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Stargazing Apparent vs actual position of stars

  1. Apr 14, 2017 #1
    When we ‘see’ the Sun, from here on Earth, what we are actually seeing is the Sun as it was over 8 minutes ago when the light we currently observe first embarked on its journey towards us. In that time the Sun has in fact arrived at a position about two degrees east of the place it now appears to be due to the rotation of the Earth that has taken place in the interim. Likewise, Saturn, being that much further than the Sun, is about twenty degrees east of where it appears in an Earth observers telescope. Whereas, an object about 173 times as far away as the Sun would appear in the same vicinity of the sky as it was 24 hours earlier.

    Based on this aberration of location due to Earth’s rotation and the limited speed of light, it would seem constellations are simply a happy coincidence which to my mind is conceivable. What I can not wrap my head around is why then do galaxies appear as organized clumps in the night sky instead of being smeared across the heavens much like our own Milky Way?

    Where has my thinking obviously crossed over into . . . ‘The Twilight Zone‘?
  2. jcsd
  3. Apr 14, 2017 #2


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    I will let others comment on your earlier questions

    but what I have quoted is incorrect ..... what makes you think the milky way is "smeared across the sky ?
    that comment couldn't be further from the truth but I am interested in your reasoning :smile:

  4. Apr 14, 2017 #3
    Perhaps it would help if I adjusted the focus knob a little better :wink:
  5. Apr 14, 2017 #4
  6. Apr 14, 2017 #5


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    You can easily get motion blur if you expose a photo of a moving object. Racing cars and athletes need fast shutter speeds. The same thing occurs (star trails) with long (seconds up to hours) exposures, due to the rotation of the Earth. But our eyes see sharp images. The relative (angular) motions of the other stars in our galaxy is very slow and taking photos, separated by hours / days / years doesn't show any change in position of the most distant astronomical objects. (Telescopes need to be mounted on an Equatorial mount, which takes out the rotation for Earth) Planets, comets and asteroids move around with respect to the 'fixed' stars. Your suggestion about 'smearing' has some legs, though but for a different reason than you suggest. We also get relative motion of nearby stars against the background of very distant stars as Earth orbits the Sun (moving along a baseline of 300 million km). This effect is called parallax and, over the year, the relative positions of near and far objects can be many seconds of arc. (Look up Parsec, which is a measure of astronomical distances). But any motion of very distant objects is very slow in angular terms and the explanation is purely down to geometry and not the delay time of the light reaching us.
    As to the point about the delay in what we see, all objects at the same distance have the same delay and when light from a distant galaxy passes by a nearby star, there is the same delay on the journey of both images to our telescope.
  7. Apr 14, 2017 #6


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    not really .... it's your term "smeared " that is giving the wrong idea

    our galaxy, the Milky Way, is no different to many other large spiral galaxies out there
    we are just seeing our galaxy from a different perspective .... namely from within it, rather than from millions of light years away

    take the Andromeda Galaxy (2.5 million light years away) ...... (not my photo)

    if you were on the outer edges of it, and looking towards it's centre, it would look much like what our galaxy does (give or take a bit) ..... (my photo)


  8. Apr 14, 2017 #7


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    I did wonder, for a minute or two. :wink:
  9. Apr 14, 2017 #8

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    This whole thread has started on the wrong premise. The position of an object on the sky due to the rotation of the earth has nothing to do with the light travel time.
  10. Apr 14, 2017 #9


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    You're overthinking. What matters is what the light looks like when it gets here: they send straight lines (rays) of light in all directions and it doesn't matter how long it took to get here, they are still straight. By they time they get to Earth, you have a sky saturated with parallel (in focus) rays for you to intercept where-ever your eye happens to be. The fact that your eye is moving doesn't change that, it just means your eye intercepts a different ray because it is in a different spot.

    But if you want motion blur, take a long exposure photo without a tracking mount!
  11. Apr 14, 2017 #10
    I realize my thinking must be skewed somehow given the way distant galaxies appear in one locality despite the varying distance of its individual light sources. I would just like to wrap my mind around why this is so given that more distant objects are not where they appear from the vantage point of our rotating platform. Suppose (ignoring defraction of the atmosphere for the sake of this discussion) I pulsed a laser at the apparent position of the Sun (as viewed say from the Equator during an equinox) that pulse would not arrive at the Sun's disc ~8.3 minutes later since the Earth has rotated two degrees to the east in the time the Sun's apparent position is observed. Ignoring the odd exception, distant objects are not where they appear and yet their relative positions (at the time of their light's origin) are somehow in large part retained. Perhaps what I've failed to reconcile is that for any give time frame everything else has actually turned by the same degree of rotation.

    What we see in an illusion albeit as Albert would say a persistent one. It's in understanding that which creates the illusion we come one step closer to grasping the reality.

    Thanks to everyone for helping me to resolve an apparent non-sequitur.
  12. Apr 14, 2017 #11
    Galaxies really are 'organized clumps'.
    Andromeda is visible to naked eye (though you do need good eyes), most galaxies are not, fine instruments are needed.
    Constellations on the other hand are just recognizable patterns of stars, like 'Orion's belt', the 'big dipper', or 'the plough',
    Those stars are within our own galaxy but unrelated to each other, just a recognizable pattern.
    These patterns proved to be very useful to early navigators trying to estimate where they were on the Earth,
    and of course there are the mystical interpretations as well. which make no sense because they make no sense,
    Last edited: Apr 14, 2017
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