Talk:Aberration of light

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[edit] 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)

[edit] 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)

[edit] 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)

Aberration of light.
Animation

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 . 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)

Aberration of star light. The north pole of the ecliptic lies within the constellation Draco. This point is very close to the galaxy NGC 6552 and within 10 arcminutes of the Cat's Eye Nebula - NGC 6543.
(size is only 188k) other animations

--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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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″ 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)

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)