Talk:Aberration (astronomy)/Archive 1

Abeerration of light

Is not Abeerration of light (along with redshift) of one the two effects of travelling at relativistic speeds (as seen in the StarStrider(R) program)?

I don't know about StarStrider, but yes. See http://www.fourmilab.ch/cship/aberration.html .

Figures

Where are the figures the text talks about????

All figures were in the original printed version of the 1911 Encyclopaedia Britannica article, but were obviously not included in the digitized version. Someone with access to the printed version would need to scan them, or, possibly better, reword the text to eliminate any need for them. — Joe Kress 03:47, Nov 1, 2004 (UTC)

Needs diagrams

This page is one of the most difficult to read pages on Wikipedia that I've read in a long time. Why? There needs to be diagrams. Probably half a dozen or so. Mjm1964 12:11, 8 September 2005 (UTC)

I agree totally with the above comment, and I've had a bash at rewriting part of it, including diagrams (well, one diagram actually). It could do with more. I've also tried to get rid of some of the more antiquated Victorian language, but again it could be edited a bit better. My gut feeling is that the historical bit is overdone and could be removed to another page. --Portnadler 16:30, 17 December 2005 (UTC)

Thanks for your help. I have clarified some of your wording, especially changing the generic "motion" to the specific kind of motion, velocity or displacement, which is critical to understanding aberration. In general, Wikipedia articles need not be split unless they approach 32KB, which is the limit of some editors like Notepad in Windows. This article is now only 16KB. — Joe Kress 07:30, 18 December 2005 (UTC)

moving in an inertial frame?!

The article starts off with claiming that aberration is due to "that an observer on Earth is moving in an inertial frame". In reality stellar aberration is due to the fact that the earth is not moving in an inertial frame. I can both explain it and refer to at least one book, but it's easy to understand that inertial motion does not result in an "apparent motion of celestial objects". Before I make corrections I like to know who originated that phrase, and why. Harald88 08:46, 18 April 2006 (UTC)

I think it is trying to say that "the earth is moving relative to an inertial frame". --Portnadler 08:53, 18 April 2006 (UTC)

OK, that makes sense, thanks! Thus I also didn't express it well enough. I'll think about how to state it more clearly; anyway the subtlety is missing that inertial motion relative to an inertial frame is also not called stellar aberration - it's the change of velocity that does it. Harald88 19:59, 19 April 2006 (UTC)

I've made a simple change along the lines I suggested, but please feel free to reword it if you can make it clearer. --Portnadler 07:50, 21 April 2006 (UTC)

OK I did an attempt, and I hope it's OK (if not just revert and explain why). But I see that in the article more is to be done. For example, "The star's apparent position is hence displaced from its true position by the angle SES’" is a bit strange, as no "true position" can be determined - only a displacement of apparent positions. Harald88 13:01, 21 April 2006 (UTC)

No change in velocity is needed to produce aberration—a constant velocity through any inertial frame of reference also produces aberration. Indeed that is what the graphic is illustrating. The misunderstanding comes from its application to the apparent elliptical or circular paths followed by stars. But those paths are due to Earth's (nearly) circular orbit, not due to aberration per se. An extreme example of aberration would be experienced by an observer in a space ship travelling at almost the speed of light in a straight line. Almost the entire observable universe will be shifted from a celestial sphere into a celestial disk directly in front of the ship. Almost everything behind the ship will be observed at the edge of the disk in front of it. Only objects directly astern will still be behind it. A good discussion is at [1]. — Joe Kress 00:19, 23 April 2006 (UTC)

That's erroneous, and indeed the figure is misleading. The lead of the article is correct: what one commonly means with stellar aberration is the apparent motion of celestial objects - stellar aberration is the apparent elliptical or circular paths followed by stars. Ignoring parallax, no apparent motion occurs: they remain fixed in place. If you read your example again you'll notice that you only get a "shifting" by a change of speed. Similarly, the reference of Fourmilab states:

After a while, the apparent motion Bradley observed was discovered to be the result of the aberration of light. On each point in its orbit, the Earth is travelling with a velocity with respect to the distant stars that differs by 60 kilometers per second from the velocity at the opposing point--where the Earth will be six months hence.

We can only observe relative velocites that correspond to relative positions, see below. If you are right that there are books that state things differently, then we should quote the different sources and indicate that there are two incompatible definitions, wihout mixing them as is now the case. Harald88 10:03, 23 April 2006 (UTC)

I have already given my major reference in the article under References: Explanatory Supplement to the Astronomical Alamanc. — Joe Kress 08:07, 24 April 2006 (UTC)

Regarding "true position": Astronomy calculates the position of solar system objects by applying Newton's theory of universal gravitation. The results are their "true positions". To determine their observed positions, small corrections are applied, including aberration as well as relativity and light-time correction (and even refraction for a topocentric rather than a geocentric frame of reference), resulting in their "apparent positions". When applied in the reverse, the process is called "reduction", so an observed "apparent position" is reduced to the star or planet's "true position". This distinction is simply extended to 'fixed' stars. "True position" and "apparent position" have specific technical meanings in astronomy, and both are used correctly here. — Joe Kress 02:03, 23 April 2006 (UTC)

A "true position" is only possible to work with if one defines an "absolute frame". In practice (correct me if I'm wrong) astronomers use the solar frame as reference, well knowing that it's not a true rest frame. Harald88 10:03, 23 April 2006 (UTC)

The International Celestial Reference Frame uses extra-galactic radio sources to establish an inertial frame of reference relative to the solar system barycenter. — Joe Kress 08:07, 24 April 2006 (UTC)

Harald88, I agree with Joe Kress's last two comments. The example of the space ship moving at nearly the speed of light is a good one. The aberration is caused by the velocity itself -- not a change of velocity. I think you are confusing the displacement of a star from its true position (which is aberration) and the change in this displacement during the course of a year, which causes a star's apparent position to describe (in the general case) an ellipse about its true position. --Portnadler 11:03, 23 April 2006 (UTC)

No I'm confusing nothing. Which text book do you use for you definition, and which reference frame? We have no means to measure what you call "true position", as we don't know the "absolute speed" of the earth. Side note: it might one day become popular to use the CMBR frame, or the estimated centre of mass frame of the universe; but these result in completely different stellar aberrations than the ones that are now calculated. Harald88 12:01, 23 April 2006 (UTC)

The article mentions secular aberration, which is due to the motion of the solar system relative to the galaxy (and the remainder of the universe, if you wish). It also states that "secular aberration can be regarded as constant for all practical purposes, and so is usually ignored." The corrollary of this is that we are regarding the barycentre of the solar system as our reference frame when discussing annual aberration, which is the main focus of the article. It follows from this that the "true position" of a star can be regarded as its position when observed from the barycentre.

OK, that deviation from special relativity should be clarified in the article: it should explain that "true" is with reference to the solar frame. And it remains to be made clear again (I had made it clear...) that for near infinite objects only a velocity change results in an observable aberration. Harald88 18:07, 23 April 2006 (UTC)

I have my copy of Newcomb's Compendium of Spherical Astronomy by my side. It states the Law of Aberration as The apparent position of an object seen by an observer in motion is displaced from the true position in which it would be seen if the observer were at rest by an amount equal in linear measure to the observer's motion at constant speed during the time occupied by light in passing from the object to the observer. The direction of the displacement is that of the observer's motion at the moment of observation. --Portnadler 14:03, 23 April 2006 (UTC)

Thanks, that's interesting; Monday I'll look up what my textbook says of it. Harald88 18:07, 23 April 2006 (UTC)

Everyone is correct. A velocity consists of a speed and a direction. Earth's speed in its orbit around the solar system barycenter is virtually constant at 30 km/s, yet its velocity is always changing because its direction is always changing. It is this change in direction that causes the stars to follow elliptical paths due to aberration. If Earth's velocity was constant, meaning both its speed and direction were constant relative to a solar system barycenter that was not moving through inertial space (if the Sun and its gravity ceased to exist causing Earth to fly off on a tangent to its former orbit), then aberration would produce a shift in the position of each star that would vary depending on the angle between the direction to the star and the direction of Earth's constant velocity. The shift would be greatest at a constant 20.5" if the angle was 90° (moving broadside to the star) and least at 0" if the angle was either 0° (directly toward the star) or 180° (directly away from the star). The latter also describes the secular aberration due to the motion of the barycenter of the solar system through the Milky Way towards the star Vega at about 217 km/s, which would produce a maximum aberration at right angles to that motion of 150". The anisotropy of the cosmic background radiation of about 0.1% means that the solar system is moving relative to the inertial reference frame of the universe at 600 km/s,[2] producing a maximum aberration of roughly 400" at right angles to that motion. The latter includes the motion of the solar system relative to the Milky Way, so stars within the Milky Way would have some constant aberration, and distant galaxies would also have some constant aberration. But as the article states, the aberration due to the motion of the solar system itself (exclusive of Earth's motion around the barycenter) is usually ignored—the constant shift in the positions of the stars and distant galaxies due to the (linear) motion of the solar system itself (during our lifetime) is simply accepted. Because the International Celestial Reference Frame relies on extra-galactic radio sources, which are presumably well outside the Milky Way galaxy, yet nowhere near the observable edge of the Universe, we don't quite have a true inertial reference frame. — Joe Kress 11:20, 24 April 2006 (UTC)

Good to see that we agree about the fundamentals; remains the details of how to explain it in a simple way that is nevertheless correct. Note that I haven't looked at the last changes yet, I just have time to read the above comment, and to add two references of my own, as Ii promised. But I should correct an important misconception in the above comment by Joe:

as I already stressed, if the earth were in inertial motion, no stellar aberration would occur, as the stars would remain fixed at the same angles. For it's indeed this change in direction that causes the observation of aberration, and it's the observed circular motion of the stars that is (or at least, was!) called aberration. Aberration of the solar system would be (should be) related to an assumed cosmic non-inertial motion of the sun, which has an observable effect in principle.

My references:

- [3] (Found. Phys. Lett, by Liebsher et al [4] in which they define aberration as "the difference between the apparent positions found by observers in relative motion" - which is good in that it stresses that it's a relative measurement between two observation velocities, but it's crappy in that it obsures the fact that commonly it's not by different observers but the same one who changed velocity; and

- Jenkins&White, , internat. students edition 4th ed. :

"Bradley discovered an apparent motion of the stars which he explained as due to the motion of the earth in its orbit. This effect, known as aberration [...]

"Aberration, which depends on the earth's velocity [...]. Let the vector v represent the velocity of the telescope relative to a system of coordinates fixed in the solar system, and c that of the light relative to the solar system. [..]

tan alpha = v/c "

Thus they correctly explain that it's due to the orbital motion, but next they use the solar frame for calulation without explaining that it's simply the most convenient, and that in principle any inertial frame can be used to calculate annual stellar aberration as observed from the earth.

Cheers, Harald88 22:13, 24 April 2006 (UTC)

I think we've flogged this dead horse enough. Anyway, please take a look at the changes I made to the article as a whole, and see what you think. --Portnadler 17:26, 25 April 2006 (UTC)

I think that the article is very good overall. Still, the intro is again crappy: "that an observer on Earth is moving relative to an inertial frame of reference" is a faulty definition of stellar aberration as I explained above exhaustively, and I dug up (after only 10 minutes searching) a quality source that also explains why it's wrong - thus avoiding Original Research. Harald88 20:56, 25 April 2006 (UTC)

New diagram and rewrite of historical section

I have added a new diagram that attempts to explain aberration using the example of a star at the ecliptic north pole, and how this relates to the initial discovery of aberration by Bradley. I have also reworked most of the historical section to remove some of the 1911 archaic language and wordiness. It could do with still more work on it. --Portnadler 16:44, 24 April 2006 (UTC)

Hi Portnadler, I use a resolution of 1024x768 on my monitor. I believe a considerable portion of computer-users use 1024x768 resolution. The image you have uploaded is 849x539 pixels. The picture does not fit in my browserwindow. In the picture tutorial resizing section it is stated:

In general, overly large pictures should not be put into articles. Most pictures are between 100 and 400 pixels wide. Generally, pictures should not be wider than that.

I think it would be best to do a remake, writing the months with bigger letters, so that the image can be smaller. --Cleonis | Talk 10:39, 26 April 2006 (UTC)

Point taken. I'll give it a try when I get a bit of time. --Portnadler 18:22, 26 April 2006 (UTC)

I went ahead and redid the image myself. I hope that's OK with you. --Cleonis | Talk 21:36, 26 April 2006 (UTC)

No problem. Thanks for your help. --Portnadler 11:54, 27 April 2006 (UTC)

Planetary aberration ?

I'm unfamiliar with "Planetary aberration" as set forth in the artile, please provide a reference. I would appreciate it if the editor who wrote that part could send me a pdf of the source article (or a link to it). Harald88 21:52, 26 April 2006 (UTC)

The following is from the Explanatory Supplement to the Astronomical Almanac, p. 23

Because the velocity of light is finite, the direction in which a moving celestial object is seen by a moving observer (its apparent position) is not the same as the direction of the straight line between the observer and object (its geometric position) 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 orbital and rotational motion of the observer, both of these motions being referred to a standard coordinate frame. The former part, which is independent of the motion of the observer, is referred to as the correction for light-time; the latter part, which is independent of the motion or distance of the object, is referred to as stellar aberration. (For stars the light-time is ignored). The sum of the two parts is called planetary aberration because it is applicable to planets and other members of the solar system.

I think that is a very clear definition. --Portnadler 12:06, 27 April 2006 (UTC)

Thanks, yes that is very clear, and also in full agreement with the standard definition of stellar aberration ("orbital and rotational motion of the observer"). Harald88 19:06, 27 April 2006 (UTC)

Expressions that are used in specific technical meanings

I copy and paste from above:

"True position" and "apparent position" have specific technical meanings in astronomy, and both are used correctly here. — Joe Kress 02:03, 23 April 2006 (UTC)

I assume that the International Celestial Reference Frame as referenced to intergalactic radio sources, and a frame that is referenced on the cosmic background radiation coincide (to within the currently attainable measurement accuracy.)

Recapitulating:
Arguably, the cosmic background radiation is as close as it gets to defining the inertial frame of the universe. It is possible to calculate exactly what stellar positions would be measured by a telescope in space that moves in such a way that there is no Cosmic Background radiation anisotropy for that telescope platform. The cosmic background radiation frame is the only unique frame. I gather that positions as related to the unique frame are by convention categorized as "true positions" and that positions as related to any other frame than the unique frame are by the same convention categorized as "apparant positions".

As I understand it, the first usage of the word 'aberration' is about the reception of light. There is a large number of celestial objects (stars), and there is an observatory platform, with a significant velocity with respect to the barycenter of the large number of stars. The first usage of the word 'aberration', I gather, is about the physics of the proces of light arriving at an observatory platform.

There is something odd about some textbooks referring to 'stellar abberration'. If aberration is supposed to be about reception of light, how then is aberration also to be a property of stars? Clearly, the expression 'stellar aberration' in textbooks is physics shorthand, an expression that is only superficially related to the primary expression.

Harald88 is still somewhat dissatisfied with the wording. It seems to me that clarity can be achieved/improved by distinguishing between the primary meaning of 'aberration', and derived usages of that expression. --Cleonis | Talk 12:08, 27 April 2006 (UTC)

Cleon, can you cite any publication in which stellar aberration is calculated relative to that "absolute" frame; or, for that matter, relative to any other inertial reference frame that does not correspond to the preferred reference frame for periodic motion? (See also below). Harald88 19:24, 27 April 2006 (UTC)

Have a look at the definitions I posted above. I think we crossed in the post (so to speak). I have also reverted Harald88's last version of the introduction, because it did not properly introduce or cross-reference the concept of an inertial frame. Also, I repeat from the definion from the Explanatory Supplement, aberration is caused by the fact that the observer has instaneous velocity relative to the observed object. There is no need to complicate the explanation further. --Portnadler 12:17, 27 April 2006 (UTC)

You are right, I had more complexity than necessary. When two spacecrafts in space are only receiving signals from each other, they can deduce from measurements their relative velocity (up to deducing from measurements relative transversal velocity). Given a known relative transversal velocity, the magnitude of aberration can be deduced. --Cleonis | Talk 12:53, 27 April 2006 (UTC)

Sorry but no, Portnadler is not right this time: stellar aberration is the same for all stars. The explanation of the "Explanatory supplement" corresponds with the other sources that I cited on this page, and not with the paraphrase above by Portnadler: I stress once more that "orbital and rotational motion of the observer" differs from "the fact that the observer has instaneous velocity relative to the observed object". In fact, Portnadler here copied Einstein's erroneous explanation of 1905, which has been disproved by De Sitter when observing double stars: the velocity of the star is irrelevant for stellar aberration, as the distant stars are "fixed" for all practical purpose.

You lost me here. You are ascribing a point of view to Portnadler that is not to be found in Portnadler's writing. All through it has been clear to me that integral part of Portnadler explanation is that the velocity of the star is irrelevant for stellar aberration, as exemplified in the observation of double stars. --Cleonis | Talk 20:39, 27 April 2006 (UTC)

I copy-pasted his definition of just above your comment on this page (just before his signature of 12:17, 27 April), and which refers to his apparently new (but still erroneous) understanding, and which he also edited into the intro of the article; see also the new subject below, "velocity relative to the object" - thus I changed the intro back and made more improvements. In contrast, the explanation of the "Explanatory supplement" agrees with the majority of references, which (happily) are quite correct, although not all formulate it equally well. Harald88 20:59, 27 April 2006 (UTC)

To get back to the essentials: stellar aberration is in all modern calculations that I have seen only a function of the cyclic motion of the observer (thus the preferred solar frame which is very nearly the centre point of the annual motion). Harald88 19:21, 27 April 2006 (UTC)

Another rewrite of the intro

I have had another bash at rewriting the introduction. As others have pointed out, the definition of aberration depends upon the precise definitions of the true position and apparent position of an object. Stellar aberration (and light-time correction, when relevant) causes the difference between these two positions. Stellar aberration is caused by the fact the the observer has velocity relative to the object at the instant of observation. It is not necessary to introduce relavistic concepts, such as inertial frames. At velocities which are small relative to the speed of light, aberration can be calculated using classical methods. Remember that Bradley was nearly 200 years before Einstein. --Portnadler 15:14, 27 April 2006 (UTC)

Thanks for the try but it's still erroneous, eventhough I agree that no relativistic corrections need to be accounted for. The way Bradley formulated it is still perfectly valid - and I'll include it now. I hope that you don't think that I'm nitpicking, but all those at first sight subtle differences in formulation hide big differences in meaning. See above for a clarification. Harald88 19:30, 27 April 2006 (UTC)

The parts about the velocity of the observed object and the cosmic background are irrelevant, but aberration really is a displacement of apparent position, not a motion, caused by the Earth's velocity relative to a hypothetical stationary-by-definition observer on the Sun. There's no need for the "non-inertial" or "periodic" motion you've got in the intro. Any two observers in relative motion will, if they compare notes, see that the objects they see are in different positions. See for instance, http://scienceworld.wolfram.com/physics/StellarAberration.html

Now, because the Earth's velocity is changing as it goes around its orbit, the vector of the aberration is also constantly changing, but the phenomenon doesn't need that.

—wwoods 07:47, 28 April 2006 (UTC)

The point is that it's the relative motion between the two observations that matters. At constant speed the angle is constant; there is no phenomenon at constant angle. Harald88 16:41, 28 April 2006 (UTC)

Yes there is! If a spaceship travelling at constant velocity observes a distant star, the apparent position will be displaced from the true position. This is aberration. This is the point I have been trying to make, and why I rewrote the intro. --Portnadler 17:45, 28 April 2006 (UTC)

You just overlooked that you here considered two positions, as exhaustively explained and multiply cited above. But evidently I wasn't clear enough, and I agree with the above cited astronomers that it can be confusing.

Thus, another attempt to make it clear: You are on a spaceship travelling at constant velocity and you observe a distant star under a certain angle; based on your observations you draw an atronomical chart of the stars. One year later you look again. What phenomenon (how much stellar aberration) do you observe? Harald88 21:59, 28 April 2006 (UTC)

Exactly the same amount. —wwoods 06:17, 29 April 2006 (UTC)

That's correct: and if your explanation was correct, there would be no change of position observed - that is, no phenomenon at all.

The confusion is that of confusing between Apparent Displacement of Stars (= change of apparent position) and their apparent position itself. Indeed, your new reference http://www-spof.gsfc.nasa.gov/stargaze/Saberr.htm also shows that rather well. Harald88 11:16, 29 April 2006 (UTC)

Or, to continue the running-in-the-rain analogy, it doesn't matter whether you're running in a straight line or around a track--the apparent direction from which the raindrops are coming is the same.

—wwoods 18:21, 28 April 2006 (UTC)

See above for my additional explanation to Portnadler (if finally one explanation "clicks", then that one is likely good enough to copy to the article space!). But about this last one, it's again erroneous (and equally erroneous for rain drops!): the earth moves in the solar frame at nearly constant velocity v, resulting (schematically) in for example +v in January and -v in July. This velocity difference delta_v between consecutive measurements causes an observable angular shift of all stars on the photographic film, and that is called stellar aberration.

Some sources are sloppy, but several of the abovementioned sources emphasize this. As a reminder:

• On each point in its orbit, the Earth is travelling with a velocity with respect to the distant stars that differs by 60 kilometers per second from the velocity at the opposing point--where the Earth will be six months hence - Fourmilab
• the difference between the apparent positions found by observers in relative motion - Liebsher [5]
• the orbital and rotational motion of the observer - Explanatory Supplement to the Astronomical Almanac

Harald88 21:59, 28 April 2006 (UTC)

No, the aberration is the displacement observed because the Earth is moving, compared to what would be observed if it were not. (For some relevant definition of 'not'. In this case, the average position of the Earth is defined to be at rest.)

The effect depends on the observer's velocity at the time of observation; the observer's position or velocity six months hence is irrelevant. [Nitpick: the Earth moves in the solar frame at nearly constant speed, not velocity.]

• http://www-spof.gsfc.nasa.gov/stargaze/Saberr.htm
• http://www.daviddarling.info/encyclopedia/A/aberration_starlight.html
• http://www.fourmilab.com/cship/aberration.html You quoted this yourself, but note that it's about the effect observed from a spaceship traveling at constant velocity v, not from the Earth. And yet the effect exists--look at the diagrams showing the apparent positions of distant objects at various speeds. That's aberration!
  —wwoods 06:17, 29 April 2006 (UTC)

Wwoods, I find it amazing that you claim that 'the observer's position or velocity six months hence is irrelevant ', while Fourmilab -which you've read yourself- correctly explains that "On each point in its orbit, the Earth is travelling with a velocity with respect to the distant stars that differs by 60 kilometers per second from the velocity at the opposing point--where the Earth will be six months hence". Harald88 16:06, 29 April 2006 (UTC)

The fact that the Earth's velocity is changing over the course of the year is irrelevant; the effect of aberration exists at every instant, though its value changes with the Earth's velocity. That comparison with the Earth six months hence just determines the range over which the aberration varies. Go back to fourmilab and read the rest of the page.

If a spacecraft is cruising along at a constant velocity of 0.99c, is the aberration it observes (a) huge, or (b) zero?

—wwoods 00:18, 30 April 2006 (UTC)

Indeed, aberration is about observed changes in star positions. Thus that spacecraft observes zero aberration in its own system, while the astronauts observe a huge aberration between those as observed in the space craft and those as observed on average on earth (if they have an earth map to compare it with of course). Harald88 11:37, 30 April 2006 (UTC)

[reset indentation] Better to say 'observed differences in position'. And six months later, if they keep the same velocity, they'll observe the same huge aberration. Or, if they change velocity, they'll observe a corresponding change in aberration. Likewise, as the Earth's velocity changes over the course of a year, astronomers on Earth observe changing values of the aberration of stellar positions, relative to those "observed on average on earth".

—wwoods 20:05, 30 April 2006 (UTC)

Almost: it's inaccurate to talk of a "change of aberration, as the change of apparent position is the phenomenon that is called stellar aberration - but I already cited sources that point that out and explained why as well... Harald88 20:40, 30 April 2006 (UTC)

No! As you yourself quoted above, aberration is "the difference between the apparent positions found by observers in relative motion". Just look at the formulas: fr:Aberration, fourmilab, wolfram.com. Aberration is a function of velocity, not acceleration. A change of velocity produces a change of aberration.

—wwoods 00:25, 1 May 2006 (UTC)

I totally agree with Wwoods. What I really don't like about Harald88's intro is the reference to some arbitrary "inertial frame". All you need is the fact the observer has velocity relative to the object at the instant of observation. In the case of an earthbound observer, the direction of this velocity vector changes during the course of a year – hence annual aberration. I propose to revert to my previous version of the intro, but will await other's comments before doing so. --Portnadler 08:11, 29 April 2006 (UTC)

There is an obvious confusion or misunderstanding here: I fully agree that it's not good to refer to some arbitrary inertial frame, as Wwoods does here above, and which leads to the miscomprehension that the phenomenon would be observed when in inertial motion.

So far I have improved the intro only. But, as both I and Cleon also pointed out above, it's an historically known error to refer to "the object at the instant of observation". For a summary of one abovementioned references that also discusses that error, see http://www.astro.uni-bonn.de/~pbrosche/aa/acta/vol03/acta03_096.html Harald88 16:21, 29 April 2006 (UTC)

PS I quite like the simple but still correct description of the "Explanatory Supplement to the Astronomical Almanac" , according to which stellar aberration is due to the orbital and rotational motion of the observer. If you prefer that formulation too, please don't hesitate to improve the intro accordingly. Harald88 16:10, 29 April 2006 (UTC)

But the orbital and rotational motion of the Earthbound observer is a special case. Aberration occurs if the observer has any velocity component that is perpendicular to the straight line between the observer and object. That is the simple point that you do not appear to understand.

Consider the following example. An observer is on a spaceship which has an instrument that is capable of measuring very precisely the angular separation of two objects. Close to a distant star is another star whose angular separation from the first star is exactly one degree, when the spaceship is at rest relative to the two stars (we assume they have no relative velocity between them either). This is the true angular separation. The spaceship now starts to move directly towards the first star. There is no tangential velocity relative to the first star, but there is to the second star. Aberration will therefore make the angular separation between the two stars to be slightly less than one degree (as an aside, this effect is differential aberration and perhaps warrants a paragraph in the main article). If the spaceship turns around and moves directly away from the first star, the angular separation will appear to be slightly more than one degree. --Portnadler 17:14, 29 April 2006 (UTC)

Portnadler, we obviously continue to disagree about the usual meaning of the term stellar aberration, making an agreement on correct paraphrasing of the sources difficult. Happily that is not essential, as long as we can agree on citations. Note that in your last criticism you confused me with one of the sources that you supplied - and we both think that it's a good source.

Anyway, here my take on your example (note that I I'll be happy to see a source that distinguishes beteen "aberration" and "differential aberration").

Your example is fine, except for the "therefore" which is misleading in this context. Apparently you have difficulty with the Almanac's clarification that stellar aberration is independent of the motion of the object, and your example can be extended to illustrate that fact as follows:

Consider a third star with the same angular separation from the first star as between the first and second star, but which has a velocity equal to that of the space ship on its travel. Eventhough the velocity between the spaceship and that star becomes zero, the stellar aberration is still identical to that of the second star.

Regards Harald88 11:37, 30 April 2006 (UTC)