Stellar aberation, a One way measurement of c?

[Tracer]

Why isn't Stellar aberation considered to be a one way measurement of c?

if the angle of aberation (Theta) is 20.5 arcseconds and the Earth's orbital speed is 29.79 Kilometers/second normal to the arriving star light. The value of c should be:
c= Vt/tan(Theta) = 29.79/9.94E-5 = 299737.98 Kilometers/second.

If more acurate values of Vt and Theta are used why would this not confirm a one way measurement of c?

 

[Bill_K]

Sure, in fact the guy who discovered stellar aberration, James Bradley, used his measurements to determine a more accurate value for the speed of light. This was in 1725.

 

[Tracer]

Sure, in fact the guy who discovered stellar aberration, James Bradley, used his measurements to determine a more accurate value for the speed of light. This was in 1725.

Yet there are persistent comments claiming that the measurement of the One Way speed of light is not possible.

 

[ghwellsjr]

Yet there are persistent comments claiming that the measurement of the One Way speed of light is not possible.

Yes, many, starting with Einstein.

 

[Bill_K]

Anyone who claims the one-way speed of light cannot be measured is, to put it mildly, wrong. We learn as freshmen that the speed of light was first determined by Romer's observations of the moons of Jupiter. It's true that many determinations are round-trip, but that's for added accuracy.

 

[ghwellsjr]

From the wikipedia article on the One-way speed of light:

Rømer's measurement

The first experimental determination of the speed of light was made by Ole Christensen Rømer. It may seem that this experiment measures the time for light to traverse part of the Earth's orbit and thus determines its one-way speed, however, this experiment was carefully re-analysed by Zhang, who showed that the measurement does not measure the speed independently of a clock sychronization scheme but actually used the Jupiter system as a slowly transported clock to measure the light transit times.

 

[Bill_K]

I agree that measuring the one-way speed of light requires clock synchronization. That does not mean it can't be done, or that there is anything suspect about the result. Without the use of clock synchronization, solar system observations would make no sense.

 

[ghwellsjr]

Do you also agree with this quote from the same article:

The one-way speed

Although the average speed over a two-way path can be measured, the one-way speed in one direction or the other is undefined (and not simply unknown), unless one can define what is "the same time" in two different locations. To measure the time that the light has taken to travel from one place to another it is necessary to know the start and finish times as measured on the same time scale. This requires either two synchronized clocks, one at the start and one at the finish, or some means of sending a signal instantaneously from the start to the finish. No instantaneous means of transmitting information is known. Thus the measured value of the average one-way speed is dependent on the method used to synchronize the start and finish clocks. This is a matter of convention.

 

[Bill_K]

All right, I guess I see the point. Stellar aberration basically measures the ratio v/c where v is the Earth's orbital velocity. But the determination of v rests on the observed Doppler shift of stellar spectral lines. Which in fact only gives you v/c also, so the argument is circular.

However I still maintain that the limitation is only a practical one. In principle one could determine the Earth's velocity directly from Kepler's laws, by measuring the astronomical unit with a meter stick and counting the number of atoms in the sun!

 

[harrylin]

All right, I guess I see the point. Stellar aberration basically measures the ratio v/c where v is the Earth's orbital velocity. But the determination of v rests on the observed Doppler shift of stellar spectral lines. Which in fact only gives you v/c also, so the argument is circular.

However I still maintain that the limitation is only a practical one. In principle one could determine the Earth's velocity directly from Kepler's laws, by measuring the astronomical unit with a meter stick and counting the number of atoms in the sun!

Kepler's laws are classical. How would you measure v= (x2-x1)/(t2-t1) without clocks and without knowing the speed of the sun?

 

[Bill_K]

harrylin, I see there's another long thread elsewhere on this same topic, so further discussion should probably be done there. If a discussion is going to go anywhere, it's important to read the previous comments carefully, or else most of it degenerates into misunderstandings of the form "I didn't say that".

"Kepler's laws are classical." - the orbital equations of motion, if you must. But the situation we are talking about, the Earth's orbit about the sun, is classical. Relativistic corrections are completely negligible.

"without clocks" - Without clocks?? You can't time anything without clocks. I said the 'slow transport' of synchronizing clocks was a good one. However it's not relevant. To determine the orbital velocity of the Earth, we only need to (hypothetically) measure the radius or circumference of the orbit with a meter stick, and time the period with an Earthbound clock.

"without knowing the speed of the sun" - this is all done in the sun's reference frame.

 

[TrickyDicky]

What I fail to understand is why the stellar aberration formula gives us the ratio between the Earth's speed and c precisely in the sun's reference frame. What is special about the sun reference frame in this context?

 

[TrickyDicky]

Stellar aberration is explained by SR as being due to the relative motion between the Earth and the distant star the aberrated light is sent from. But I don't understand why in the usual formula for stars at 90º from our observation point: tan theta=v/c, v should refer to Earth's speed wrt the sun, what does the sun have to do with the relative motion between the Earth and a remote star?
Also why are all astronomical charts corrected for the motion of the Earth and not for the motion of the light source which in the case of some binaries is well known and important enough to produce significant corrections?

 

[PAllen]

Stellar aberration is explained by SR as being due to the relative motion between the Earth and the distant star the aberrated light is sent from. But I don't understand why in the usual formula for stars at 90º from our observation point: tan theta=v/c, v should refer to Earth's speed wrt the sun, what does the sun have to do with the relative motion between the Earth and a remote star?
Also why are all astronomical charts corrected for the motion of the Earth and not for the motion of the light source which in the case of some binaries is well known and important enough to produce significant corrections?

We don't know the 'true' angular position of stars. We observe, for example, a seaonal variation in the position. Independent of the motion of the solar system as a whole, the seasonal variation depends only on the Earth's orbital speed.

 

[harrylin]

harrylin, I see there's another long thread elsewhere on this same topic, so further discussion should probably be done there.
[..]
"without knowing the speed of the sun" - this is all done in the sun's reference frame.

OK, I'll just end with putting attention to the fact that "in the sun's reference frame" the one-way speed of light relative to that same reference frame is defined as equal to the two-way speed - so we came full circle... The whole concept of local time arose from the fact that we can equally well assign a velocity vector to the sun, in which case the speed of light is assumed to be isotropic relative to another reference system and anisotropic relative to the sun.

 

[harrylin]

Stellar aberration is explained by SR as being due to the relative motion between the Earth and the distant star the aberrated light is sent from. [..]

Hmm not exactly, but this is a common misunderstanding.

The relative motion of Earth and star matters for the calculation of the Doppler effect. However, according to SR the motion of a distant star is irrelevant for aberration (second postulate!) and SR doesn't really try to explain - it just predicts the necessary consequences of the postulates.

Stellar aberration is predicted to be an effect from the change of motion of the Earth and the effect can be calculated relative to any freely chosen inertial reference system. The Sun is most convenient.

 

[TrickyDicky]

Stellar aberration is predicted to be an effect from the change of motion of the Earth

only the Earth's motion counts then? then what is "relativistic" about stellar aberration?
I guess the incoming light ray is considered to be at rest. And we are calculating the transverse velocity of the Earth wrt incoming star's light.
Here's how this is explained in wikipedia:
"stellar aberration is independent of the distance of a celestial object from the observer, and depends only on the observer's instantaneous transverse velocity with respect to the incoming light beam, at the moment of observation. The light beam from a distant object cannot itself have any transverse velocity component, or it could not (by definition) be seen by the observer, since it would miss the observer. Thus, any transverse velocity of the emitting source plays no part in aberration. Another way to state this is that the emitting object may have a transverse velocity with respect to the observer, but any light beam emitted from it which reaches the observer, cannot, for it must have been previously emitted in such a direction that its transverse component has been "corrected" for. Such a beam must come "straight" to the observer along a line which connects the observer with the position of the object when it emitted the light"

and the effect can be calculated relative to any freely chosen inertial reference system. The Sun is most convenient.

I still can't see it. How exactly is the sun chosen as a reference for the Earth's speed relative to the distant star light ray received on the earth? How is that implicit in the formula Tracer mentioned in the OP?
The Earth is also moving wrt the galaxy and wrt the CMB with different speeds, why is precisely the Earth's orbital speed that shows up in that formula?
The wikipedia article mentions this as Secular aberration, and says it is difficult to observe and therefore ignored, but considering a extragalactic star if our galaxy is moving 600 km/s, shouldn't that speed show up in the calculations?

 

[harrylin]

only the Earth's motion counts then? then what is "relativistic" about stellar aberration?
[..]
I still can't see it. How exactly is the sun chosen as a reference for the Earth's speed relative to the distant star light ray received on the earth?

We can choose an inertial system in which the star is in rest (as Einstein did for his derivation) or one in which the sun is in rest (as is commonly done) or any other inertial reference system, it doesn't matter: the predicted observed aberration from Earth will still be exactly the same.

I guess the incoming light ray is considered to be at rest.

Something went wrong here... perhaps you meant the star? But see my remark here above, we may pretend any inertial frame to correspond to a virtual light medium.

And we are calculating the transverse velocity of the Earth wrt incoming star's light.
Here's how this is explained in wikipedia:
"stellar aberration is independent of the distance of a celestial object from the observer, and depends only on the observer's instantaneous transverse velocity with respect to the incoming light beam, at the moment of observation. [..]"
[..]
The Earth is also moving wrt the galaxy and wrt the CMB with different speeds, why is precisely the Earth's orbital speed that shows up in that formula?

Wikipedia is inaccurate; I think that it was better last time that I looked at it. PAllen said it better: we observe a seasonal variation. That's why the orbital speed shows up in the formula. Zero aberration would be observed at constant observer velocity.

[..] considering a extragalactic star if our galaxy is moving 600 km/s, shouldn't that speed show up in the calculations?

No. The speed of the light rays coming from that star is not affected by its motion, and the angular change of position over time is negligible for distant stars. For sure this is why Einstein put "infinitely distant source of light", to avoid discussing such effects.

 

[TrickyDicky]

We can choose an inertial system in which the star is in rest (as Einstein did for his derivation) or one in which the sun is in rest (as is commonly done) or any other inertial reference system, it doesn't matter: the predicted observed aberration from Earth will still be exactly the same.

This is the part I have difficulties reconciling with SR, I thought only light's speed was constant regardless the reference system.

 

[harrylin]

This is the part I have difficulties reconciling with SR, I thought only light's speed was constant regardless the reference system.

I don't understand the problem... so I'll do a shot in the dark.

The most essential feature of SR is the relativity principle, which originally was formulated as the impossibility to detect absolute (uniform) motion. An alternative formulation is that the laws of nature (such as about observed aberration) must be independent of the inertial reference system that one chooses for the calculations.

If for example the sun would correspond to a preferred frame for the laws of aberration as observed from Earth, then that would break the relativity principle. Instead, it is only "preferred" for convenience (simplicity) of calculation - just as it also simplifies mechanical calculations of the orbits of planets (more precisely, the solar system's centre of mass frame).

 

[Ken More]

There are at least three different models for predicting stellar aberration that are taught at mainstream universities or promoted by government organizations. Three of the most commonly discussed models are: James Bradley's Falling Rain model, Einstein's 1905 Doppler Stellar Aberration Model and the Stellar Aberration model used by the US Naval Observatory's NOVAS software to predict the location of a given star at a given future date and time. Although all three of these models claim to be able to predict the location of a given star to within a few arc seconds (say 3 or 4 arc seconds); the predictions of these three models do not agree. In fact, their predictions for a star when it is in the plane that is perpendicular to the Earth's velocity vector disagree by more than 20 arc seconds.

Does this disagreement between the predictions of models supported by the mainstream bother any of you "Physicists" that post to the Physics Forum. If it bothers you, you may be interested in learning more about this disagreement at: http://www.ken-more.com/Stellar_Aberration.html

 

[harrylin]

There are at least three different models for predicting stellar aberration that are taught at mainstream universities or promoted by government organizations. Three of the most commonly discussed models are: James Bradley's Falling Rain model, Einstein's 1905 Doppler Stellar Aberration Model and the Stellar Aberration model used by the US Naval Observatory's NOVAS software to predict the location of a given star at a given future date and time. Although all three of these models claim to be able to predict the location of a given star to within a few arc seconds (say 3 or 4 arc seconds); the predictions of these three models do not agree. In fact, their predictions for a star when it is in the plane that is perpendicular to the Earth's velocity vector disagree by more than 20 arc seconds.

Does this disagreement between the predictions of models supported by the mainstream bother any of you "Physicists" that post to the Physics Forum. If it bothers you, you may be interested in learning more about this disagreement at: http://www.ken-more.com/Stellar_Aberration.html

The classical "falling rain model" has been replaced by special relativity. If there is a refined model model that slightly differs from that of SR, I would expect it to include a GR correction accounting for the gravitational bending of light - and that does not bother me all.

Alternatively, if that isn't the case, there could be an error in that model and I think that that would bother astronomers.

So, could you please summarize in one line what you think is the cause?

 

[Ken More]

The classical "falling rain model" has been replaced by special relativity. If there is a refined model model that slightly differs from that of SR, I would expect it to include a GR correction accounting for the gravitational bending of light - and that does not bother me all.

Alternatively, if that isn't the case, there could be an error in that model and I think that that would bother astronomers.

So, could you please summarize in one line what you think is the cause?

The SR model predicts that a star near the ecliptic pole and in the plane that is perpendicular to the Earth's velocity vector has a declination aberration of over 20 arc seconds while the NOVAS model predicts aberration close to zero. There is not any significant gravitational bending due to the Earth's gravity and the gravitational bending from the Sun is very small (less than one arc second) for stars near the ecliptic pole. The NOVAS model's predictions of aberration for stars near the ecliptic pole and in the plane that is perpendicular to the Earth's velocity vector is close to zero and actual telescopic observations verify that aberration is close to zero when the star in the plane that is perpendicular to the Earth's velocity vector.

The issue here is whether there is a problem with the SR stellar aberration model's predictions or a problem with the NOVAS model's predictions that agree with empirical telescopic observations.

So, could you send me in one line what you think is the reason that the SR model (with or without GR corrections) predicts aberration close to 20 arc seconds for all stars near the ecliptic pole while their aberration is observed to be near zero when in the plane that is perpendicular to the Earth's velocity vector.

 

[PAllen]

The SR model predicts that a star near the ecliptic pole and in the plane that is perpendicular to the Earth's velocity vector has a declination aberration of over 20 arc seconds while the NOVAS model predicts aberration close to zero. There is not any significant gravitational bending due to the Earth's gravity and the gravitational bending from the Sun is very small (less than one arc second) for stars near the ecliptic pole. The NOVAS model's predictions of aberration for stars near the ecliptic pole and in the plane that is perpendicular to the Earth's velocity vector is close to zero and actual telescopic observations verify that aberration is close to zero when the star in the plane that is perpendicular to the Earth's velocity vector.

The issue here is whether there is a problem with the SR stellar aberration model's predictions or a problem with the NOVAS model's predictions that agree with empirical telescopic observations.

So, could you send me in one line what you think is the reason that the SR model (with or without GR corrections) predicts aberration close to 20 arc seconds for all stars near the ecliptic pole while their aberration is observed to be near zero when in the plane that is perpendicular to the Earth's velocity vector.

Can you provide any reference for this alleged discrepancy between telescope observations and SR abberation model besides your personal website? If not, this discussion is in gross violation of physicsforum rules.

 

[Ken More]

My "personal website" has all of the references and explains the problems with the state-of-the-art of stellar aberration prediction models. The reference to the SR model can be found in: "Einstein, A. 1905, On the Electrodynamics of Moving Bodies, revised and translated in The Principle of Relativity, Dover, NY, 1923, pp. 35-65". You can apply Einstein's Doppler Stellar aberration model to get its prediction of the aberration of stars near the Ecliptic pole. The reference to the equation used in the NOVAS model can be found in: The Astronomical Almanac 2010, Washington: U.S. Government Printing Office, p. B28 you can apply this equation to do the calculations of stars when they are in the plane that is perpendicular to the Earth's velocity vector or you can download a free C++ or Fortran version of the NOVAS software that uses this equation shown in the 2010 Astronomical Almanac and have it do the calculations of stars when they are in the plane that is perpendicular to the Earth's velocity vector.

 

[PAllen]

My "personal website" has all of the references and explains the problems with the state-of-the-art of stellar aberration prediction models. The reference to the SR model can be found in: "Einstein, A. 1905, On the Electrodynamics of Moving Bodies, revised and translated in The Principle of Relativity, Dover, NY, 1923, pp. 35-65". You can apply Einstein's Doppler Stellar aberration model to get its prediction of the aberration of stars near the Ecliptic pole. The reference to the equation used in the NOVAS model can be found in: The Astronomical Almanac 2010, Washington: U.S. Government Printing Office, p. B28 you can apply this equation to do the calculations of stars when they are in the plane that is perpendicular to the Earth's velocity vector or you can download a free C++ or Fortran version of the NOVAS software that uses this equation shown in the 2010 Astronomical Almanac and have it do the calculations of stars when they are in the plane that is perpendicular to the Earth's velocity vector.

The issue is you're the one applying SR aberration in the way you think is correct, and claiming a discrepancy. I have a guess where your misunderstanding might be, but since you are claiming the discrepancy as fact, you must provide a reference to a reputable source describing the discrepancy. Otherwise, even if you are right (not likely), this constitutes original research at odds with peer reviewed opinion. It is therefore not allowed to be discussed here at all - unless you provide a reference for there being a discrepancy.

Note, for example, there are known anomalies in respect to expected results from GR. These can be discussed here because there are peer reviewed discussions of these discrepancies.

 

[zonde]

The SR model predicts that a star near the ecliptic pole and in the plane that is perpendicular to the Earth's velocity vector has a declination aberration of over 20 arc seconds

Am I missing something? Because ecliptic pole is always on the plane that is perpendicular to the Earth's velocity vector.
So obviously all stars near ecliptic pole have aberration at all times. The only change is in what direction their apparent position has shifted.

while the NOVAS model predicts aberration close to zero. There is not any significant gravitational bending due to the Earth's gravity and the gravitational bending from the Sun is very small (less than one arc second) for stars near the ecliptic pole. The NOVAS model's predictions of aberration for stars near the ecliptic pole and in the plane that is perpendicular to the Earth's velocity vector is close to zero and actual telescopic observations verify that aberration is close to zero when the star in the plane that is perpendicular to the Earth's velocity vector.

The issue here is whether there is a problem with the SR stellar aberration model's predictions or a problem with the NOVAS model's predictions that agree with empirical telescopic observations.

So, could you send me in one line what you think is the reason that the SR model (with or without GR corrections) predicts aberration close to 20 arc seconds for all stars near the ecliptic pole while their aberration is observed to be near zero when in the plane that is perpendicular to the Earth's velocity vector.

This (bold part) is plain wrong as far as I know.
For example if we look in wikipedia Aberration of light figure 4 shows apparent shift in star's position. A star near ecliptic pole has ecliptic latitude around 90° and it "draws" circle during a year.

Maybe you have mixed ecliptic plane with ecliptic pole somewhere along the way?

 

[Dale]

We don't know the 'true' angular position of stars.

I think that this is the key point. Abberation is a difference between a "true" and an observed angle. The angle is frame variant, so the designation of the "true" angle requires specification of some frame as being the frame whose angle represents truth. That is a matter of convention, the usual convention being the frame in which the sun is at rest.

 

[Ken More]

Am I missing something? Because ecliptic pole is always on the plane that is perpendicular to the Earth's velocity vector.
So obviously all stars near ecliptic pole have aberration at all times. The only change is in what direction their apparent position has shifted.


This (bold part) is plain wrong as far as I know.
For example if we look in wikipedia Aberration of light figure 4 shows apparent shift in star's position. A star near ecliptic pole has ecliptic latitude around 90° and it "draws" circle during a year.

Maybe you have mixed ecliptic plane with ecliptic pole somewhere along the way?

Zonde, you are not missing anything. A star near the ecliptic pole is always near the plane that is perpendicular to the Earth's velocity vector. Now can you tell me when annual aberration is zero for a star? Is it only when the star's vector is in the ecliptic plane or can a star above the ecliptic plane have zero aberration at any time of the year?

 

[zonde]

Zonde, you are not missing anything. A star near the ecliptic pole is always near the plane that is perpendicular to the Earth's velocity vector. Now can you tell me when annual aberration is zero for a star? Is it only when the star's vector is in the ecliptic plane or can a star above the ecliptic plane have zero aberration at any time of the year?

Aberration for a star is zero only when Earth is moving in direction of that star. That of course can happen only for a star that is in ecliptic plane.

 

[Ken More]

Aberration for a star is zero only when Earth is moving in direction of that star. That of course can happen only for a star that is in ecliptic plane.

So, are you saying that declination aberration is zero only when declination (δ₀) is 0° and right ascension (α₀) is 0°? If you are saying this, then it is my openion that this disagrees with the Reduction for Annual Stellar Aberration equation: Aberration (δ - δ₀) = - x/c.cos α₀.sin δ₀ that is on page B28 in the 2010 Astronomical Almanac where x is the Earth's velocity and c is the velocity of light. When I do the math, declination aberration is zero when α₀ = 90° and δ₀ = 0° as well as when right ascension (α₀) is 0° and δ₀ = 0°. Also, since cos α₀ is always zero when α₀ = 90° then declination aberration is always zero for any declination (δ₀ from 0° to 90°) and this occurs for all stars three months from the time when α₀ = 0° when the star is in the plane that is perpendicular to the Earth's velocity vector.

If you or anyone else at Physics Forum has enough interest in this subject to get access to a copy of the 2010 Astronomical Almanac and check out my interpretation of its Reduction for Annual Stellar Aberration equation then I would be very grateful if you can tell me where I am going wrong.

 

[zonde]

So, are you saying that declination aberration is zero only when declination (δ₀) is 0° and right ascension (α₀) is 0°?

No, I was not talking about "declination aberration", I was talking about "aberration".

Astronomers probably care much more about "declination aberration" because that's much easier to measure than "right ascension aberration". So they might (I guess) shorten "declination aberration" for "aberration" but that would not be correct of course.

 

[Ken More]

No, I was not talking about "declination aberration", I was talking about "aberration".

Astronomers probably care much more about "declination aberration" because that's much easier to measure than "right ascension aberration". So they might (I guess) shorten "declination aberration" for "aberration" but that would not be correct of course.

zonde, I am very impressed! I believe you understand aberration better than anyone with whom I have ever discussed the subject. Since you are apparently an advanced stellar aberration guru, I would like to know if you think Einstein's Doppler Stellar Aberration equation in his 1905 paper "On the Electrodynamics of Moving Bodies" yields an accurate estimate of "declination aberration" or "right ascension aberration" or "intrinsic aberration" or neither. I would also like to know if you know who has an accurate model for estimating both "declination aberration" and "right ascension aberration" other than the USNO as described in the 2010 Astronomical Almanac?

 

[Ken More]

No, I was not talking about "declination aberration", I was talking about "aberration".

Astronomers probably care much more about "declination aberration" because that's much easier to measure than "right ascension aberration". So they might (I guess) shorten "declination aberration" for "aberration" but that would not be correct of course.

zonde, I will assume that you are acknowledging that astronomers can measure "right ascension aberration" as well as "declination aberration". Can they also measure "aberration" (which I will call "intrinsic aberration" henceforth)? Also, can you please tell me what kind of aberration is predicted by the Doppler Aberration equation that is in Einstein's 1905 paper "On the Electrodynamics of Moving Bodies"?

 

[PAllen]

Einstein's paper deals with total aberration, which is the combination of declination and ascension aberration (as a total displacement angle). I assume astronomers can detect both aberrations now. Having reviewed astronomic coordinates, I notice that stars at certain positions on the ecliptic will never undergo seasonal declination aberration; they will only have right ascension aberration. Stars at 90% on the ecliptic to these will have both aberrations, but the declination aberration will be smaller than the ascension aberration. Of course, for stars on the ecliptic, there will be no aberration at all twice a year.

For a star perpendicular to the ecliptic, there will always be aberration (of constant magnitude relative to position in a solar system frame), but twice a year it will include no declination aberration (it will be pure ascension aberration).

Stars in between, will have varying total aberration, and will also have no declination aberration twice a year.

Finally, I don't think any of this has to do with relativistic aberration per se. The size of the relativistic correction to classical aberration is, last I checked, undetectable for Earth's orbital speed. My guesstimate for the correction due to the difference between classical and relativistic aberration for seasonal aberration from Earth's motion would be of the order .002 arcseconds. Thus, none of this discussion has anything to do with special relativity. It only relates to the finite speed of light (Galilean versus Special relativity is not distinguished).

 

[TrickyDicky]

zonde, I will assume that you are acknowledging that astronomers can measure "right ascension aberration" as well as "declination aberration". Can they also measure "aberration" (which I will call "intrinsic aberration" henceforth)? Also, can you please tell me what kind of aberration is predicted by the Doppler Aberration equation that is in Einstein's 1905 paper "On the Electrodynamics of Moving Bodies"?

Not sure what you guys are going on about here, right ascension and declination are simply the equatorial coordinate components of the aberration, the direction northward-southward or eastward-westward of the apparent shift of the star. It looks like you are asking what the true position of a point on the Earth's sphere is, its latitude or its longitude.

EDIT: Pallen was faster to clarify it anyway.

 

[harrylin]

[..] Otherwise, even if you are right (not likely), this constitutes original research at odds with peer reviewed opinion. It is therefore not allowed to be discussed here at all - unless you provide a reference for there being a discrepancy. [..]

This is not Wikipedia and its rules don't apply - however you are completely right about the benefit of giving good references.
Regretfully I'm not familiar with such astronomical terms as "ecliptic pole", so I can't comment on the question...

 

[harrylin]

[..] I don't think any of this has to do with relativistic aberration per se. The size of the relativistic correction to classical aberration is, last I checked, undetectable for Earth's orbital speed. [..] My guesstimate for the correction due to the difference between classical and relativistic aberration for seasonal aberration from Earth's motion would be of the order .002 arcseconds. [..]

Yes that is also what I remember to have read in textbooks; last time I read about it, the difference between classical and relativistic prediction was claimed to be negligible. Thanks for confirming that.

 

[PAllen]

This is not Wikipedia and its rules don't apply - however you are completely right about the benefit of giving good references.
Regretfully I'm not familiar with such astronomical terms as "ecliptic pole", so I can't comment on the question...

There used to be an original research forum. It was discontinued. Under the guideline against 'overly speculative posts' note the following:

"It is against our Posting Guidelines to discuss, in the PF forums or in blogs, new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional mainstream scientific discussion."

and:

"Unfounded challenges of mainstream science and overt crackpottery will not be tolerated anywhere on the site."

Claiming a discrepancy between SR and observation based on own research definitely falls into those. Changing the tone (as occurred here) to 'how is the discrepancy between x and this calculation I do resolved' is perfectly ok.

 

[TrickyDicky]

I think I understood the replies to my doubts, but still I'd like to be sure, so for instance the motion of the solar system wrt the remote stars is not observed as aberration because unlike the motion of the Earth around the sun, it has a constant velocity, is that right?

 

[PAllen]

I think I understood the replies to my doubts, but still I'd like to be sure, so for instance the motion of the solar system wrt the remote stars is not observed as aberration because unlike the motion of the Earth around the sun, it has a constant velocity, is that right?

Yes. We can't detect 'absolute' aberration any more tha 'absolute rest' or 'absolute motion'. We can detect change in apparent position between two reference frames. We could detect (in principle) aberration of some distant quasar over a period of time in which the solar system's motion around the galactic center is significant.

 

[harrylin]

There used to be an original research forum. It was discontinued. Under the guideline against 'overly speculative posts' note the following:

"It is against our Posting Guidelines to discuss, in the PF forums or in blogs, new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional mainstream scientific discussion."

and:

"Unfounded challenges of mainstream science and overt crackpottery will not be tolerated anywhere on the site."

Claiming a discrepancy between SR and observation based on own research definitely falls into those. Changing the tone (as occurred here) to 'how is the discrepancy between x and this calculation I do resolved' is perfectly ok.

Yes indeed - please simply stick to the rules and avoid Wikipedia jargon.

 

[Ken More]

---Einstein's paper deals with total aberration, which is the combination of declination and ascension aberration (as a total displacement angle). I assume astronomers can detect both aberrations now. Having reviewed astronomic coordinates, I notice that stars at certain positions on the ecliptic will never undergo seasonal declination aberration; they will only have right ascension aberration. Stars at 90% on the ecliptic to these will have both aberrations, but the declination aberration will be smaller than the ascension aberration. Of course, for stars on the ecliptic, there will be no aberration at all twice a year.

For a star perpendicular to the ecliptic, there will always be aberration (of constant magnitude relative to position in a solar system frame), but twice a year it will include no declination aberration (it will be pure ascension aberration).

Stars in between, will have varying total aberration, and will also have no declination aberration twice a year.

Finally, I don't think any of this has to do with relativistic aberration per se. The size of the relativistic correction to classical aberration is, last I checked, undetectable for Earth's orbital speed. My guesstimate for the correction due to the difference between classical and relativistic aberration for seasonal aberration from Earth's motion would be of the order .002 arcseconds. Thus, none of this discussion has anything to do with special relativity. It only relates to the finite speed of light (Galilean versus Special relativity is not distinguished).

Stars in between declination θ₀ = 90° and θ₀ ≈ 0° will have varying declination aberration and varying right ascension aberration. However, stars in this range will have zero declination aberration when α₀ = 90° and 270° according to the Declination Aberration Model described in the 2010 Astronomical Almanac (AA). I agree there is no declination aberration for a star in the ecliptic plane; however, the attached "AA Declination Aberration Model for a star near the ecliptic plane" the declination aberration is zero two times a year (when α₀ = 90° and 270°). As for right ascension aberration, I can only say that the AA Right Ascension model seems to confirms that right ascension aberration is zero for a star in the ecliptic plane only when α₀ = 0.

According to Einstein’s 1905 paper: If we call the angle between the wave-normal (direction of the ray) in the moving system and the connecting line “source-observer” θ, the equation for θ assumes the form cos θ = - (cos θ₀ − v/c)/(1 − cos θ₀• (v/c)). This equation expresses the law of aberration in its most general form. If θ₀ = π/2 (i.e. if θ₀ = 90º) the equation becomes simply: cos θ = − v/c.

Therefore: When θ₀ = 90º, v = -29.783, c = 299792.458, then cos θ = -29.783/ 299792.458 = -0.005692072º = -20.491458542 arc seconds. This value agrees very closely to James Bradley’s Constant of Aberration (= -atan(v/c) = -20.491458475 arc seconds) which is the declination aberration for a star at the zenith (at θ₀ = 90º) according to Bradley’s Falling Rain Model. Bradley's model may be referred to as the "Classical" aberration model because it was the first since Bradley discovered stellar aberration in the late 1720's. Therefore, you can see that Bradley's "Classical" model and Einstein's 1905 "Relativistic" model closely agree to within 0.000001 arc seconds: That is, they both agree that the declination aberration of a zenith star (at θ₀ = 90°) has a declination aberration of -20.491458 arc seconds. I believe the Bradley model assumes a declination aberration close to 20.5 arc seconds every day of a year. Since many physicists claim that Bradley's "Classical" model and Einstein's "Relativistic" model closely agree, I must also assume that Einstein's "Relativistic" model predicts that declination aberration is about 20.5 arc seconds every day of a year.

The attached "AA Declination Aberration Model for a star at the ecliptic pole" disagrees with those who say that declination aberration is about 20.5 arc seconds every day of the year for a star at or very near an ecliptic pole. This attachment shows that declination aberration is zero when right ascension (α₀ = 90° and 270°). Also, the "AA Declination Aberration Model for declination = 75°" and "AA Declination Aberration Model for a star near the ecliptic plane" show that declination aberration is near zero when right ascension is 90° and 270°.

Finally, I believe that the AA Declination Aberration Model is the most accurate model because the Astronomical Almanac is a universally accepted authority on prediction of the precise apparent location for a star on a specific future date and time. Also, the AA Models predicted locations of important stars such as Polaris and gamma-Draconis at θ₀ = 75° (the star Bradley studied) (see attached AA Declination Aberration for Declination = 75 degrees) have been verified many times by telescopic observation.

 

[PAllen]

I don't have time to go into the details of you discussion, but I note you are repeating the same error Zonde first note, and I tried to explain in more detail:

The Bradley and Relativistic aberration formulas you quote determine total aberration not declination aberration. They say for a star at the ecliptic pole, the total aberration is constant. However, twice a year it is all right ascension aberration, twice a year it is all declination, with a mix in between.

Also note that the AA tables you quote are considered to be derived by their authors from the aberration model you dispute. They are an application of the one accepted aberration model, not an alternative model.

If you want to discuss the details of the AA conventions and calculations, you should take this over to the astronomy forum.
Further, this whole discussion of 'supposed discrepancy' has nothing to do with the topic of this thread (one way measurement of light speed using aberration).

PLEASE open a new thread in astronomy on the topic of derivation of Almanac tables from aberration formulas.

 

[Dale]

Also, as previously requested, please post a mainstream scientific reference to demonstrate that this conflict actually exists. I completely doubt it.

 

[Ken More]

I don't have time to go into the details of you discussion, but I note you are repeating the same error Zonde first note, and I tried to explain in more detail:

The Bradley and Relativistic aberration formulas you quote determine total aberration not declination aberration. They say for a star at the ecliptic pole, the total aberration is constant. However, twice a year it is all right ascension aberration, twice a year it is all declination, with a mix in between.

Also note that the AA tables you quote are considered to be derived by their authors from the aberration model you dispute. They are an application of the one accepted aberration model, not an alternative model.

If you want to discuss the details of the AA conventions and calculations, you should take this over to the astronomy forum.
Further, this whole discussion of 'supposed discrepancy' has nothing to do with the topic of this thread (one way measurement of light speed using aberration).

PLEASE open a new thread in astronomy on the topic of derivation of Almanac tables from aberration formulas.

Some do say that for a star at the ecliptic pole total aberration is constant but twice a year it is all right ascension aberration and twice a year it is all declination aberration with a mix in between. However, for an ecliptic pole star right ascension aberration is only an "apparent" spin of the pole star. This apparent spin does not change its apparent location. Therefore, we are left with a total aberration that is declination aberration only and this applies at all times during a year for an ecliptic pole star.

I have noted that the AA tables I quote are considered to be derived by their authors from the SRT relativistic aberration model. Also, they do say that their AA model equations are an application of Einstein's SRT relativistic aberration model. However, when I do the AA model math and the SRT model math Einstein's relativistic model gives very different predictions from the AA model (I am not saying that their is a discrepancy, you can do the math to see for yourself whether there is a discrepancy).

Finally, I will take your advice and open a new thread in astronomy on the topic of the aberration of ecliptic pole stars and stars near the north ecliptic pole and the south ecliptic pole. Therefore, I will hereby terminate this discussion and will not respond to any future replies to my posts on this thread.

 

[PAllen]

Some do say that for a star at the ecliptic pole total aberration is constant but twice a year it is all right ascension aberration and twice a year it is all declination aberration with a mix in between. However, for an ecliptic pole star right ascension aberration is only an "apparent" spin of the pole star. This apparent spin does not change its apparent location. Therefore, we are left with a total aberration that is declination aberration only and this applies at all times during a year for an ecliptic pole star.

This is incorrect. The ecliptic pole is not the celestial pole. It is only the north polar star for which right ascension is undefined. The ecliptic pole will differ in declination from the polar star by the tilt of Earth's axis.

I have noted that the AA tables I quote are considered to be derived by their authors from the SRT relativistic aberration model. Also, they do say that their AA model equations are an application of Einstein's SRT relativistic aberration model. However, when I do the AA model math and the SRT model math Einstein's relativistic model gives very different predictions from the AA model (I am not saying that their is a discrepancy, you can do the math to see for yourself whether there is a discrepancy).

All this shows is that they know what they are doing and you don't. Since this is purely a question of the details of applying aberration in a particular astronomic coordinate system, the appropriate forum to clarify the calculations and find your error is the astronomy forum.

Finally, I will take your advice and open a new thread in astronomy on the topic of the aberration of ecliptic pole stars and stars near the north ecliptic pole and the south ecliptic pole. Therefore, I will hereby terminate this discussion and will not respond to any future replies to my posts on this thread.

Fine.

 

[ghwellsjr]

Now that the hijacking is over, can any of you experts on aberration please answer the OP's question?

 

[harrylin]

Now that the hijacking is over, can any of you experts on aberration please answer the OP's question?

Oops you're right. OK then, let's make a start. I think that it has already been clarified that aberration is caused by the velocity difference due to the Earth's orbit. The Earth's orbital speed is simply its speed relative to the Sun; and the Sun does not designate an absolute reference frame. Instead we could for example choose the combined speed of Earth + Sun through the Milky way. That would yield a very different one-way speed of light if we use the same t.

It always boils down to the same thing: if we choose a certain reference system as "rest" frame, then we will measure the speed of light relative to it as c, but else we won't. And then it depends on what one means with "one way measurement of c".

 

[PAllen]

Oops you're right. OK then, let's make a start. I think that it has already been clarified that aberration is caused by the velocity difference due to the Earth's orbit. The Earth's orbital speed is simply its speed relative to the Sun; and the Sun does not designate an absolute reference frame. Instead we could for example choose the combined speed of Earth + Sun through the Milky way. That would yield a very different one-way speed of light if we use the same t.

I don't think this is true. I think a very careful analysis (which I have not done) would show that this case is equivalent to slow clock transport - the one way speed would always be measured as c, but this measurement would be an artifact in the theories that have underlying anisotropy of one way c in most frames.