What frame(s) of reference are used to measure the rotation of the Sun around the galaxy ?

Question

I can find various speeds and estimated durations listed at numerous places but none specifically describe the frame of reference.

Possible options as example of kind of answer I expect.

• Local Galactic cluster
• Distance quasars
• The cosmic background radiation?

--------- UPDATE ---------

Thanks AIB and voithos. Lot of reading for me.

Though technically, I still don't have an answer that meets the following criteria.

1. rotational velocity(average preferably) of sol around best estimate of center of Milky-way galaxy.
2. publicly available reference(I don't have immediate access to some of the books given)
3. frame of reference external to Milky-way galaxy.

As I note below the only reference (wmap5basic_reprint.pdf) I can read that uses an external frame of reference doesn't specifically state the vector is rotational (despite wikipedia article assuming such). The topic is barely touched on in that paper.

I realised the speeds are variable. What I had not realised is that the whole idea of a (relatively) clearly defined x orbiting y system doesn't really scale up well from the local solar system to the galactic scale. The galaxy is more like a whirlpool or tornado compared to the "clockwork" appearance of the solar system. Although both the solar system and galaxy are constantly(very slowly) changing "fluid" rotational systems, the galaxy is obviously far more fluid than the solar system. Also we have not yet been able to observe anything about it's center.

Or in other words, we are not "orbiting" the galaxy, we are part of the galaxy.

I suspect the topic is more in the realm of "fluid dynamics" than "orbital mechanics"

I've accepted AIB's answer as the most enlightening to me personally. Also, it would appear I have wiki-sidebar blindness. Apologies for that.

FYI
The paper referencing the speed relative to the CMB, as mentioned in wikipedia article can be found here http://cmbdata.gsfc.nasa.gov/product/map/dr3/pub_papers/fiveyear/basic_results/wmap5basic_reprint.pdf The relevant section appears to be 7.3.1. "... implies a Solar System peculiar velocity of 369.0 ± 0.9kms-1 with respect to the CMB rest frame." Although it's not obvious to me what vector that velocity is along.

Though Dipole Anisotropy in the COBE DMR First-Year Sky Maps gives a specific velocity(including vector) for the local galactic group in relation to the CMB rest frame "implied velocity of the Local Group with respect to the CMB rest frame is 627 +/- 22 km/s toward (l,b) = (276 +/- 3 deg, 30 +/- 3 deg)."

FYI
Other reference frames that are external to the local galaxy are "The Supergalactic coordinate system"

Answer 1

The Wikipedia page on Sun gives these three velocities,

• ~220 km/s (orbit around the center of the Galaxy)
• ~20 km/s (relative to average velocity of other stars in stellar neighbourhood)
• ~370 km/s(relative to the cosmic microwave background)

So my inference is that 220km/s is the estimated orbital velocity. It is not constant velocity because the orbital motion around galactic center is not circular.

The velocity of Sun around the Milkyway is in fact same as the spin motion of Milkyway around itself.

All stars in the galaxy rotate around a galactic center but not with the same period. Stars at the center have a shorter period than those farther out

Sun's orbital motion is calculated with galactic north pole as the frame of reference. It is called the galactic coordinate system. See this

It's a complicated calculation, because stars have arbitrary motion in local regions, which need to be subtracted out.

Answer 2

Very good question- Einstein stated that there is no "preferred frame of reference" in any given context. So, ideally, relative velocities should be verified with multiple frames of reference.

As far as our Sun's orbit in the galaxy, I believe that typically the frame of reference is the galactic center. There are various journal articles that detail how this measurement can be accomplished. It is difficult to measure our own motion within our galaxy, but we can use the motions of neighboring stars relative to the galactic center to obtain a rotation rate for the Milky Way, although we must be careful, as AIB pointed out. From there, it is simply a matter of checking and double checking this velocity in comparison to other frames of reference. The Wikipedia article mentions the Sun's rotation a bit.

Of course, even though there is no true "preferred frame of reference", there is a relatively absolute frame: the cosmic microwave background.

Answer 3

We need to determine what is the velocity of the Sun relative to Vc, the velocity of an object in a circular orbit around the galactic center. There are two approaches:

1) We look at the average velocities of the youngest stars; these have recently formed out of the interstellar gas which is presumed to the have the desired circular velocity. Older stars have been perturbed into elliptical orbits, and the mean of these do not give the circular velocity but something less since we see more stars from closer in near their 'apogalacticon' than stars from further out near their 'perigalacticon'.

2) We look at the mean velocity of the 21-cm spectral line of the interstellar neutral hydrogen.

While the answers from 1) and 2) are not identical, they agree that the Sun is moving with respect to Vc at 15-20 km/sec in the direction of the constellation Cygnus.

Historically, Vc was very difficult to determine and results varied from 190 to 250 km/sec. More recently, observations of the apparent motion ("proper motion") of the galactic center radio source Sgr A* have determined that Vc is about 220 km/sec.

The dipole of the Cosmic Background Radiation is about 370 km/sec, that is, the Sun is moving at a speed of 370 km/sec relative to the CMB. But it points more or less in the opposite direction to Vc with the result that the center of mass of our Galaxy is moving at about 575 km/sec relative to the CMB, in the direction of the distant galaxies in the constellation Carina.