Apparent vs actual position of stars

[Dmstifik8ion]

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‘?

 

[davenn]

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?

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 Dave

 

[Dmstifik8ion]

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 Dave

Perhaps it would help if I adjusted the focus knob a little better

 

[Dmstifik8ion]

http://thetechnews.com/wp-content/uploads/2016/06/milky-way-wallpaper-24.jpg

That better?

 

[sophiecentaur]

smeared across the heavens

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.

 

[davenn]

http://thetechnews.com/wp-content/uploads/2016/06/milky-way-wallpaper-24.jpg

That better?

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)

Dave

 

[sophiecentaur]

. (not my photo)

I did wonder, for a minute or two.

 

[Vanadium 50]

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.

 

[russ_watters]

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‘?

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!

 

[Dmstifik8ion]

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.

 

[rootone]

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,

 

[Leos Friend]

Here's where it gets a little quirky for me: some galaxies are 100K or 200K light years across. Let's say we're observing a galaxy that's 2.5 million ly away [on "center"]. If positioned at a certain angle, the stars we see at the closest edge actually reflect their radial positions 2.4 million-years in the past.

Yet, the stars from the far edge are actually their radial positions 2.6 million-years in the past. So in that sense, what we're seeing is a bit of a jumble (and more so for the hinders). What we see is in no way a "snapshot" of what the galaxy really looks like. In fact, if the radial speeds of all the stars were the same [which they aren't], the apparent positions of the stars toward the back are 200K years more removed from their true [real-time] positions than those we are seeing from the front. The front edge is a "more accurate depiction" than the back edge.

If tessering were a reality, the farther we go in an apparent straight line, the greater the curve [or wobble] from start to finish.

 

[sophiecentaur]

Here's where it gets a little quirky for me:

Is it any more quirky than the experience we have of the apparent position of a passing fast aircraft does not coincide with the direction that its sound appears to come from, due to delay in the path of the sound? If you listen to the sounds of two aircraft on a similar bearing, you could get the impression that they are on different bearings.

 

[mjc123]

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.

Not so. Suppose the Sun emits a photon when the observer on Earth is at position A, and it reaches Earth when he is at position B. Because it has traveled in a straight line, the direction of the Sun is that where the photon appears to come from, and sending a laser beam back in that direction will hit the Sun, because the Sun has not moved its actual position due to the rotation of the Earth. (Its apparent position moves slightly due to the Earth orbiting the Sun, but in 8 minutes that movement is about 1/200 of a degree, much smaller than the size of the Sun's disk.)

To look at it another way: At all points in its journey, the photon is in a straight line between Sun and Earth, thus in a frame where Earth is not rotating and the Sun is moving around the Earth, the photon's path is not a straight line, and when it arrives at Earth it appears to be coming from the direction the Sun now is (B), not where it was when the photon was emitted (A). Similarly, a photon sent in the direction of B will appear to follow a curved path and hit the Sun at its apparent position in 8 minutes.

 

[russ_watters]

Here's where it gets a little quirky for me: some galaxies are 100K or 200K light years across. Let's say we're observing a galaxy that's 2.5 million ly away [on "center"]. If positioned at a certain angle, the stars we see at the closest edge actually reflect their radial positions 2.4 million-years in the past.

Yet, the stars from the far edge are actually their radial positions 2.6 million-years in the past. So in that sense, what we're seeing is a bit of a jumble (and more so for the hinders). What we see is in no way a "snapshot" of what the galaxy really looks like.

Your thought experiment is missing a key component: how fast are the stars moving in their orbits around the galaxy center? Or, rather, in those 200,000 years, how far do they move in their orbits? I suspect you will find, that distortion you are referencing (which actually is real), is a lot smaller than you think.

 

[sbrothy]

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‘?

Lightspeed delay and standard candles should surely be factored into your calculations or your observations may end up ambiguous or just basically puzzling. The story of this particular star "lightens" this vividly:

"Due to the uncertainty in the value, this age for the star may or may not conflict with the calculated age of the Universe as determined by the final 2015 Planck satellite results of 13.799 ± 0.021 billion years."
- - - - https://en.m.wikipedia.org/wiki/HD_140283

 

[sophiecentaur]

There is no 'actual position' of anything. Position is totally dependent on its relative direction and distance from each observer. Here is a story that Einstein quotes in his short book on special relativity. I looked for a copy of the image in the book but I can't find it but the following was a copy of a copy of an original tale:

Einstein’s Theory of Relativity came to him in a dream about cows. He dreamed he was walking through a farm when he came upon some cows by an electric fence. He then saw the cows jump at the same time as the fence gave them an electric shock. But a farmer, who had been standing at the other end of the field, saw them jump one by one, like a Mexican wave. Einstein realized their views of the same event had been different. This lead to the Theory of Relativity, the idea that events look different depending on where you’re standing because of the time it takes the light to reach your eyes.
(this assumes the Farmer switched on the fence at his end and the pulse travels at c)

If the galaxy is rotating then the stars' positions and progress round the orbit will look different from here, compared with where they appear from somewhere else. Neither observation can be 'actual'. Nearest thing would be to observe the galaxy from somewhere on it axis of rotation (at a great distance).