Talk:Aberration (astronomy)

message: Following discussions with Harald88, I have done a further rewrite. We both consider that the Explanatory Supplement is a good source, and I have used a form of words based on its description of aberration in the introduction. I note that it mentions motion relative to an inertial frame of reference, and I therefore withdraw my previous objection to using this term. I have also added some other new material. --Portnadler 07:45, 4 May 2006 (UTC)
thread_title:  Yet another rewrite 

message: I'm not sure how it might be phrased, but the secondary conclusion from Bradley's survey was that the aberration of starlight was diminishing over time.
It seems to me an interesting sidenote to the discussion. My initial sense is that this diminution may be a result of an expanding universe.
Westmiller 19:36, 16 October 2006 (UTC)
thread_title:  Diminishing k? 

message: :Digges is mentioned in connection with parallax. Aristarchus referred to the same
:much earlier.  —Preceding unsigned comment added by 86.139.212.139 (talk) 13:36, 9 October 2007 (UTC)
replies: text:  See the article on Aristarchus, which mentions parallax.  —Preceding unsigned comment added by 86.139.212.139 (talk) 13:48, 9 October 2007 (UTC)
thread_title:  Digges 

message: I think the following paragraph from the article is nearly incomprehensible (and, I suspect, either false or meaningless):
:A special case of annual aberration is the nearly constant deflection of the Sun from its true position by κ towards the west (as viewed from Earth), opposite to the apparent motion of the Sun along the ecliptic. This constant deflection is often erroneously explained as due to the motion of the Earth during the 8.3 minutes that it takes light to travel from the Sun to Earth. The latter is a type of parallax, and actually causes the apparent motion of the Sun along the ecliptic towards the east relative to the fixed stars. (8.316746 minutes divided by one sidereal year (365.25636 days) is 20.49265", very close to κ, but of opposite sign, east vs. west.) Nor is this the Sun's light-time correction because the Sun is almost motionless, moving around the barycenter (center of mass) of the solar system by usually much less than 0".03 (as viewed from Earth) during 8.3 minutes.
The way I see it, the aberration of light coming from the Sun is exactly the same (i.e., is the same phenomenon) as the light-time correction for the Sun, but seen in a different reference frame:
* in the Earth's reference frame (or, rather, a reference frame that is nonrotating wrt. distant stars and fixing the Earth's center), the Sun rotates around the Earth, light leaving it takes 8.3 minutes to reach the Earth, so an Earth-bound observer sees the Sun where it was 8.3 minutes earlier (i.e., 20.5″ west along the ecliptic),
* in the Sun's reference frame (ditto), the Earth rotates around the Sun, light leaving the latter reaches the former along a straight Earth-Sun light at the time of arrival but it is seen at an angle because of aberration of light.
The two points of view are equally valid, hence the angle of annual aberration must be equal to the path swept by the Sun along the ecliptic in the time it takes for light to travel from one to the other.  This is not a coincidence (as the above-quoted paragraph might lead one to think).  Note two things, however (before someone tells me I forgot one or the other): first, the Earth's reference frame is not inertial, but this does not matter since we are talking kinematics and not dynamics; second, this whole reasoning is equally valid in Galilean or special (i.e., Einsteinian) relativity, except that in the first case one must take care that the light's velocity is only fixed wrt. the electromagnetic ether and in the second case one must beware of time dilations, space contractions and simultaneity problems; but in the first order of approximation, all this can be neglected.  (I'm making the second point because otherwise someone is going to come up with one of the endless paradoxes of special relativity, e.g.: “but what if the Earth were to suddenly stop along it's orbit: by reasoning in the Sun's reference frame one would conclude that the aberration would stop suddenly, so the apparent position of the Sun would change discontinuously at the time of the stop, whereas in the Earth's reference frame one would conclude that the Sun would cease to move along the ecliptic only 8.3 minutes later”—I will dispense, if I may, with the usual refutation of such simpleminded paradoxen. :-)
Anyway, I can't make heads nor tails of the sentence "The latter [the latter what?] is a type of parallax, and actually causes the apparent motion of the Sun along the ecliptic towards the east relative to the fixed stars."  Maybe this is just meant to say that the reason we see the Earth travel along the ecliptic is that the Earth moves around it (a parallax effect, indeed), but that is the most obscure way of stating it I could ever think of—plus, it has nothing to do with the aberration of light.  Nor can I figure out what to do with this "east versus west" remark.
So I would like to rephrase all of this perhaps something like this:
:A special case of annual aberration is the nearly constant deflection of the Sun from its true position by κ towards the west (as viewed from Earth), opposite to the apparent motion of the Sun along the ecliptic. This constant deflection is often erroneously explained as due to the motion of the Earth during the 8.3 minutes that it takes light to travel from the Sun to Earth: this is a valid explanation provided it is given in the Earth's reference frame, whereas in the Sun's reference frame the same phenomenon must be described as aberration of light.  Hence it is not a coincidence that the angle of annual aberration be equal to the path swept by the Sun along the ecliptic in the time it takes for light to travel from it to the Earth (8.316746 minutes divided by one sidereal year (365.25636 days) is 20.49265", very close to κ).  Similarly, one could explain the Sun's apparent motion over the background of fixed stars as a (very large) parallax effect.
Am I missing something?  --Gro-Tsen (talk) 21:25, 17 December 2007 (UTC)
replies: text: Well, since it's been a week and nobody replied, I made the change I suggested. --Gro-Tsen (talk) 00:59, 24 December 2007 (UTC)
thread_title:  Annual aberration versus light-time correction 

message: The introduction talks about an 'apparent motion'. While this may be technically correct based on the aberrational ellipse traced out in the sky over a period of one year, would it not be more correct to be talking about an 'apparent deflection'? David Tombe (talk) 11:25, 15 November 2008 (UTC)
thread_title:  Apparent motion or apparent deflection? 

message: The Earth moves in an elliptic orbit, and maximum aberration occurs at maximum speed (perihelion). Minimum aberration occurs at aphelion.  For stars located at either ecliptic pole, the path traced is geometrically similar to the Earth's orbital path, rotated by 90 degrees. Even though the hodograph of an elliptic orbit is circular, the resulting aberration is elliptical.
Figure 2 is incorrect/misleading, since it applies only to circular orbits.
Aberration can be calculated by a Mobius transformation. Since every Mobius tranformation has two invariant poles, there are in fact two places on the celestial sphere where stars DO trace out circles. 
________
Next, The very first sentence of the article is incorrect:
"<aberration> is an astronomical phenomenon which produces an apparent motion of celestial objects about their REAL LOCATIONS."
The apparent positions and motions of any celestial object is wholly unrelated to it's "real location".  Objects that emitted light billions of years ago may not even exist today. Light observed at Earth tells us nothing at all about the emitter's "real location".
The concept of the true/actual location (real position, geometric position) should be replaced with the concept of "Astrometric Position".  The Astrometric position is the result of applying the light time correction to the geometric/true position. 
_______
Next, Special Relativity spam.
"According to the special theory of relativity, the aberration only depends on the relative velocity v between the observer and the star."
This utterly irrelevant assertion is 100% true. However, contrary to Einstein's asseverations, aberration is a function of the velocity of the observer relative to the Solar System Barycenter, NOT of the observer relative to the source. Again, the source may not even exist at the time of observation. Einstein begrudgingly admitted this truth, and so should his parrots. 
But why even bring up Special Relativity's inapplicable assumptions of light speed invariance? In the real world, light slows, twists, and refracts in the ISM and gravitational fields. Light speed is C only in a vacuum universe, not ours. 
Special Relativity in this context is spam. Like most people, I have no need for V1agra, a new mortgage, or another hackneyed rationalization of S.R. paradoxes. Enough already with the batshit relativity digressions.  —Preceding unsigned comment added by 66.135.173.139 (talk) 06:50, 13 February 2009 (UTC)
thread_title:  A star on an ecliptic pole traces an ellipse, not a circle 

message: I doubt if this article is ripe for a merger with the article on relativistic aberration.  They are both rather complicated subjects, and I suggest that the descriptions of both of them need to be made clearer (and perhaps on certain points also more correct) than they currently are. (At the moment, the relativistic aberration article appears cryptic and theoretical rather than explanatory, specially its practical implications are not reliably spelled out.)
In the meantime, I suspect that a merger would make the aggregated text  less clear and accessible. Terry0051 (talk) 19:56, 3 November 2009 (UTC)
thread_title:  Comment on merge proposal 

message: I am removing recent edits by Docjudith because they are based upon an article of his entitled Discrete Field Model (now available at Wikibin, a home for deleted Wikipedia articles) which was deleted as original research or not notable. Without that article, there is no basis for "another possible explanation". — Joe Kress (talk) 22:14, 23 June 2010 (UTC)
thread_title:  Discrete field model 

message: Observed direction, or apparent topocentric direction: The actual measured direction of the object, the direction of the telescope. The position and velocity of the frame of reference are topocentric. (Topocentric means "with respect to a frame fixed to a point on the Earth's surface"). 
Proper direction: The observed direction after removing the effect of atmospheric refraction. The direction the telescope would be pointing if the Earth had no atmosphere. The position and velocity of the frame are topocentric.
Apparent geometric direction: The proper direction after correcting for diurnal parallax and diurnal aberration. The position and velocity of the frame are geocentric. (Geocentric means "with respect to the geocenter").
Natural direction: The apparent geometric direction after correcting for annual aberration. The position is geocentric, but the velocity is barycentric. (Barycentric means "with respect to the center of mass of the solar system").
Astrometric direction, or isotropic direction: The natural direction after correcting for light bending due to General Relativity and coronal refraction. The position is geocentric, and the velocity is barycentric.
Barycentric direction at epoch of date, The astrometric direction after correcting for annual parallax. The position and velocity of the frame are barycentric.
Barycentric direction at epoch Jxxxx.xx, The barycentric direction after correcting for proper motion. The barycentric direction at an arbitrary time. The position and velocity of the frame are barycentric.
Catalog direction or barycentric direction at catalog epoch:  Same as above. These are the coordinate directions printed in astrometric catalogs. For Hipparcos the catalog epoch is J1991.25
True direction: For objects in the solar system, this is the barycentric direction after correction for light-time. For objects outside of the solar system, the "true" direction is synonymous with the barycentric direction -- it is not the actual physical direction. The actual physical direction to distant objects is not known. In astronomy it is conventional to neglect the light-time correction for objects outside of the solar system. For example, gravitational lensing can cause multiple images of a single star; in astronomical parlence each image has a unique "true" direction. If known, the actual real direction can be described in any reference frame (topocentric, geocentric, barycentric, etc), while the "true direction" is understood to be with respect to solar system barycenter.
Note that the phrase "apparent direction" is ambiguous, and should be avoided.
Technically, the aberration corrections are Lorentz transforms, or "boosts". Aberration corrections are simply a change in the velocity of the reference frame. Aberration occurs for all physical objects as well as light. It's best to consider the aberration of physical particles before contemplating light.
In both classical and relativistic physics, aberration has nothing to do with the "relative velocity of the source and observer". Contrary to Einstein's 1905 paper, Lorentz boosts are entirely independent of the motion of the sourcehttp://www.aip.de/~lie/Publikationen/366.ThreeTraps.html. A modern and proven analysis is provided in the Hipparcos catalog documentation"Ephemerides, Timing, and Calculation of Celestial Directions" http://badc.lamost.org/archives/cats/i/239/version_cd/docs/vol3/ch12.pdf.
NOrbeck (talk) 18:43, 24 November 2010 (UTC)
thread_title:  Nomenclature of celestial directions. 

message: Quotation from article:
"In contrast, 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."
This seems to contradict galilean relativity in that there is a difference between effects of the movement of the observer and movement of the observed. This difference would go away if we assumed that the incoming light ray had INDEED a transversal velocity component originating in the movement of the light source.
Note 5 (which has direct relevance to this) seems to be coming from a website of physics crackpots and originates from a dubious (late) canadian professor who sets out to disprove Einstein. 
LATER: It seems that this is REALLY complicated and that my view expressed above is too simple - albeit the paragraph in question and especially the analogy with raindrops done in the article seem still to be WRONG!!
I found an interesting link: http://www.aip.de/~lie/Publikationen/366.ThreeTraps.html
Seems to be respectable - but it seems you have to have a firm grasp of the subject to really understand what is said, let alone find out if this is the correct view. Could someone WITH A FIRM GRASP look into the subject??
TommyKat (talk) 10:49, 4 November 2011 (UTC)
replies: text: Aberration can indeed be understood, to first-order, by neglecting relativity and just considering Galilean relativity. The problem is in thinking that "relative velocity" is as important for the emitter as the observer, and that isn't true in cases where they must intrinsically perform different actions (pitching and catching), and one has a harder "job" than the other. Galilean relativity is only important for situations in which there is symmetry and constant velocity, and cannot be applied to those where one person accelerates or decelerates during the experiment (as Galileo himself took care to point out).  If you consider an emitter firing laser pulses or (even better) pellets of birdshot, you see that he must (somehow) manage to figure out where the observer is going to be, when the particles arrive at his location. If he misses, there is nothing the observer can "do" to fix things and collect the signal. Thus, the only way the emitter can be sure to be seen for an artibrarily acceclerating observer (say Earth in Earth orbit) is to emit particles in every direction. The observer then must pick a direction to look, in order to capture a particle.  Sometimes it's helpful to consider aberration in a situation where emitter and observer are in constant transverse relative motion through the entire transfer of particles, so we CAN apply Galilean relativity. Then it's helpful to notice that in the emitter's rest-frame where the observer is moving, the emitter has to "lead" the observer and fire at a point ahead of him (like leading a duck with a shotgun blast) or else he will miss. He sees a charge of shot (or a photon of light) go straight up the telescope barrel (assume this is used to focus his laser) toward where the target will be. He sees it goes down the (tilted) barrel at the other end, only because the target is moving. At the target the observer sees something else: he sees light come straight down the barrel not because his telescope is moving (for him, it isn't), but rather because the emitter managed to send it on a straight line trajectory even though moving transversely, by the trick of pointing his telescope/shotgun ahead, so the charge came out not in the exact direction it was pointing, but rather in a direction toward the observer. However, that direction is on a diagonal. Here the observer thinks he's "tilting" his own telescope ahead (his "ahead") only because when he gets the signal it doesn't come from from exactly lateral (the x direction), but rather from a position behind where the emitter is at the moment-- so it's only tilted toward where the emitter was when he sent the signal (the observer is now "trailing" the real position of the emitter, in order to see him or his signal). In this symmetrical situation, the observer can exchange signals with the emitter if they fire at the same time, each with their telescopes pointed in the same direction as regards the x axis (each using telescope both to focus laser and receive impulse).  Diagram this out if it will help-- you should see that for constant velocity observer/emitters, exchanging a signal along the x axis but moving along the y axis in opposite directions so that each one emits before they get to the x axis, but each one observes at the x axis, it's exactly symmetrical, but each sees himself doing something different for emission (where he has to lead the target) than reception (where he doesn't, because the aiming was done for him at the sending end, but he has to tilt his telescope because the light is coming from a point not on the x axis). Even though the signal goes along the x axis exactly for one observer, it doesn't for the other, but goes diagonally. So each observer/emitter must start in the y direction (+y or -y) with telescope tilted "ahead" (leading the other's perceived direction of motion) to do each job.  In unsymmetrical situations you see the symmetry of the problem is broken by the fact that one person accelerates and the other does not. For example, start with two spaceships at rest, a light year from each other on the x axis. One accelerates to high velocity (almost) instantaneously, emits a pulse at the other, then decelerates to a stop, all without moving any appreciable transverse distance. Now you see that relative velocity here when the pulse is emitted, only matters to the emitter, not the observer (who sees the pulse coming in from the same direction whether the emitter was moving or not). But the moving emitter has to severely correct his aim while he has his transverse velocity, or he'll miss the observer: he has to aim ahead, exactly as if the observer was moving, not himself (so his AIM depends on relative, not absolute velocity). But none of this ever affects the observer, so long as the emitter corrected for it. He sees the implse direction unchanged by the transverse velocity, and in Galilean physics he can't tell the emitter was moving. In relativity he can, but not by the direction-- there is no aberration. The observer does see a funny Doppler shift down in frequency, due to the emitter's clocks running slow relative to him when the pulse was sent (transverse Doppler effect). But that's all. For the symmetrical situation, both emitter and observer see the Doppler slowing, and it does depend on their relative velocity, and it's the same for both of them. SBHarris 20:41, 4 November 2011 (UTC)
thread_title:  Doubts about correctness 

message: Astronomical almanac:
The Astronomical Almanac Online!  http://asa.hmnao.com/AsA/
ASTRONOMICAL CONSTANTS K7
http://asa.hmnao.com/AsA/SecK/2012/Astronomical_Constants_2012.pdf 
Constant of aberration at epoch J2000•0 ĸ= 20”.495 51  — Preceding unsigned comment added by 84.155.226.78 (talk) 17:39, 21 January 2012 (UTC)
thread_title:  Aberration constant citation needed 

message: Although the article was factually (nearly) correct, it did never explain the importance of frame of reference, nor the use of Newton or Minkowski pictures was specified. One piece of the "explanation" section even confusingly suggested that one should consider the frame of reference of the light source – I hide this. Though, the article still needs several fixes to specify reference frames explicitly, especially in Aberration of light#Types of aberration.
BTW is there a volunteer to create an article on important Sun's frame of reference (if not only to catch such terms as heliocentric velocity) and also  (now a redirect) disambiguation page? Incnis Mrsi (talk) 19:25, 12 July 2012 (UTC)
thread_title:  Light source, its velocity and various frame of reference 

message: The equations at the begining are erronous.
 Star S' - observed image pos.
 |.../
 |../
 |./
 |/ f' < f = 90
 Earth ----> v
:\cos(\phi') = \frac{\cos(\phi) + v}{1 + v\cos(\phi)}
Your equation is for completely diferent situation:
 S <---- v; 
 |..
 |..
 |.
 | f' = f = 90
 Earth
There is no aberration of light, but for close sources, like the moon, there is the yime-light correction, due to the tangential and relative motion of source-observer.
:\cos(\phi') = \frac{\cos(\phi) - v}{1 - v\cos(\phi)}
after the time: t = d/c, the object S moves to the negative direction, ie. x = -vt, thus: cos(f') = -v, f' > f = 90 
This is simple motion effect, there is no aberration.
The classical Bradley's verision is correct, but it's first-order approximation only.  — Preceding unsigned comment added by 83.10.228.220 (talk) 21:03, 22 February 2014 (UTC)
thread_title:  Wrong sign in equatins... and erronous explanation in general 

message: Is this animation still considered a decent way to illustrate aberration?
If not, what are the best animations you can recommen?
https://en.wikipedia.org/wiki/File:StellarSky3sm.gif
or how about this?
StellarSky1m.gif
Thanks for your time,
RK (talk) 18:09, 10 July 2015 (UTC)
thread_title:  Simple explanations 

message: Looking at the previous Talk discussion, it's not clear to me whether the mathematical nonsense in this article represent unnoticed vandalism or good faith misunderstanding.
Take a look at this equation for classical aberration:
:\tan(\phi) = \frac{u_y'}{u_x'} = \frac{u_y}{u_x-v} = \frac{\sin(\theta)}{\cos(\theta)-v/c}
The sign in front of v/c is the opposite of a published version of the equation and is inconsistent with the first figure. What the equation implies is that with increasing v, the star is increasingly aberrated in the opposite direction from the direction of movement.
The relativistic equation is also messed up. Before this series of edits, the signs were correct, and I would have concluded that this was just unnoticed serious vandalism from years ago, but from the previous talk discussion, it appears that multiple eyes have looked at these equations. 
So I'm not going to revert these messed-up changes without input. I invite commentary. Prokaryotic Caspase Homolog (talk) 12:53, 21 November 2018 (UTC)
replies: text: One editor commented on my personal talk page. Basically, I am going to go by my own judgement. Prokaryotic Caspase Homolog (talk) 10:15, 22 November 2018 (UTC)
replies: text:  I made the reverts. Since the derivations themselves may be original research, I will have to double-check them. The final results now match with published versions. Prokaryotic Caspase Homolog (talk) 10:43, 22 November 2018 (UTC)
thread_title:  Vandalism or good faith misunderstanding? 

message: Now I have to dig back and figure out who removed this.  Oh well -- no rest for the wicked, I guess. But it would be nice if the originator of the SFN could help out :) LaurentianShield (talk) 02:32, 4 February 2019 (UTC)
thread_title: Why is there an unlinked SFN?