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patrick
2007-Oct-13, 01:03 PM
Hi all
I have a question.

Earth evolves around the sun, and the sun around the galaxy, and the galaxy moves also, which brings me to my question: at what speed are we (on earth) moving at?

Noclevername
2007-Oct-13, 01:17 PM
Hi all
I have a question.

Earth evolves around the sun, and the sun around the galaxy, and the galaxy moves also, which brings me to my question: at what speed are we (on earth) moving at?

I'm sitting still.

Seriously though, all motion is relative. So the question is, how fast are we moving compared to ____? What point of reference should we use?

patrick
2007-Oct-13, 01:25 PM
True indeed what point of reference should we take.(?)

But if we look at it this way: as far as I am aware of, the universe is expanding near * or at * the speed of light (correct me please if I am wrong here). Would that mean we ourselves are moving near the speed of light (since we are part of that universe)?

In addition, assuming we are moving close to the light speed, would the universe not be much much older in relation to a (theoretically) motionless point of reference?

patrick
2007-Oct-13, 01:46 PM
one more (side) question that came up:
How fast is our Sun moving away from e.g. Proxima Centauri (at 4.22 light years)

antoniseb
2007-Oct-13, 01:49 PM
True indeed what point of reference should we take.(?)
I like to use the cosmic microwave background as a reference for this one.


But if we look at it this way: as far as I am aware of, the universe is expanding near * or at * the speed of light (correct me please if I am wrong here). Would that mean we ourselves are moving near the speed of light (since we are part of that universe)?
You might think of it as expanding away from us... and it is a misunderstanding to think it is expanding *at the speed of light*. It is expanding. The galaxy clusters near us are getting further away from us at much less than the speed of light. Things much more distant are getting further away at much faster than the speed of light.


In addition, assuming we are moving close to the light speed, would the universe not be much much older in relation to a (theoretically) motionless point of reference?
Are you trying to use Special Relativity here? That doesn't really apply on a cosmic scale, and, as I noted above, we can be thought of as moving very little.

antoniseb
2007-Oct-13, 01:52 PM
one more (side) question that came up:
How fast is our Sun moving away from e.g. Proxima Centauri (at 4.22 light years)
Any relative movement of Proxima Centauri and the Sun has nothing to do with Universal Expansion. It is moving away at 20.3 kilometers/second.

01101001
2007-Oct-13, 02:33 PM
But if we look at it this way: as far as I am aware of, the universe is expanding near * or at * the speed of light (correct me please if I am wrong here). Would that mean we ourselves are moving near the speed of light (since we are part of that universe)?

You might find some of these fundamental cosmology questions answered at Sean Carroll's Cosmology Primer FAQ (http://preposterousuniverse.com/writings/cosmologyprimer/faq.html).

Like Are distant galaxies moving faster than the speed of light? Wouldn't that violate relativity? (http://preposterousuniverse.com/writings/cosmologyprimer/faq.html#ftl):


A profound feature of relativity is that two objects passing by each other cannot have a relative velocity greater than the speed of light. An even more profound feature, one which has received much less publicity, is that the concept of "relative velocity" does not even make sense unless the objects are very close to each other. In Einstein's general theory of relativity (which describes gravity as the curvature of spacetime), there is no way to define the velocity between two widely-separated objects in any strictly correct sense. The "velocity" that cosmologists speak of between distant galaxies is really just a shorthand for the expansion of the universe; it's not that the galaxies are moving, it's that the space between them is expanding. [...]

eburacum45
2007-Oct-13, 03:01 PM
The concept of relative velocity may be a tricky one, but certain relative movements can be discerned.
See this section on Wikipedia;
http://en.wikipedia.org/wiki/Milky_Way#Velocity

Another reference frame is provided by the Cosmic microwave background (CMB). The Milky Way is moving at around 552 km/s[30] with respect to the photons of the CMB.

But as far as I am concerned, I'm just sitting here.

Tim Thompson
2007-Oct-13, 05:36 PM
The equatorial circumference of Earth is 40,074.8 km and the sidereal period of rotation is 23.9345 hours. That translates into an equatorial speed, relative to the axis of rotation of Earth, of about 0.465 km/sec (1,040.4 miles/hr). Multiply that speed times the cosine of your latitude to get your speed. At the latitude of Los Angeles, we are going about 862 miles/hour.

Relative to the center of mass of the solar system, Earth orbits the sun with an average speed of 29.78 km/sec (66,616 miles/hour). Since the orbit is slightly elliptical, that varies from a minimum of 29.29 km/sec to a maximum of 30.29 km/sec.

The stars in the disk of the Milky Way orbit the center of the Galaxy at about 220 km/sec (492,000 miles/hour), and our sun takes about 250,000,000 years to make one orbit.

Our Milky Way Galaxy is part of the local group of roughly 50 galaxies, gravitationally dominated by the Milky Way & M31. The Local Group is falling into the more massive Virgo Cluster of galaxies, at a speed of about 240 km/sec (536,900 miles/hour), relative to the Virgo Cluster.

Our Local Group is also moving with respect the the rest frame of the cosmic microwave background, at about 627 km/sec (1,402,600 miles/hour).

A proper vector addition of all those velocities is beyond the limit of my patience at the moment. The orbital motions all vary in direction with different periods, so the resultant will be a complicated function of time. But you get the picture. Depending on the frame you choose, we are moving through space really fast.

The motion of the local group compared to Virgo and the CMB is discussed in Kocevski & Ebeling, 2006 (http://adsabs.harvard.edu/abs/2006ApJ...645.1043K), but see also Jerjen & Tammann, 1993 (http://adsabs.harvard.edu/abs/1993A%26A...276....1J). The data on the size & rotation of Earth comes from the Lunar & Planetary Sceinces page (http://nssdc.gsfc.nasa.gov/planetary/planetary_home.html) at the NSSDC. The rotation of the Milky Way comes from my own memory, having once worked on the problem long ago.

eburacum45
2007-Oct-14, 11:13 AM
The stars in the disk of the Milky Way orbit the center of the Galaxy at about 220 km/sec (492,000 miles/hour), and our sun takes about 250,000,000 years to make one orbit.

Is 220 km/s pretty much constant, regardless of distance from the galactic centre, or is it more complex than that?

Tim Thompson
2007-Oct-14, 04:47 PM
Is 220 km/s pretty much constant, ...
I remember 220 km/sec as the standard for the sun. I assume that it is "pretty much constant"; the Milky Way, like other spirals, should have a fairly flat rotation curve. But I don't know, and it's an interesting question, so I did a little "research".


Using Open Clusters to Trace the Local Milky Way Rotation Curve and Velocity Field (http://adsabs.harvard.edu/abs/2006AAS...20916501F); Frinchaboy & Majewski, December 2006; abstract only. They report: "Through analysis of the cluster sample, we find (1) the rotation velocity of the LSR is 221 (+2,-4) km/s, (2) the local rotation curve is declining with radius having a slope of -9.0 km/s/kpc ...". The results come from Frinchaboy's PhD thesis (abstract (http://adsabs.harvard.edu/abs/2006PhDT........12F)) from the University of Virginia.

LSR is the "local standard of rest" for the sun and the local stars, so that is not necessarily the rotational speed of the sun itself. While I remember 220 km/sec as the standard, I have seen higher estimates.


A Large Local Rotational Speed for the Galaxy Found from Proper Motions: Implications for the Mass of the Milky Way (http://adsabs.harvard.edu/abs/1999ApJ...524L..39M); Mendez, et al., The Astrophysical Journal Letters 524(1): L39-L42, October 1999. They report: "The SPM data are best fitted by models having a solar peculiar motion of 5 km/s in the V-component (pointing in the direction of Galactic rotation), a large LSR speed of 270 km/s, and a disk velocity ellipsoid that points toward the Galactic center. We stress, however, that these results rest crucially on the assumptions of both axisymmetry and equilibrium dynamics. The absolute proper motions in the U-component indicate a solar peculiar motion of 11.0+/-1.5 km/s, with no need for a local expansion or contraction term." That's the highest estimate of the LSR rotational speed that I know of. But they also report the "peculiar velocity" of the sun, which is its speed relative to the LSR.

Most galaxy people don't buy a rotation speed that large, so I stick to the 220 km/sec that I remember (and with the excuse that Frinchaboy's PhD thesis agrees).

astromark
2007-Oct-14, 06:40 PM
If you could consider the sun a motionless mass then this question is simplified. It is not. Regardless of Earths rational movement giving us a velocity around the planets center of mass definable. We know the diameter and length of day. We also know the length of a year and our distance from the sun. So these velocities are obtainable from simple mathematics. Complicated as it might be by small irregularities. Not a perfect circular orbit... and so on. Work those numbers and put aside as that is only part of your answer. The solar system is not orbiting just the central mass of this Galaxy. This orbital velocity is further complicated by the spiral arms mass we are near. As Sol moves up and down through the plain of this galactic medium. You see what a complicated question you have asked. The galactic velocity is just to hard to deal with as we have no relative motionless mass to calculate from or to.

Moonhead
2007-Oct-14, 07:01 PM
Any relative movement of Proxima Centauri and the Sun has nothing to do with Universal Expansion. It is moving away at 20.3 kilometers/second.

That sucks! :sad:

eburacum45
2007-Oct-14, 07:26 PM
Interesting; I have just checked David Nash's Distant Star mapper at Astronexus
http://www.astronexus.com/node/69
according to that, Proxima will only be 3.77 light years away in the year 10,000 after present.

That suggests to me that Proxima is moving towards us, or the Astronexus database is wrong...

Moonhead
2007-Oct-14, 08:34 PM
[...] Proxima will only be 3.77 light years away in the year 10,000 after present.

That suggests to me that Proxima is moving towards us

Now that's cool! (unless it decides to speed up and wreck the Solar system... :shifty: )

astromark
2007-Oct-15, 06:05 AM
Astronomers are often wrong. Above is evidence of that fact. So pin pointing the velocity of planet Earth is at this time unsupported by irrefutable fact... We do not have all the information we need to answer this with absolute certainty. Really quick is not good enough but, true.

Kaptain K
2007-Oct-15, 02:12 PM
I think "wrong" is too strong a word! If I measure a bar to be 99 cm long, and it is "really" 99.3 cm, am I wrong? There is a difference between precision and accuracy.

astromark
2007-Oct-15, 06:22 PM
incorrect then... we just said the same thing. I was referring to proxima's recession or advance. one of these is 'wrong'.Do we know.?

John Mendenhall
2007-Oct-15, 06:48 PM
The galactic velocity is just to hard to deal with as we have no relative motionless mass to calculate from or to.



I was just kicking this idea around wrt another question thread. Do we have to look to distant quasars for a reference? Tell us, oh professional members who actually do this in your work.

Ken G
2007-Oct-16, 02:32 AM
I was just kicking this idea around wrt another question thread. Do we have to look to distant quasars for a reference? Tell us, oh professional members who actually do this in your work.
The most clear thing to look to for a reference is the cosmic microwave background radiation, and our galaxy does have an obvious relative velocity to that. I don't recall, but 300 km/s jumps into my mind. Quasars should work too, but not with the precision of the CMB. Note that relativity only says that all reference frames work equally well for the formal application of the laws of physics, it never claims that a particular one can't jump out and wave its hands and say "choose me, choose me!".

By the way John, are you going to get back to the CERN answer to those "rockets on a string" question at some point? I'm very curious about all that.

EvilEye
2007-Oct-16, 04:16 AM
You are moving relative to what you see. (light)

Anything beyond that is inaccessable and unimportant.

You drive on the road at 55mph relative to the ground, which at the equator is moving at a speed, and the earth is going at a speed around the sun, and the solar system is moving around in the galaxy which is moving, and the ....

well...you get the idea.

Space is expanding at the speed of light, so tell your friends you are already time traveling.

Thanatos
2007-Oct-16, 05:25 AM
I like Tim Thompson's answer - 627 km/s wrt the CMB. That is about as independant a measure as you can get. The complicating factor is the CMB does not appear to be static. We cannot say with great certainty what it was a billion years ago. The expansion of the universe appears to be inhomogenous over time.

astromark
2007-Oct-16, 06:12 AM
So to be as precise as we obviously like to be... 627 km/sec. and continuing to accelerate.

Occam
2007-Oct-16, 06:41 AM
...Things much more distant are getting further away at much faster than the speed of light...
Really? How does that get measured? :whistle:

astromark
2007-Oct-16, 08:57 AM
Really? How does that get measured? :whistle:


If I might sagest... If a far distant Galaxy seems to fade and disappear. Has it ? Or has it simply red shifted right out of view, and now exceeds the speed of light so can never be seen again?
Is there evidence this has happened?

antoniseb
2007-Oct-16, 10:19 AM
Really? How does that get measured?
One way to think of the mainstream view is that little bits of new space are showing up everywhere all the time at the slow pace of 22 millimeters per second per lightyear. Given this, there is some distance away where objects are increasing their distance from us at the speed of light.

If there was a galaxy at this distance, some light would be emitted from it in our direction, traveling much faster than the local expansion rate, and would soon be in a region that is moving toward us at less than the speed of light, and so would eventually arrive at our location (significantly red-shifted).

I would recommend looking up clearer explanations, with pictures, which can be found easily enough with Google, or Wiki.

John Mendenhall
2007-Oct-16, 02:04 PM
By the way John, are you going to get back to the CERN answer to those "rockets on a string" question at some point? I'm very curious about all that.

I found it very difficult to not get hostile about that one, to say nothing of the possibility that I could be wrong. Let me work on it a while. I might even be daring and open an ATM string (pun intended).

Thanks for the CMB answer, it makes sense.

Ken G
2007-Oct-16, 02:38 PM
I say go for it, it could only lead to deeper relativity insights.

Ken G
2007-Oct-16, 02:47 PM
I like Tim Thompson's answer - 627 km/s wrt the CMB. The question was how fast are "we" moving, so that sounds like the Earth, and is more in the ballpark of a bit over 300 km/s. That includes various cancelling velocities in various directions, one of which could be chosen to get 627 km/s.
The complicating factor is the CMB does not appear to be static. We cannot say with great certainty what it was a billion years ago. The expansion of the universe appears to be inhomogenous over time.The expansion of the universe has little to do with either the 627 km/s or the 367 km/s numbers, they are all due to what would be called "peculiar" velocities that are unique to our vantage point, are not cosmological, and don't evolve according to any cosmological rules-- it's just our own little backyard of the cosmos. The acceleration effects the overall redshift of the CMB, not its variation in different directions due to our own peculiar motion.

Tim Thompson
2007-Oct-16, 03:48 PM
The question was how fast are "we" moving, so that sounds like the Earth, and is more in the ballpark of a bit over 300 km/s. That includes various cancelling velocities in various directions, one of which could be chosen to get 627 km/s.
As I noted in my earlier post, 627 km/s is reported by Kocevski & Ebeling, 2006 (http://adsabs.harvard.edu/abs/2006ApJ...645.1043K) as the velocity specifically of the Local Group towards the CMB dipole. They in turn reference Kogut, et al., 1993 (http://adsabs.harvard.edu/abs/1993ApJ...419....1K), which is the primary reference. Kogut, et al., give 627 +/- 22 km/s and do discuss the orbital motion of Earth. This is of course not a reference of any kind towards the expansion velocity of the universe, but can only be interpreted as the peculiar velocity of the barycenter of the Local Group with respect to the rest frame of the CMB.


I like Tim Thompson's answer - 627 km/s wrt the CMB. That is about as independant a measure as you can get. The complicating factor is the CMB does not appear to be static. We cannot say with great certainty what it was a billion years ago. The expansion of the universe appears to be inhomogenous over time.
The CMB is not static, but we certainly can say what it was, with "great" certainty, a billion years ago (with the caveat that "great" is subject to interpretation). Batistelli, et al., 2002 (http://adsabs.harvard.edu/abs/2002ApJ...580L.101B) use spectral measurements of the Sunyaev-Zeldovich effect (http://astro.uchicago.edu/sza/primer.html) on galaxy clusters to derive the CMB temperature as a function of redshift. They find T = 2.789 (+0.080 -0.065) Kelvins at redshift 0.0231 (313 million years ago) for the Coma Cluster (http://www.solstation.com/x-objects/coma-sc.htm), and T = 3.377 (+0.101 -0.102) at redshift 0.203 (2.44 billion years ago) for Abell 2163 (http://nedwww.ipac.caltech.edu/cgi-bin/nph-objsearch?objname=Abell+2163&extend=no&out_csys=Equatorial&out_equinox=J2000.0&obj_sort=RA+or+Longitude&of=pre_text&zv_breaker=30000.0&list_limit=5&img_stamp=YES). Their results are consistent with the redshift dependence that is required by standard cosmology.