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How far

  1. Apr 24, 2003 #1

    wolram

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    hi, has anyone calculated how far the earth traveles in say one year,not just orbiting the sun,i mean our galaxy is rotating and space is expanding also, how fast are we travelling? if there is all this dark matter in space why dont gas giants show some distortion to the direction of travel?
     
  2. jcsd
  3. Apr 24, 2003 #2
    The effect by comparatively homogeneous dark matter on gas giants or any other object smaller than the galaxy by 1010 gives negligible distortion due to gravitation. Velocity (inertial) effects from a dark matter medium would not arise for linear stellar motion.
     
  4. Apr 24, 2003 #3

    marcus

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    There is an absolute rest frame and people need to know the earth's direction and speed with respect to it in order to observe the CMBR.

    The motion is a few hundred km/sec. I don't remember but I will try to look it up. It is a vector sum of our galaxy's speed of travel and the sun's motion within the galaxy (it orbits galactic center) and then whatever the earth's orbital velocity vector happens to be at that time of year.

    Being absolutely at rest means with respect to the Hubble flow or with respect to the cosmic background. An observer who is not at rest will see a blue-shifted "hotspot" in the CMBR ahead of him and a red-shifted "coldspot" behind him, due to doppler. And indeed such is actually observed in the CMBR (and must be subtracted out to get the real picture) and this is how we know
    how fast and in what direction we are going wrt the restframe.
    Somebody else said essentially this on some other thread recently.

    But I would like to find an answer to your question. I think I recall that the combined velocity of the galaxy plus the velocity of the sun within the galaxy is a vector pointing in the direction of the constellation Leo and the speed is (heck, wild guess) 500 km/s.
    So you would multiply by the number of seconds in a year to get the kilometers per year. The earth's orbital speed around the sun is only 30 km/s so it does not perturb the main speed by more than around ten percent. I will try to get a better answer later.
     
  5. Apr 24, 2003 #4

    marcus

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    I found several sources on the web, not perfectly consistent

    http://www.opencourse.info/astronomy/introduction/36.universe_evolution/

    [[Analysis of the data indicates that we are moving at a speed of 390 km/s towards Leo.


    Taking into account our motion around the galaxy (horizontally to the right), this means that the entire galaxy must be moving at a speed of 600 km/s in the direction of Centaurus, somewhat closer to the center of this figure (in the green).]]

    I still have doubts and will try for something more authoritative.
    Earlier I found a 1994 announcement, but it was unclear whether they had subtracted out the sun's motion or were talking about combined motion. They say here the direction is in Leo, which sounds like the combined velocity, but they say the speed is something equivalent to 600 km/s, which sounds like the center-of-galaxy motion.

    http://hubblesite.org/newscenter/archive/1994/15/text


    Here is a quote from that possibly unreliable Hubblesite.org page

    [[Astronomers generally assume that the diffuse glow of microwave radiation left over from the Big Bang provides the backdrop or rest frame of the universe. In the mid 70's astronomers found that temperature of this radiation is slightly hotter towards the direction of the constellation of Leo.

    This effect has been interpreted to mean that the Milky Way is drifting with respect to the rest of the universe at about 380 miles per second in this direction. It has also been assumed that most of this motion is due to the gravitational attraction of more distant galaxies; however, these galaxies have never been positively identified.]]

    Still need to pursue this.
     
    Last edited: Apr 24, 2003
  6. Apr 24, 2003 #5

    wolram

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    thanks for link, there is somthing gnawing me about useing the microwave remnant from the bb to base calculation on, i just cant put it into words as yet, also the word "drifting" seems wrong, ah well a trip to the local may organise my thoughts
     
  7. Apr 24, 2003 #6

    marcus

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    I agree, don't like the word "drifting". Our galaxy and the local group to which it belongs are going hell-for-leather towards Centaurus. Your question got me interested so I continued surfing and found something more---in case you are interested, if not ignore it. There is a good graphic online:

    http://aether.lbl.gov/www/projects/u2/

    It shows the CMB dipole anisotropy against a map of stars
    with the "hot spot" right on constellation Leo.
    This is the best graphic way to communicate our motion
    relative to CMB that I've found.

    Relative to galactic center, as part of galactic rotation, the solarsystem is heading in direction of Cygnus (which also shows on their starmap) at speed of around 250 km/s.
    But this is partly canceled out because the galaxy as a whole is going some 600 km/s towards Centaurus (where there is a big attractor supercluster of galaxies) and the two directions are partly opposed. So the net result is that the solarsystem is moving in the Leo direction at around 370 km/s.
    These 3D vector addtions are potentially confusing and I cant find anywhere on web where things are presented simply and clearly. Also, different sites dont always agree on the speeds.
     
  8. Apr 24, 2003 #7

    wolram

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    all good info marcus, as you say its difficult to find agreement on this topic,"one that is fundamental to our understanding the universe", i am still searching.
     
  9. Apr 24, 2003 #8
    Marcus-

    Don't we have to assume some n-pole symmetries of the CMBR to determine the Earth's absolute peculiar velocity?
     
  10. Apr 24, 2003 #9

    marcus

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    I believe that is a caveat which I read at one site reporting COBE and MAP results. It said something like "we can not distinguish between earth motion and [intrinsic] bipolar anisotropy of CMB".
    Please do not hold me to absolute wording. But yes as far as I know yes. What the experts seem to be doing is attributing all the bipolar (aka "first order") anisotropy to the earths motion. Which amounts to assuming first order symmetry---as you might say.

    Maybe there is more to the story and eg Cragwolf knows, maybe you know more about it. I just naively assume the CMB is isotropic at that level and that the earth is going 370 km/s at Leo.

    I just found a website
    http://www.virtualstar.fsbusiness.co.uk/andromeda.htm
    that says that Andromeda is approaching MW at 120 km/s and
    is 2.3 million LY away and will collide in 5 billion years. And it gives an animated movie of the simulated splat and entanglement.
    They look like two croissants trying to copulate and I think "this is my galaxy doing this? this is where we live?"
    I guess its well known that Andr'a will collide with MW but seeing an animation...
     
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