How do we know where the stars really are?

[David Mc]

Here's one that's bugged me for awhile.

Light can be bent passing through the gravitaional field of an object in space.

If there is light from a star some X Million light years away and its course changes by 1 degree everytime it passes an object, how can an astronomer know that that a star is directly overhead if its light has passed by 45 objects on its way to Earth?

How do we know that it's not 45 degrees to the left or right of where it appears to be?

[Celestial Mechanic]

Gravity doesn't deflect light by very much. A light ray skimming the surface of the Sun is deflected by about 1.7 seconds of arc, and the deflection is even less farther away from the Sun.

You might want to check out this thread, Things Aren't Where They Seem ... for a discussion of the effect of various forms of aberration and how they affect the apparent positions of objects ranging from artificial satellites to distant galaxies.

[Faultline]

The majority of stars you see in the sky are part of our local neighborhood. They are close by and light from them rarely gets a chance to get deflected by the method that Celestial Mechanic mentioned.

[eburacum45]

This is something we have discussed a little over at OA; although I don't remember any of the exact numbers. If one imagines a very tight beam, non-divergent laser sending messages from one solar system to another (hey- this is a thought experiment- bear with me) where the stars are ten light years apart.
That beam might be aimed at a particular receiver on a particular planet on the other solar system in order to maximise the signal strength.
But where do you aim it? You know where the star was ten years ago, and you know the proper motion, so you should know the position of the star in ten years time.
But you don't- not really. The target star is being pulled this way and that by the gravity of the stars of the rest of the galaxy- this is a chaotic effect, and it means that you don't really know the precise location of the star and its planet after twenty years of proper motion. Your ultra-tight laser beam would almost certainly miss the target by a greater or lesser extent- and you would'nt know how accurate your aim was until twenty years after you sent your message.

This is important for one reason in particular; it means you can't send a time signal to a nearby star and synchronise your clocks exactly even if you attempt to compensate for the difference in velocities between the two stars. There really is no simultaneity between distant objects, and any attempt to establish a common time such as UTC would be literally impossible.

[Jeff Root]

David,

When light from a star is deflected by the gravity of something that
it is passing by, the amount of deflection depends on the nearness
of the encounter. Light passing at different distances is deflected
by different amounts. That means that the image you see will be
distorted. So you can get a pretty good idea how much the light
has been deflected from the degree of distortion. Most objects in
the night sky have no detectable distortion.

-- Jeff, in Minneapolis

[Squashed]

David,

When light from a star is deflected by the gravity of something that
it is passing by, the amount of deflection depends on the nearness
of the encounter. Light passing at different distances is deflected
by different amounts. That means that the image you see will be
distorted. So you can get a pretty good idea how much the light
has been deflected from the degree of distortion. Most objects in
the night sky have no detectable distortion.

-- Jeff, in Minneapolis

Is distortion how we can tell which image is real and which image is a copy of a gravitationally lensed object? I have seen pictures of gravitational lensing that included 4 or more images and I wondered how to tell which is what.

[ToSeek]

Is distortion how we can tell which image is real and which image is a copy of a gravitationally lensed object? I have seen pictures of gravitational lensing that included 4 or more images and I wondered how to tell which is what.

They're all "real" in the sense that they're all actual light from the object. In the cases I've seen, all the images are distorted - there's no unaffected light path.

[David Mc]

***1.7 seconds of arc***
Not much, unless it occurs at the beginning of a 500 million lightyear journey.
Do we really care about where they are, or is the make up and direction of a extremely distant object (or galaxy) all that matters now?

***The majority of stars you see in the sky are part of our local neighborhood. ***

So even if we get that 1.7 degrees from a pesky Dark object, it is (or was) just about where it appears to be now if it's a more "local" star. Right?

[antoniseb]

***1.7 seconds of arc***
So even if we get that 1.7 degrees

1.7 was kind of a worst case. Most stars positions are deflected by less than a milliarcsecond, though that will be taken into account by the computers handling the data from the Gaia project.

I'm getting, from what you're writing that somehow you want this to be a bigger issue than it is.

[Fazor]

Sounds to me like he wants to just show that the stars as we see them aren't at that moment where they look like they are.

But more than light bending is the time it took to get here. to the eye observing the stars in the sky (i.e. no computer modeling or whatever), I would say that yeah, they stars aren't where they look like they are. the farther the star is away, the more it has moved since it emited the light we're now seeing. Could be wrong...i usually am...but hey that's my 2 1/2 cents.

[jlhredshift]

Sounds to me like he wants to just show that the stars as we see them aren't at that moment where they look like they are.

But more than light bending is the time it took to get here. to the eye observing the stars in the sky (i.e. no computer modeling or whatever), I would say that yeah, they stars aren't where they look like they are. the farther the star is away, the more it has moved since it emited the light we're now seeing. Could be wrong...i usually am...but hey that's my 2 1/2 cents.

No, your not wrong. The pragmatist will say that we can only know what we can see and measure. This is true. If I see the original question in the OP as
a question of philosophy, I would say that a human can take avail of the marvelous ability to step out of 4D spacetime and see where the stars and galaxies might be and what they might have occurring "now" in what some have called "The Minds Eye", our imagination. One then could imagine a simultanity that is not allowed in our reality, and gaze upon all existance, to the farthest reaches, as they are in that imaginairy "now".

It' sorta like a child imagining what the first trip to Disneyland will be like. Ahh... the marvels of unfettered youth.

[Kaptain K]

the farther the star is away, the more it has moved since it emited the light we're now seeing.

...and the less it affects where the star appears to be.

[mugaliens]

The variations in angular distance are accounted for in the almanacs used by both aeronautical and marine navigators. While various bodies do deflect the light, the process is exceedingly slow, is thoroughly understood, and is well accounted for by the masters of navigation, the team at the US Naval Observatory, who publishes the almanacs.

Simple sight errors (accuracy of the sextants and the navigator) account for many orders of magnitude over an errors due to light-bending.

[Grashtel]

The variations in angular distance are accounted for in the almanacs used by both aeronautical and marine navigators. While various bodies do deflect the light, the process is exceedingly slow, is thoroughly understood, and is well accounted for by the masters of navigation, the team at the US Naval Observatory, who publishes the almanacs.

Simple sight errors (accuracy of the sextants and the navigator) account for many orders of magnitude over an errors due to light-bending.

Um, you really should check the dates of the thread that you are replying to, the last post before yours in this thread is almost a year and a half old so replying to it is kinda pointless.

[mugaliens]

Um, you really should check the dates of the thread that you are replying to, the last post before yours in this thread is almost a year and a half old so replying to it is kinda pointless.

So you're saying that questions arising in person's minds two years ago are no longer relevant?

Fine.

Let's throw out Einstein and the many thousands of other researchers beyond the 2-year barrier...





Uh.. DUH?

I could care less how "old" a thread might be. 1 month, 10 years.

Whoopee!

Bring it on. What in the world does "longevity" have to do with anything?

All it makes me think is that you're hiding behind some sort of veil...