have a graphics glitch in the latest Kubuntu and Stellarium has no text. In any case, I could only do an approximation there and Stellarium does not have facility to measure angular distance.

Thanks.

Mentor
dotancohen
Thanks, Borek!

dotancohen
That page makes the distinction between the Sun's movement in the galaxy and the Sun's movement in the Local Standard of Rest. That is interesting, and makes me wonder in which direction is the entire galaxy moving? Is it in the direction of Andromeda, as I know that we will collide with Andromeda some day, or will we meet Andromeda in some other location in the sky?

I know that there is no origin in space from which to derive a direction of movement, so the question of in which direction is the Milky Way travelling might not even be relevant. I suppose that if I had to pick a reference frame I would pick the distant galaxies as an envelope and consider the Milky Way's movement inside that envelope. Or is that also impossible, seeing how those distant galaxies are apparently moving away from us?

Gold Member
The easy way is to use the CMB rest frame. Now that we have the WMAP data, we just look for the warmest point on the CMB map. That is the 'absolute' direction we are moving relative to the CMB - re: http://apod.nasa.gov/apod/ap090906.html

Gold Member
Dearly Missed
That page makes the distinction between the Sun's movement in the galaxy and the Sun's movement in the Local Standard of Rest. That is interesting, and makes me wonder in which direction is the entire galaxy moving? Is it in the direction of Andromeda, as I know that we will collide with Andromeda some day, or will we meet Andromeda in some other location in the sky?

I know that there is no origin in space from which to derive a direction of movement, so the question of in which direction is the Milky Way travelling might not even be relevant. I suppose that if I had to pick a reference frame I would pick the distant galaxies as an envelope and consider the Milky Way's movement inside that envelope. Or is that also impossible, seeing how those distant galaxies are apparently moving away from us?

The "solar apex" is not quite the direction you were asking for. That is a slight motion of the sun relative to its immediate neighborhood. It is only some 16-20 km/s.

The direction of that orbital motion (which we share with the surrounding crowd of stars except for minor deviations) can be calculated by finding a direction which is perpendicular to the direction of galactic center and also perpendicular to the galactic north pole.

Our general orbital direction is therefore about 21 hours (RA) and 48 degrees celestial "North latitude".

I would say that our direction is more or less towards the star DENEB in the constellation CYGNUS. I could be wrong but you probably can check easily. Please tell me if I am way off.

That is not the star Vega. A lot of internet sources seem to think it is the direction of Vega, or the constellation Hercules. I think they've confused it with the Solar Apex (our drift within the overall motion of our neighbors).

Here is a source:

==quote==
Below we give some data for the Galactic Center (this and all following positions for epoch 2000.0):
Right ascension 17 : 45.6 (h : m)
Declination -28 : 56 (deg : m)
Distance 28 (kly)

The Galactic North Pole is at
Right ascension 12 : 51.4 (h : m)
Declination +27 : 07 (deg : m)
The coordinate data given here were extracted from the online coordinate calculator at Nasa/IPAC's Extragalactical Database (NED) (also available by telnet).

Our Sun, together with the whole Solar System, is orbiting the Galactic Center at the distance given, on a nearly circular orbit. We are moving at about 250 km/sec, and need about 220 million years to complete one orbit (so the Solar System has orbited the Galactic Center about 20 to 21 times since its formation about 4.6 billion years ago).

In addition to the overall Galactic Rotation, the solar system is moving between the neighboring stars (peculiar motion) at a velocity of about 20 km/s, to a direction called "Solar Apex," at the approximate position RA=18:01, Dec=+26 (2000.0); this motion has been discovered by William Herschel in 1783.

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).
==endquote==
http://seds.org/messier/more/mw.html [Broken]

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dotancohen
The easy way is to use the CMB rest frame. Now that we have the WMAP data, we just look for the warmest point on the CMB map. That is the 'absolute' direction we are moving relative to the CMB - re: http://apod.nasa.gov/apod/ap090906.html

Thanks, Chronos, I had heard of the CMB dipole but never seen it expressed as in that map. I subscribe to apod, but I might not have been at that time in 2009. I agree, the CMB would be the "rest frame" I was looking for in my previous post. Thanks!

dotancohen
The "solar apex" is not quite the direction you were asking for. That is a slight motion of the sun relative to its immediate neighborhood. It is only some 16-20 km/s.

Yes, I did realise that the solar apex is not the direction of the sun around the galactic center. But it did lead me some keywords to google.

The direction of that orbital motion (which we share with the surrounding crowd of stars except for minor deviations) can be calculated by finding a direction which is perpendicular to the direction of galactic center and also perpendicular to the galactic north pole.

That is of course assuming that the sun is right on the symmetrical plane of the galactic disc, orbiting right in the disc. As I understand it we have a bit of an inclination, and even though right now we are near the vertical center of the disc, we are oscillating up and down.

Our general orbital direction is therefore about 21 hours (RA) and 48 degrees celestial "North latitude".

I would say that our direction is more or less towards the star DENEB in the constellation CYGNUS. I could be wrong but you probably can check easily. Please tell me if I am way off.

That is not the star Vega. A lot of internet sources seem to think it is the direction of Vega, or the constellation Hercules. I think they've confused it with the Solar Apex (our drift within the overall motion of our neighbors).

Here is a source:

==quote==
Below we give some data for the Galactic Center (this and all following positions for epoch 2000.0):
Right ascension 17 : 45.6 (h : m)
Declination -28 : 56 (deg : m)
Distance 28 (kly)

The Galactic North Pole is at
Right ascension 12 : 51.4 (h : m)
Declination +27 : 07 (deg : m)
The coordinate data given here were extracted from the online coordinate calculator at Nasa/IPAC's Extragalactical Database (NED) (also available by telnet).

Our Sun, together with the whole Solar System, is orbiting the Galactic Center at the distance given, on a nearly circular orbit. We are moving at about 250 km/sec, and need about 220 million years to complete one orbit (so the Solar System has orbited the Galactic Center about 20 to 21 times since its formation about 4.6 billion years ago).

In addition to the overall Galactic Rotation, the solar system is moving between the neighboring stars (peculiar motion) at a velocity of about 20 km/s, to a direction called "Solar Apex," at the approximate position RA=18:01, Dec=+26 (2000.0); this motion has been discovered by William Herschel in 1783.

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).
==endquote==
http://seds.org/messier/more/mw.html [Broken]

I'm sorry, I had orange juice instead of coffee this morning. Is this suggesting that the Sun (and our local group) is travelling in the opposite directions of most of the stars in the galaxy?

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Gold Member
Dearly Missed
I'm sorry, I had orange juice instead of coffee this morning. Is this suggesting that the Sun (and our local group) is travelling in the opposite directions of most of the stars in the galaxy?

No :rofl:
I'm not sure how you got that idea. I could have misspoken (don't see that though) or another possibility is that you don't happen to know that a convention was broken when astronomers set up galactic coordinates. What they call "galactic north pole" is the direction perpendicular to the plane such that if you travel out along that direction you can look back and see the galaxy rotating clockwise.

Otherwise, say in the solar system context and by analogy, we tend to have the opposite convention, well a kind of righthandrule convention. The northpole is where if you make a fist and do "thumb up" in that direction your fingers tell the sense of rotation. Or if you travel out in space along that direction and look back you see counterclockwise rotation.

You surely know all that (but others might be reading--I want to be clear). Your other comments are astute and accurate. Indeed the sun is bobbing up and down thru galactic plane. So the Cygnus direction is just an approximate one based on what all the stars in our neighborhood are known to be doing on average. but that is a 250 km/s velocity and the individual deviation is only on the order of 10%. Less, I don't want to put too fine a point on it (seem overly precise.)

So if you want a bright star that marks direction that the sun and surrounding crowd are heading, I'd say Deneb. It is the tail-feathers of the flying swan, right? The brightest star in Cygnus.

I value your precision and skill with coordinates. If I've made some error here please correct, also would be glad to see more accurate detailed analysis if you want to make one. Probably other readers here would too.

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popsjabo
Why don't we just admit that we do not have the least notion as to where we are with respect to the overall universe???

dotancohen
No :rofl:
I'm not sure how you got that idea. I could have misspoken (don't see that though) or another possibility is that you don't happen to know that a convention was broken when astronomers set up galactic coordinates. What they call "galactic north pole" is the direction perpendicular to the plane such that if you travel out along that direction you can look back and see the galaxy rotating clockwise.

No, I did not know that galactic coordinate systems have the north and south poles reversed. I wonder why they did that, the Wikipedia article did not have much information. But I'm certain that Google will help me find some. Thanks, that is interesting.

So if you want a bright star that marks direction that the sun and surrounding crowd are heading, I'd say Deneb. It is the tail-feathers of the flying swan, right? The brightest star in Cygnus.

Yes, Deneb is close enough. I don't need enough precision to guide a starship!

I value your precision and skill with coordinates. If I've made some error here please correct, also would be glad to see more accurate detailed analysis if you want to make one. Probably other readers here would too.

That's not quite my field, but if I do ever come across anything relevant and accurate I will post back.