Talk:Aberration (astronomy)

Deciding what to categorize as aberration
Let two spaceships, A and B, be in inertial motion in space. (In the region of space where they are in gravitional influences are negligable.) The two spaceships have a velocity relative to each other. Both spaceships are astronomic observatories, and they want to cross-reference their respective star charts. For the purpose of cross reference, they both agree to recalculate star charts to the star positions that would be measured by an observatory platform for which there would be zero Cosmic Background radiation anisotropy. What is not disputed, I gather, is that the difference in position as actually measured, and the calculated position that would be measured by a zero-anisotropy observatory platform, can be recognized as a form of 'aberration'.

(Condition: the following discussion only applies when any observatory platform moves at a fraction of the speed of light; space-ships moving at close to the speed of light do not figure in the discussion below)

Harald, if I understand you correctly, you regard the following distinction of foremost importance: (1) in the case of a star and an observatorial platform with a (transversal) uniform velocity relative to that star, any aberration is inferred, rather than directly observed. (2) If the observatorial platform is a planet orbiting a star (in general: if the platform velocity vector changes direction over time) then over the course of time the aberration can be directly observed.

Harald, if I understand you correctly, you are advocating that only the directly observable aberration should be categorized as actual aberration. If I understand you correctly, you are advocating that it is better to not categorize the inferred aberration under the umbrella 'aberration'.

My personal preference is to categorize both the inferred aberration and the directly observable aberration under the umbrella 'aberration'. --Cleonis | Talk 19:35, 30 April 2006 (UTC)


 * First of all, I'm advocating to follow the Wikipedia rules: in a nutshell, to summarize what usually is meant with "aberration". As far as I know the liturature is pretty uniform about what is meant with stellar aberration, and of course different uses of aberration exist that should all be mentioned or referred to in an article called "aberration" - thus also for example chromatic aberration.
 * About stellar aberration, points 1 and 2: You misunderstand me.
 * As far as I read from our sources, the common meaning of stellar aberration is the apparent motion of stars as seen on earth. The aberration between the CBR frame and an observation on earth is in principle also stellar aberration, also IMO - but it's not what any of the usual terms such as annual aberration stands for. If you know a source such as an astronomy handbook that provides other values for stellar abberation than the common one, thus corresponding to such an "inferred" stellar aberration, please cite it here and we can include that.
 * I also like to point out that I agree with the first sentence of the article -and which I didn't modify- as it corresponds to what appears to be the usual meaning, but it doesn't correspond to the inferred aberration that you propose to include. Thus, if we find significant sources that discuss the "inferred" meaning, then the first sentence should allow for that and be modified with a qualifyer such as "generally". Harald88 22:39, 30 April 2006 (UTC)


 * My personal style is that I am more interested in physics than in the habits of the physics community. If an obviously appropriate physics categorisation is habitually not mentioned in the literature, then I see no reason to follow that habit. I follow the ramifications of existing physics knowledge, wherever that leads me. In contributing to wikipedia articles, I follow the guideline that synthesis of existing knowledge does not count as introducing novel ideas. --Cleonis | Talk 06:25, 1 May 2006 (UTC)


 * What is your opinion of the way WP:NOR is formulated (in particular the part on not "advancing a new position")? If you disagree, please participate in the discussion there! Harald88 06:57, 1 May 2006 (UTC)


 * Collecting and organizing the existing scientific framework of thought is the very purpose of the science-related wikipedia articles. for any encyclopedia that is where good editors make a difference: Gathering, arranging and occasionally, when appropriate, re-categorizing available knowledge in such a way that communication of science is maximized. I judge such editing as not constituting 'advancing a new position'. --Cleonis | Talk 20:29, 1 May 2006 (UTC)

The title of this article is the Aberration of light, of which stellar aberration is one particular case. However, despite everything, I think we are beginning to reach some kind of consensus. I hope we are all agreed on the basic cause of the displacement of the apparent position due to aberration. Harald88's case is that we can only measure this effect when the displacement changes over time, because the direction of the observer's velocity relative to the object changes over time. In effect, he is saying that annual aberration and diurnal aberration are observable phenomena, whereas secular aberration is not. The aberration of a star's light as seen by a spaceship moving at a constant velocity can be considered to be similar to secular aberration. If this is all we are arguing about, then I can live with it and hopefully find a form of words that is acceptable to all. --Portnadler 16:24, 2 May 2006 (UTC)


 * I agree that we do seem to come to a consensus about how the different usages of the (rather general) term "aberration" are best categorized in this article. I'm not sure if secular stellar aberration isn't observable in principle (on earth, although over millions of years); but obviously, planetary aberration is in part deduced, based on calculations in the solar frame. The more I think of it, the more I admire the way the Explanatory Supplement to the Astronomical Almanac formulated so much so correctly with so few words. It's a pity that we don't have the right to just copy-paste it in the article.
 * Note: we pointed out that for stellar aberration neither the velocity relative to the object nor a change of that velocity matters - but I leave it to someone else to try to explain that better, as it's evident that my efforts as well as that of our sources were insufficiently clear, and no doubt it's unclear for many readers (probably it's one of those things that are more often misunderstood than understood). Harald88 21:10, 2 May 2006 (UTC)

As far as I am aware, there is nothing to stop me from copying the relevant paragraph from the Explanatory Supplement on to this talk page, so here it is from page 127:


 * The velocity of light is finite, and so the apparent direction of a moving celestial object from a moving observer is not the same as the geometric direction of the object from the observer at the same instant. This displacement of the apparent position from the geometric position may be attributed in part to the motion of the object, and in part to the motion of the observer, these motions being referred to an inertial frame of reference. The former part, independent of the motion of the observer, may be considered to be a correction for light-time; the latter part, independent of the motion or distance of the object, is referred to as stellar aberration, since for the stars the normal practice is to ignore the correction for light-time. The sum of the two parts is called planetary aberration, since it is applicable to planets and other members of the solar system.

I hope we are all agreed that this is the most authoritative and up-to-date source. --Portnadler 08:45, 3 May 2006 (UTC)


 * I would not call any source about a term that is owned by nobody "most authorative", and I would hope that "up-to-date" is irrelevant (there is no speculation involved, or is there?). But I noticed nothing wrong and it looks concordant with other sources.
 * Thus I'm happy to see that you also think that they sketch it rather well. Harald88 10:00, 3 May 2006 (UTC)

The only reason I said 'up-to-date' is that it later goes on to discuss relativistic effects, which for obvious reasons are not described in earlier references, such as Newcomb. --Portnadler 10:11, 3 May 2006 (UTC)

Yet another rewrite
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)

Figure 4 Misrepresents Aberration
Figure 4 does not belong in this article. Although it is a pretty graphic, its presence can confuse readers and misrepresent the character of aberration. It should be included in the article on the Doppler effect. 1. The Doppler effect has almost nothing to do with aberration. Although they are both effects of relative velocity, one measures the apparent frequency shift of light and the other measures the apparent position of the light source. They vary concurrently, but are not the same measure. 2. Representing the relative frequency shift as a function of earth velocity implies that the measure of aberration is the same as the measure of frequency shift. But, no matter the velocity of the observer, aberration at the polls of the directional vector is always zero, whereas the doppler effect with be at its extremes in opposing directions. 3. By designating the velocity vector as earth orbital velocity, the caption implies that the earth could orbit the sun at .7c, which is impossible. Instead, the vector should represent the speed of a space ship, independent of the earth or any orbital motion. 4. The graphic can easily lead a reader to believe that aberration is variable when viewed from different positions on the earth. However, the aberration is a function of the angle of ascension or declination, not geographic viewing position. 5. Although the caption is superficially accurate, there is no explanatory text that clarifies the relationship between the Doppler and aberration effects. A clear explanation would diminish the value of the article by introducing a topic that is only incidental to stellar aberration. Therefore, I would recommend deleting Figure 4 from this entry entirely and moving it to the Doppler Effect entry, perhaps changing the caption to indicate a demonstration of extreme velocity in a space ship. Bill Westmiller on 26 August 2006


 * You raise some interesting points. However, I believe the graphic (or something similar) has some value to this article. To that end, I've clarified that the grid shows aberration. One possible compromise is to delete the color from the graphic, thus removing the Doppler effect from the discussion. This still leaves the problem that the Earth cannot orbit the Sun at 0.7c. Furthermore, the ecliptic and poles of the celestial sphere shown on the grid would by useless on a spaceship for specifying the positions of celestial objects. A better figure would show the shift in positions due to aberration relative to the direction of the motion of the spaceship, forward and aft. By the way, aberration is indeed slightly variable from different positions on Earth—it is called diurnal aberration, not discussed in the article. — Joe Kress 05:43, 27 August 2006 (UTC)


 * I tend to agree with the original criticism of Figure 4. I don't think it illustrates annual aberration very well, and it is more likely to confuse. Hence, I would vote to delete it. As an aside, diurnal aberration is mentioned in the article – about two paragraphs underneath! --Portnadler 16:47, 27 August 2006 (UTC)

Joe: "I've clarified that the grid shows aberration." The only problem being that it doesn't show aberration, so the accuracy of the graphic is now less than it was before. It's now relevant and wrong. I'm not even sure that I'm adding this comment correctly, but who would take a 'vote' tally and determine that the graphic should simply be deleted? Westmiller 08:00, 28 August 2006 (UTC)


 * I see your point. Because Earth orbits the Sun, the graphic is only valid for an instant in time—it cannot describe a full year of annual aberration, which, at a substantial fraction of the speed of light, would cause stars to describe a very wide circle around the undeflected pole of the ecliptic. But the grid does show annual aberration for Earth's tangential velocity at a single instant. It does this by distorting the entire celestial coordinate system of ecliptic latitude and longitude (not right ascension and declination) instead of the coordinates of the stars themselves. Thus a star that has a latitude of 90°, so is at the pole of the ecliptic, would still be at that pole at 0.7c if the pole itself was appropriately shifted.


 * Voting on a single Wikipedia article is entirely informal. Your vote is imbedded in your comment. So we know your vote is to delete the graphic. Mine would be to keep it but modify its caption as I've described above—remove the color and note that the grid only describes instataneous aberration (or straight line aberration as viewed from a spaceship), not a full year of annual aberration. I've also noted that a better graphic would not even show the distortion of ecliptic coordinates, but show distorted coordinates related to the motion of the spaceship itself.


 * Regarding diurnal aberration, I was thinking about the old article prior to the many changes made this spring. Nevertheless, the current description of diurnal aberration is still quite brief.


 * I've indented the recent comments to correspond to the writer of each (no indent for Westmiller, one indent for myself, and two indents for Portnadler), so indentation does not necessarily indicate a comment on the immediately preceding comment.


 * — Joe Kress 17:28, 28 August 2006 (UTC)


 * I've given it further consideration. The graphic is very pretty and someone has clearly spent a bit of time in producing it. However, I still don't think it adds anything to the explanation of aberration. Also, in its current position, it gets in the way of Figure 3 and its associated explanation underneath. I might be more inclined to keep Figure 4 if we move it below the explanation of Figure 3 and take the explanatory text outside the picture frame, to be consistent with the other figures in the article.--Portnadler 08:45, 29 August 2006 (UTC)

I've got a pretty picture of Madonna too, but I don't think that's relevant to the shifting positions of stars. The graphic could be modified to show aberration, perhaps with a gradation from black at the polls of the vector [zero] to green at the circumference [maximum aberration], but then the velocity would be irrelevant. If it's to show apparent aberration from earth, the grid would have to be modified to represent the rotation of the north pole. Whatever the effort involved in generating the graphic, it is a misrepresentation of aberration and should be deleted until a proper substitute can be found. Westmiller 23:58, 29 August 2006 (UTC)


 * There's only 3 of us, but we have a 2 to 1 majority, so I've deleted it. --Portnadler 15:35, 2 September 2006 (UTC)


 * I didn't see this discussion, sorry. I was sure that this talk is in my watch list but it is not. A few month ago I've made couple pictures in 4d-space just for myself. It was fast moving objects and tachyon. I thought that they could be useful to somebody else. First my image in Wikipedia was [[Image:Rel05s.gif|100px]]. But it wasn't good enough without any explanation. I'm no good to find a right words for explanatory text even in my native language. So, I added some colors and made a new image instead . However,  it still wasn't easy to find an appropriate spot for it. Then, I decided to hold only light directions in this image and got Celestial Sphere. I agreed that this image gives just a little value to this article, but it was a fun to make the graphics anyway. Now, I've made a new image, hope it will be useful to the article. I'll be thankful for any comments and it wont be difficult to make any changes to the image.--TxAlien 21:46, 10 September 2006 (UTC)

--TxAlien 07:01, 13 September 2006 (UTC)


 * Such a large animated image is unacceptable because it takes too long to load over a phone connection. Indeed, after a suitable period, I will even remove it from this talk page. The smaller animated images in your last post would have been acceptable if they made any sense (without an explanation). — Joe Kress 19:16, 13 September 2006 (UTC)


 * Ok, I'm sorry. I've changed size of it. I'll make jpg instead and link to animation.--TxAlien 19:31, 13 September 2006 (UTC)


 * Thanks. However, the Starsky03 figure with a random distribution of stars does not convey any understanding of aberration, certainly not in its static form. Even in its dynamic form, the figure, at least initially, appears to be a randow movement of stars. It requires intensive study to see what each star is doing. Your StarsSky01c figure with regularly spaced stars is much better, both in its static and dynamic forms. — Joe Kress 18:40, 15 September 2006 (UTC)
 * I've put this image (with a random distribution of stars) into the article. I'll remove it if you think that it still does not convey any understanding of aberration. --TxAlien 22:09, 19 September 2006 (UTC)


 * I really don't think it's helpful to put this graphic right at the top of the article. Either put in much further down after the other figures, or take it out. My preference would be for the latter. --Portnadler 16:11, 20 September 2006 (UTC)


 * I concur. The animated random distribution of stars conveys no knowledge of aberration—my first impression is that it is alive, like a beating heart. I greatly prefer the regular spacing of stars with ellipses, which does convey the concept of aberration. — Joe Kress 22:16, 20 September 2006 (UTC)


 * I've removed it. --Portnadler 11:08, 21 September 2006 (UTC)

Bradley
That stuff about Bradley has to be wrong. Clocks at that time were not good enough for anyone to find the East-West part of aberration, which is why Bradley found only the North-South part. But that is less than the 40" arc total by the sine of the inclination of the Earth's axis (about 23°) or about 0.4 . So the full excursion North and South would be of order 16". I don't want to just fix up this number but whoever wrote it or somebody please check a good historical reference. Or go ahead and change "March, when it took up a position some 20" more southerly than its December position " to "March, when it took up a position some 16" more southerly than its December position" etc if nobody can look up a good discussion. Carrionluggage 23:20, 3 September 2006 (UTC)


 * You are quite right about the fact that only the north-south component of aberration could be detected in Bradley's time, but your analysis of the magnitude of this component is wrong. It is proportional to the sine of the ecliptic latitude of the star, not the inclination of earth's axis to the ecliptic.


 * A star at the ecliptic north pole has the co-ordinates R.A. 270, Dec. 66.5 (roughly). γ Draconis, which transits at the zenith at the latitude of London has co-ordinates R.A. 270, Dec. 51.5 (roughly), so its ecliptic latitude is 75 degrees. The sine of 75 degrees is 0.966, so the amplitude of the north-south component of aberration would be nearly the whole 20.5 arcseconds that is the constant of aberration.


 * Portnadler 10:49, 4 September 2006 (UTC)

Oops - I learned something - thanks. Carrionluggage 19:37, 4 September 2006 (UTC)

Diminishing k?
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)

Digges

 * 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)
 * See the article on Aristarchus, which mentions parallax. —Preceding unsigned comment added by 86.139.212.139 (talk) 13:48, 9 October 2007 (UTC)

Annual aberration versus light-time correction
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&Prime; 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”&mdash;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&mdash;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)


 * 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)

Apparent motion or apparent deflection?
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)

A star on an ecliptic pole traces an ellipse, not a circle
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:

" 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)

Comment on merge proposal
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)

I agree. This needs to retain a page of it's own. I've added a much more accurate and clearer diagram, though this is still going to be difficult to understand for many as it still relies on visualisation of the motion.Docjudith (talk) 17:25, 15 February 2010 (UTC)

Discrete field model
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)

Nomenclature of celestial directions.
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 source. A modern and proven analysis is provided in the Hipparcos catalog documentation.

NOrbeck (talk) 18:43, 24 November 2010 (UTC)

Doubts about correctness
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)


 * 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. S  B Harris 20:41, 4 November 2011 (UTC)

Thanks for your quick and enlightened (!) answer. I will state what I have learned from you and other sources here for the benefit of perhaps achieving a clearer explanation in the article.

First of all - stellar aberration doesn't depend on the movement of the source of light. This movement doesn't imprint anything (relevant in this context) on the emitted light - neither velocity (of course, because of special relativity) nor direction of the light beams as seen by the observer. Put bluntly, the "light rays do not move with the source". This is most clear to me when I think of this in the context of quantum electrodynamics (QED), where photons take all possible paths from source to observer - but all paths but the shortest cancel themselves out. So the only relevant observable fact is a (relative) position of the star at the time the light ray was created. For stars, there is always a detectable ray since light is sent out in all directions.

Stellar aberration is noticeable only because multiple observers (or one observer at different times) move with different velocities with respect to a common system (i. e. the solar system). So the relativity in this case does not originate from movement of source relative to observer, but movement of multiple observers relative to one another.

Why is this so difficult to grasp? I think one of the reasons is that an explanation of a single observation in classical terms needs to refer to the light as wave front (in its way from source to telescope) AND as particle (inside the telescope). According to Liebscher/Brosche this was it what Fresnel had to cope with in his description of stellar aberration.

Most important consequences for the article:

Note 5 Paul Marmet(1996) should be removed because the view expressed in this article is FALSE (which is also clearly stated in the reference below).

A reference to the Liebscher/Brosche article/poster should be included because this is a really valuable resource for readers trying to grasp stellar aberration, although the authors use the scientific jargon to full extent. There is also a lengthier version, but this is in German. --TommyKat (talk) 14:51, 5 November 2011 (UTC)

Aberration constant citation needed
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)

Light source, its velocity and various frame of reference
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.

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)

The explanation (and diagrams) in the article are INCORRECT
I want to try and correct it, so I'm putting my thoughts on the correct explanation here.

The explanation invoking the distance that the telescope travels while the light beam is inside it is wrong, as discovered in 1871 by Airy by filling the telescope with water. In that case, the formula derived in the article,


 * $$\tan(\phi) = \frac { h\sin(\theta)}{\gamma(hv/c + h \cos (\theta))}=\frac { \sin(\theta)}{\gamma(v/c + \cos (\theta))} \, $$

will predict a different angle, as (following the derivation in the text) the speed of light would no longer be c but c/n, where n is the index of refraction of water. But as Airy found the angle does not change.

The true explanation was only found after 1905 due to special relativity. It turns out that in the moving reference frame of the earth the _angle_ of the light will be different (a well known result of special relativity). We can derive this change in angle by using the lorentz transforms of velocity applied to the light beam: In the reference frame of the sun, consider a light beam coming down with x and y velocity components $$u_x$$ and $$u_y$$. Let us find the components in the frame of the earth moving at a velocity $$v$$ in the x direction. By the lorentz transform rules,
 * $$u_x' = (u_x + v)/(1-u_x v/c^2)$$
 * $$u_y' = u_y/\gamma(1-u_x v/c^2)$$.

This gives us $$u_x'$$ and $$u_y'$$, the velocity components of the light beam in the frame of the earth. The new angle is thus
 * $$\tan(\theta') = u_y'/u_x' = u_y/\gamma(u_x+v) = \sin(\theta)/\gamma(\cos(\theta) + v/c)$$

where $$\theta$$ is the angle in the sun's frame, $$\tan(\theta) = u_y/u_x$$, and using the fact that $$u_x = c \cos(\theta)$$

Bradley's old explanation is wrong, but just happens to give the right equation! BlankAxolotl (talk) 06:19, 15 April 2013 (UTC)
 * If both explanations give the correct result, why is one of them wrong? I mean, we don't suddenly invalidate Newton's laws because of relativity... CodeCat (talk) 12:35, 15 April 2013 (UTC)


 * Hah, interesting point. But Newton's laws can be regarded as approximations to the true laws - you can take relatavistic laws and get newtons laws by taking the right limits. Newton's laws give a valid 'mental model' of life at low velocities. On the other hand, Bradley's equations give dramatically incorrect answers in some cases (eg the water telescope), and I think it's fair to say they're an incorrect mental model of what's going on and they're not the limiting case of a more accurate theory. So I really do think Bradley's result is "wrong". Bradley's result is like a broken clock, right twice a day, while Newton's laws are a clock that works but drifts a little. BlankAxolotl (talk) 17:45, 16 April 2013 (UTC)

I disagree. Let me tease your brain by asserting that neither Newton nor Einstein predicts dependency of aberration angle on index of refraction of telescope fill medium. But as well known the Newtonian prediction is sine theta and Einstein is tan theta. Okay, you say, why doesn't Newton require MORE tilt angle (aberration) with water because light travels more slowly down the tube? Well he does, but once the tube is tilted, classical physics now requires a further modification of LESS tilt to compensate for angle of refraction at the vacuum water surface where light enters the telescope. Add both of these up (subtract one from the other) and guess what? You're back at the old Newtonian angle for vacuum only. S B Harris 22:10, 16 April 2013 (UTC)
 * So what is the classical explanation for this, if there is one? I'm sure that using classical theory would be a lot more useful for this article because it tends to be accessible to a lot more people. And what I also wonder is, if classical theory doesn't work, why doesn't it? Why does relativity cause a noticeable discrepancy even at low velocities, when we have no such problem with other classical theories of light? CodeCat (talk) 23:39, 16 April 2013 (UTC)


 * Could you give me a ref? I don't find this to be the case. Maybe I am being dense, but it seems like refraction would _increase_ the aberration. Consider a telescope pointed at 45 degrees, on a planet going with such velocity that these 45 degrees are needed to see a star directly above a planet ignoring refraction. If you now include refraction, the location the telescope sees is now _behind_ it, at some negative angle from the vertical. Here's a little ascii diagram (light ray is *, telescope at two different instants is /) BlankAxolotl (talk) 04:56, 17 April 2013 (UTC)

*                 *      *                   *      *                    *     /*  /                 /*  /    / * /                 / * /   /  */          vs     /  */ no refraction        refraction


 * Update: Oh I figured one thing out in my sleep, you're right. If I add refraction to the relatavistic result, I get the newtonian result (at least numerically). The last bit I don't get is why we don't consider refraction in the newtonian case too, since it causes the increased aberration mentioned above. BlankAxolotl (talk) 15:24, 17 April 2013 (UTC)

I gave the classical explanation above. Classical theory predicts the same aberration, water or not, so long as the telescope is filled with water but not moving through water, and of course so long as the water interface is more of less orthogonal to the telescope axis. This gives an arcsin v/c relation for aberration angle that cannot be told from arctan v/c at low velocity. S B Harris 00:00, 17 April 2013 (UTC)

Half rewritten article, new explanations
(and quite a few deletions). I've updated the article based on my comments above, so that the article focuses more on the 19th century and relatavistic explanations, and to explain the historical importance of aberration to E&M and special relativity. Suggestions welcome. BlankAxolotl (talk) 20:56, 19 April 2013 (UTC)
 * Thanks. I've corrected some statements regarding partial aether dragging by Fresnel, and complete aether dragging by Stokes. Note that the image showing the failure of complete aether drag is correct, because Stokes hypothesis of an irrotational aether in order to explain aberration was shown to be contradictory by Lorentz (see Whittaker p. 413 first edition; p. 387 second edition). See also the description by Lorentz:


 * The Relative Motion of the Earth and the Aether (1892).
 * Attempt of a Theory of Electrical and Optical Phenomena in Moving Bodies/Introduction (1895)
 * as well as the secondary sources in the article etc.. --D.H (talk) 11:07, 20 April 2013 (UTC)


 * Thanks! I was planning to add the image back in later, I had removed it because I had to leave before finishing my edits and didn't want to leave the article in an incomplete state.


 * I disagree slightly with your summary of Stoke's theory. Schaffer says that Stokes theory was quite well accepted for a long time (2nd chapter on aberration). Lorentz did raise objections, but only 40 years later, and the objection is not captured by that picture. As per Schaffer, Stokes theory is perfectly able to account for aberration through complete drag within the earth, since aberration is acutally caused by partial drag above the earth's surface. Stokes derivation is given in Schaffer, and I think it is simple enough to derive in a few lines in the article, which I plan add when I have time. I also plan to put a representative picture of Stokes flow to show how there is partial dragging aove the earth. Lorent'z objection was that although a fluid moving according to stokes' theory could explain aberration, no fluid could actually move the way stokes said it moved.


 * While the picture is not appropriate to disprove stokes' theory, I think it is appropriate to put next to young's theory, as it clearly shows his reasoning that the aether must be immobile. — Preceding unsigned comment added by 69.125.58.77 (talk) 18:25, 20 April 2013 (UTC)


 * Yes, the image fits into the Young section. Regarding Stokes: I didn't say that Stokes model doesn't work if all conditions a met. The problem is rather related to this if: Lorentz showed that it is almost impossible that all of those conditions are met. But I see that you now included this objection to Stokes's explanation in the Lorentz section; and a link to the important paper by Janssen/Stachel, which supersedes the older descriptions of Whittaker and Schaffner, is provided. Thanks. --D.H (talk) 09:21, 21 April 2013 (UTC)

The moving magnet and conductor problem was certainly more important than aberration for Einstein, see the historical paper by Norton, who cites the following Einstein quote:
 * My own thought was more indirectly influenced by the famous Michelson-Morley experiment. I learned of it through Lorentz’ path breaking investigation on the electrodynamics of moving bodies (1895), of which I knew before the establishment of the special theory of relativity. Lorentz’ basic assumption of a resting ether did not seem directly convincing to me, since it led to an [struck out: to me artificial appearing] interpretation of the Michelson-Morley experiment, which [struck out: did not convince me] seemed unnatural to me. My direct path to the sp. th. rel. was mainly determined by the conviction that the electromotive force induced in a conductor moving in a magnetic field is nothing other than an electric field. But the result of Fizeau’s experiment and the phenomenon of aberration also guided me.

I've included this information in the article now. --D.H (talk) 09:35, 20 May 2013 (UTC)

Wrong sign in equatins... and erronous explanation in general
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)

Simple explanations
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)

Vandalism or good faith misunderstanding?
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)


 * 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)


 * 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)

I have confirmed that the relativistic equation, as I have modified it (so that it conforms with the original), is trigonometrically equivalent to the forms that I presented elsewhere in Special_relativity. You can validate them in Rindler's textbook on relativity. Prokaryotic Caspase Homolog (talk) 10:47, 23 November 2018 (UTC)

Why is there an unlinked SFN?
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)