Position of Stars, and the shape of the known Universe

Something has been puzzling me for some time now, is this:

We know how far other stars/galaxies are from Earth. From this, we have produced images of the shape of the milky way for example.

But as all objects in space are in motion, and accounting for the time it takes for light to reach us here on earth, how do we know the stars are where they are?

Let me clarify. For arguments sake, lets use the example of the Milky Way. It is apparently a spiral Galaxy.

But as it is some 100,000 light years across, the position of some stars are seen as they where eons ago, whereas some stars, the light has only taken a few years to reach us, yet we "know" the galaxy is a spiral, and have imaged it as such in many pictures.

With the motion of the stars at the same time, have we accurately positioned the stars where they actually are, rather than their relative position to Earth?

Has this been taken into account? I realise that I am not an astronomer, and this is probably an obvious thing to take into account.

But I never hear it mentioned.

Many times during my own work (Telecoms), we have overloooked something, when trying to solve a problem, because it is so glaringly obvious....

Well, everything is in motion, so exact positions are always changing. However, since the speed of light is a constant, it's leaving it's respective orgins and arriving at it's destination (our lenses) at the same rate. Therefore the "picture" we get is an accurate one.

Not too sure that you have understood me.

For example, take two stars, one at 10ly and another at 10,000 lys.

Both are moving at the same time also, but we would see the closer one nearer to its actual position than the one further away.
For example, the one that is 10k ly away is moving at 1ly every 1000 yrs. That means it would actually be 10lys away from the position we would see it at by the time the light reached us.

On the other hand, the closer one would appear to be more or less in its actual position as its light would reach us way before it could change its position in any great way.

When plotting the position of stars, are we taking this into account?

EDIT: The speed of light is not a constant. I know that much for sure.

[edit on 5/3/05 by stumason]

Yes, I see what you're saying.

You'd be correct if you thought of them as say two lightbulds that suddenly blinked on at the same time. We'd see the light from the first one long before the second. And in the meantime, you're saying, drastic movement would have occured in terms of distance, and you'd be right. However, since they were "blinked on" so to speak eons before we were here to start looking, the light from both has already reached us, and it is continually reaching us now - from both sources. So, the discrepancy in movement has already been accounted for. Now, since you add in the constant speed of light, the "relative" distance no longer changes. Or I should say, that it changes along WITH the light. So the picture is an accurate one. Make sense now?

Actually...correction, I still don't buy it...bear with me whilst I explain...

The two stars, albeit have been around for longer than we could observe them, are still sending us different information.

For example, the light from the first star at 10ly is only 10 yrs old. Hence, that is where it was 10yrs ago.

However, observing at the same time, a star from 10,000 lys away, we are seeing the star as of 10,000 yrs ago.

So the two images are of two different time frames.

The second, further away star's light that would be the same age as the first star's (and therefore give a relative position with regards to the first star) is in fact still 9,990 years away.

So whilst we look at the two different stars, we are seeing them at completely different points in time, and therefore different positions.

Are we mapping them based solely on observations that we get now.

For example, we see one at 10ly, and another at 10,000ly and plonk them on a map in corresponding positions as we see them today? Or do we take into account the differing time frames for both of the stars individually?

[edit on 5/3/05 by stumason]

What you are saying, or questioning, is how do we know where the stars actually are...right this second. First off, time is relative. That though still won't satisfy your question.

We locate stars as we see them. No, we don't locate them as to where they are today. If for instance, the star suddenly started moving faster than it normally does, we wouldn't be able to tell until that light reached us. Therefore, we can't actually say we know where they are, just where we see them.

So, assuming still that the stars (for arguments sake) don't speed up or slow down, but travel constantly, Astronomers are just plonking them on a map based solely on their relative distance from Earth, not taking into account their motion or the amount of time the light has taken to reach us?

Again...the picture (light/movement/time) has already happened. Long before you and I were around. The difference is, NOW were looking at it. So, working backwards from what we currently see through our lenses, we can determine the distances based on SOL calculations. By doing this for each major "solar body" and putting that information together, we arrive at the accurate picture.

Again...the picture (light/movement/time) has already happened. Long before you and I were around.

Yes. I understand that. Thats part of the reason why I ask this question.

he difference is, NOW were looking at it.

Now we are looking at a mozaic of different solar bodies at different points in time.

So, working backwards from what we currently see through our lenses, we can determine the distances based on SOL calculations. By doing this for each major "solar body" and putting that information together, we arrive at the accurate picture

Don't have a problem with distance either. Its what we are seeing.

Your statement (and thankyou for taking the time to add your viewpoint) does not explain my question. What we are getting is a distorted view based on our observations of many different celestial bodies based upon images from very different points in time.

Its all very well saying that they where around ages ago, so what we see now is the picture, but in relation to each other, and not just the earth, what we are seeing is a distorted view.

If it was possible to travel instantly to another star (lets call it A, the one 10ly from earth), and look upon another (B, the one 10kly), that star would not be in the same position, or might not even look the same as it would do from Earth, as the distance between them would be different.

So from Earth, star B has a position of X. But from Star A, that position could be X+10. And neither of those positions would be the actual position of star B.

If what you say is true, and we have mapped the stars to there actual positions. Then I should be able (with my stargate thingy, as mentioned above ) to travel to that position and find a star.
However, you must remeber, that from our point of view, that star was there 10000 years ago, and will by now have buggered off to somewhere else.

You have not answered my question on wether anyone has actually taken into account distance for the light to travel, and motion of the body during the time it has taken the light to travel to earth, and therefore accurately mapped the position of the body, or wether they have just measured the distance and plonked it 10,000 ly from earth in the relative position we see it today, not where it actually is.


Hope I have made myself clear.

Originally posted by stumason
So, assuming still that the stars (for arguments sake) don't speed up or slow down, but travel constantly, Astronomers are just plonking them on a map based solely on their relative distance from Earth, not taking into account their motion or the amount of time the light has taken to reach us?

It takes over 200 million years to make one revolution around milky way (at distance on solar system) so few (ten) thousands years doesn't matter much.

I think I understand well your question, but I think it is more or less a question without a definite answer.

Indeed the maps we draw of the sky represent what we see today, where we see the objects, and it is not necessarily their position. Those maps represent the position of the objects when they sent us some light. And they sent us some light at different times, depending on their distance to us.

So, basically, yes, the maps are accurate because it is what we see today. Also, no, they're not representing what is out there now. You're right in saying that because things have moved, and obviously farther objects have probably moved more, the maps are not representing the reality.

Let's say that, if for instance, we could travel at a speed far exceeding the speed of light, in a straight line, to an object that we see at say position X, when we would reach that position, the object would not be there anymore. It has since then moved.

Originally posted by SpookyVince


Let's say that, if for instance, we could travel at a speed far exceeding the speed of light, in a straight line, to an object that we see at say position X, when we would reach that position, the object would not be there anymore. It has since then moved.

Correct. BUT since everything has moved, including us, relative to every other moving body, the discrepancy is not apparent - nor does it really matter for observations sake.

Example: Suddenly we discover that EVERYTHING is in fact upside-down and not right-side-up! Does this make a difference? Not really, because it includes EVERYTHING. The difference is certainly there, but we wouldn't notice it, nor would it matter.

Originally posted by stumason

If what you say is true, and we have mapped the stars to there actual positions. Then I should be able (with my stargate thingy, as mentioned above

) to travel to that position and find a star.

Well, not really. Positions in space are measured by the body's position relative to other body's, not on an imagined superimposed "grid" (like your normal everyday map). In those we use 2D precise points because we know where our "start point" is (the poles, or the equator, or... etc.). In space, there aren't beginning & end points, so we have to measure positions as they are relative to other positions. But, back to my point, because we use that same type of measurement with all calculations (to form the picture), the picture is accurate.

Not sure if we'll ever get what we're trying to say to eachother, but I must say it's stimulating nonetheless.

[edit on 6-3-2005 by Partyof1]

[edit on 6-3-2005 by Partyof1]

[edit on 6-3-2005 by Partyof1]

Originally posted by Partyof1
(...)
Correct. BUT since everything has moved, including us, relative to every other moving body, the discrepancy is not apparent - nor does it really matter for observations sake.
(...)

You're assuming that all the objects are moving along the same path, in the same direction and at the same speed. This is not the case, not necessarily at least. Some objects are getting closer to each other, which makes the difference appear less important, some are getting farther, which makes the difference appear to increase.

The fact is that from our own reference point, the difference would appear different, depending on whether the object we are going to is going away or coming closer. What I meant by "traveling at a speed far exceeding the speed of light" should have been understood as something close to "if we could be there in a matter of seconds". Then, because we measure in our own reference system, the place where that object was observed and the place where it is now are not the same, and the difference (the distance) between where it is and where we see it is depending on the object's relative speed to us. This makes not the "discrepancy" as you call it not apparent, it makes it sometimes appear bigger than reality, sometimes smaller.

When the sun would stop burning, we still have about 6 minutes to get a tan, I would say that the discovery of the Doppler effect has been of the greatest importance for astronemers to "normalize" all those redshifted/blueshifted signals and produce consistent maps...

Thanks for the replys peeps! It has me puzzled on many a night shift, as does other questions I may or may not post here in the future.

Not that we are any closer to a definative answer, but it has shed some light on the subject.

I think the problem lies with the acceptance that astronomers have taken into account this problem. What I have a problem with is, it hasn't been shown to me that it has.

I understand the mechanics behind measuring the universe, what I do not know is wether the map of the Milky Way (or whatever) is a picture of now, then or a combination of "thens", as each stars light has taken a different amount of time to reach us.

If someone could get me an astronomer, and get him to say(and I want him to say it, I really do):

"Of course we took that into account! We measured the direction and speed of the star, took into account the distance and time taken for the light to travel to us on earth, and placed the star on the map in its position it is today"

That would answer my question, however, no one has come forth and said that, I have just had alot of info I (and everyone else) is probably already aware of.

However, if they have just been going:

"Star X is 10 ly's away, so it goes here, and star Y is 1000 ly's away and goes here..lalalala.....doesn't that look pretty! We will just ignore the stated fact that everything is apparently moving away from everything else, and draw a map of a static galaxy."

That would be different, as the stars further away would have had greater opportunity to change their apparent positions in the time it takes for light to travel to us, more so than the stars that are closer.

If that was the case, the picture of the universe could be different to the one that we paint today.

To help some understand better, since I think there is confusion still, I'm going to paint a little picture. Let's say I am in Florida and you are in Cuba. Florida is moving really fast to the west and Cuba is moving really fast to the East. Someone in the sky takes a picture. They now have a picture of where we are at the time of that picture. This is what we see now from our telescopes, etc. But then that someone in the sky looks back down and notices that Cuba is now somewhere near southern Europe and Florida is moving through Texas. This is what is actually happening in the Universe, but the picture doesn't show us that.

Hope that helps.

Originally posted by stumason
(...)
If someone could get me an astronomer, and get him to say(and I want him to say it, I really do):

"Of course we took that into account! We measured the direction and speed of the star, took into account the distance and time taken for the light to travel to us on earth, and placed the star on the map in its position it is today"
(...)

I am not an astronomer, barely an amateur at it, but I think I can clarify it a bit for your mind. The purpose of drawing a map is precisely to show is what is seen there. Therefore, sky maps show us what is there now in our sky, and not what is really out there. What we see is what we draw...

I must agree then with staggyD: the map is what we see now (I want to emphasize the emphasized text!). To be consistent with your own words, a sky map is in a way a combination of thens. Nearly nothing of what is shown on it is really there, but it is actually where we can see it.

I am not an astronomer, barely an amateur at it, but I think I can clarify it a bit for your mind. The purpose of drawing a map is precisely to show is what is seen there. Therefore, sky maps show us what is there now in our sky, and not what is really out there. What we see is what we draw...

I must agree then with staggyD: the map is what we see now (I want to emphasize the emphasized text!). To be consistent with your own words, a sky map is in a way a combination of thens. Nearly nothing of what is shown on it is really there, but it is actually where we can see it.

That is what I was thinking, hence the thread. We're going to have to do some re-caculations when we finally figure out Interstellar travel....

Potentially, the actual Universe could look very different....it may even be the shape of Woopie Goldbergs left Breast....you never know

I think I understand you. All the stars in the galaxy may actually be in a straight line and not spiraled. The spiral we see is an effect of delayed light sources reaching us at different times? In other words the star next to us is really there the star 90000lyr away may actually be on the other side of the galaxy instead of were we see it.\


weird to think about. I wonder what the galaxy would look like if we projected all the speeds and directions of all the stars and included the distance and time taveld of the light... May look entirely different.

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