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How do we know galaxies are exactly where we see them?

  1. Aug 14, 2015 #1
    i have a doubt, how do we know far away (light years) galaxies and planets are on the right spot we see them? if light takes a lot of time to reach the earth it means we are seeing the past of the galaxy/planet. Example: If Earth is becoming extinct and we need to find an exoplanet 30,000 light years away, when we are going to the direction we saw it on the telescope it would be there because we saw the past of the galaxy from earth and wouldn't be able to find it. can someone give me a solution? thanks
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  3. Aug 14, 2015 #2
    Another example can be gravitational lensing, a massive object of something that have a lot of gravitational pulse can bend light of where we see it, so we can see galaxies from another place in front of us, the universe is a very mysterious thing..... it could be that everything we see far away could be scrambled away from all places of the universe and is nerve-racking.
  4. Aug 15, 2015 #3


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    The further away from earth a celestial object, the less accurate is our estimate of how far away it is.

    For objects whose distances can be estimated by parallax, the estimates are pretty good. For objects much further away, parallax cannot be used to estimate distances, and other means, like measuring the magnitudes of certain stars or observing supernova explosions in distant galaxies, can provide rough estimates of distance, but the uncertainty is rather large.


    I wouldn't worry too much about navigating between stars and galaxies just yet. We have not returned to the Moon in almost half a century, and that body is only a quarter million miles away.
  5. Aug 15, 2015 #4


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    It's not that we would need to know where that planet is 'now', it's that we would have to calculate where it will be in the future in order to get the right trajectory to get to this planet. It's an unavoidable effect of the finite speed of light and spaceships.
  6. Aug 15, 2015 #5


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    When we look at a star we can also tell how fast it is moving with respect to us. This is divided into two types of motion, its proper motion ( at right angles to the viewing line) and its radial motion ( along the viewing line). So let's take a star Mu Cephei which is 6000 light years away. it has a proper motion of ~ 6 mas/yr ( a mas is a milli-arcsecond or 1/3600000 of a degree) and a radial motion of - 20.63 km/sec. So in the 6000 yrs since the light left it has moved 3.9e12 km closer to the Earth, or about 1/2424 of a light year. In that same 6000 years it also will have moved 1/100 of a degree in terms of proper motion.( where we see it now is 1/100 of degree from the direction it actually is in.)

    So if we were to head to that star at 0.5 light speed for example, we would aim for for a point 3/100 of a degree from where the star is. Of course, that would just be our initial aiming point. We would likely have to adjust our course slightly as we traveled to make up for inaccuracies in our initial measurements or course heading. But the main point is that the star is not going to have moved that much that we can't just point ourselves pretty much in the direction we see it in and then make minor course corrections along the way
    Last edited: Aug 15, 2015
  7. Aug 15, 2015 #6
    ohh, nice Drakkith i liked the "now" from where will it be in the future, however thanks!
  8. Aug 17, 2015 #7
    Galaxies move slowly compared to the speed of light. If we look deep deep into space, yeah, we have no idea where things might be now. There are Hubble pictures of galaxies 12 billion years ago, we couldn't hope to guess where that galaxy actually is now. Things closer to us, however, are much easier. Part of the reason it took humans so long to realize that the stars actually moved is because they do it so so slowly. If you went 10,000 years into the future, the night sky would look more or less the same. Some of the closest stars may have move a little, but they move at tens of thousands miles an hour, not a hundred of thousand a second like light does.
  9. Aug 22, 2015 #8


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    The part I've bolded in your post, I disagree with. Their proper motion relative to us is likely to be trivial compared to their recession velocity relative to us so we have quite a good idea where they are ... they are along the same line of sight (more or less) but a known amount farther away based on their farther recession due to to 12 billion more years of expansion.
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