[tvf]
The example is intended to illustrate how arbitrary "barycenter" is. It applies to all the masses under consideration. That line can be drawn wherever we please.
A "universe" (dynamical system) consisting of Sun, planets, and Alpha Centauri is just as valid as one consisting of Sun and planets only. Barycenter can be equated with "center of mass".
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I guess I'm not making myself clear. I understand THAT we can include any mass in our definition of a dynamical system. But I'm not sure WHY we would consider adding some masses to some systems.
Rather than try to rephrase my original question, though, let me try a different approach. Just before AB asked his question about the barycenter of the solar system (to which you made your reply referencing Alpha Centauri) you had mentined the barycenter of the Earth-Moon system. When you specified "Earth-Moon" you clearly meant to exclude all other masses. True? In this case there is no ambiguity about the definition of the dynamical system or where the barycenter of that system is located. True?
AB specified the Solar system in his question. No specific masses were explicitly mentioned, so the definition of the dynamical system is ambiguous, and open to modification. True?
(Damn! You have been Tholenized! <img src=icon_smile.gif border=0 align=middle> Seriously though, it is important to be precise. Ambiguity leads to many misunderstandings and disagreements.)
AB's question was: "Is there any consistent data on the Sun's motion wrt the Solar system barycenter?" If this question were to be reworded thus: "Is there any consistent data on the Sun's motion wrt the Sol-Jupiter (or Sol-Jupiter-Saturn, etc.) system barycenter?", I'm guessing the ambiguity would go away. True?
On the topic of calculating vs observing the barycenter of a system. Aren't we observing the barycenter of a star system (the star and whatever mass is within its gravitational sphere of influence) when we use the star's wobble to detect things orbiting it?
We can also look at Sol and see it wobble. But in this case we can identify just about all of the mass causing that wobble.
Is there a different term for the center point of this wobble? Perhaps "wobblecenter"? Since this is an observable point in space, it would seem not to be the same in general as barycenter. Of course if you pick your dynamical system carefully it could be the same point.
Regards,
LB
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