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unlocking sun from solar system barycentre (Read 10289 times)
frankuitaalst
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Re: unlocking sun from solar system barycentre
Reply #15 - 06/09/08 at 09:42:03
 
You can select first of all the bodies you want at the right , and then the properties you want .  
Omit the first and only select from Rx...to Vz, so 6 variables . I usely do this with the simulation paused not to loose any data .  
Then copying into exel gives you a string which you convert ...But I see you managed this already . You can post the output file if you want or part of it .  
Be assured : units are in m and m/s.
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tallbloke
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Re: unlocking sun from solar system barycentre
Reply #16 - 06/09/08 at 09:58:10
 
Hi Frank, thanks for hanging with me.
 
You said before:
"under the first menu select "create output file ""
 
but I don't have that option under the "file" menu, just "autosave" with the option to create a data file.
There doesn't seem to be any way to select bodies or data types here, just a way of setting time intervals, and the number of autosaves. Am I on the wrong menu here? I'm using V2.0.0.
 
Looking through the forum for clues, I came across someone trying to use a 'data viewer'. Is this an additional piece of software? Where can I download it if it is the thing I need?
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frankuitaalst
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Re: unlocking sun from solar system barycentre
Reply #17 - 06/09/08 at 10:29:07
 
Maybe you have an older version . The newest version I can remember is Beta20jan2008. I think you can go through the posts to find it to download . This works fine . In the menu the ouput file option is just below the autosave .  
The dataviewer is anaother kind of postprocessing code which calculates resonances .
Here you can find the latest one :
http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1176774875
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tallbloke
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Re: unlocking sun from solar system barycentre
Reply #18 - 06/09/08 at 10:50:23
 
Great! found the beta, and I'm rerunning the sim now. See you in a bit... Cool
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tallbloke
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Re: unlocking sun from solar system barycentre
Reply #19 - 06/09/08 at 12:45:43
 
Hmmm. Reran the sim in floating mode and got the graph made. But it just shows the regular variation you'd expect from the orbital eccentricity of the earth. Here's the first few lines of data. The final value is the calculated sun-earth distance:
 
Date      Name      Rx      Ry      Rz            Name      Rx      Ry      Rz            
 1949 01 01   11 59 32      Sun      -2.79E-02      -0.830281854      7.16E-03            Earth      -33062237140      1.42908E+11      16050786.48      1.46683E+11      
 1949 01 31   23 00 52      Sun      5.838130654      -59.03008921      -0.172135874            Earth      -1.02492E+11      1.05352E+11      10694080.66      1.46982E+11      
 1949 03 03   10 02 12      Sun      47.95433189      -192.1423863      -1.100361031            Earth      -1.43037E+11      38032746237      2391547.353      1.48007E+11      
 1949 04 02   21 03 32      Sun      117.3358131      -408.4731118      -3.117500599            Earth      -1.44038E+11      -39898156024      -6500385.233      1.49461E+11      
 
The odd thing is that the barycentric motion of the sun is very small if the distances are in metres. Yet when I zoom in on the sim it shows the sun moving in a wide arc about the solar system barycentre.
 
Perhaps Tony could tell me where I'm going wrong?
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Tony
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Re: unlocking sun from solar system barycentre
Reply #20 - 06/09/08 at 17:05:58
 
You're looking at the barycenter of the Sun + 8 planets.  But since Earth orbits inside the orbits of the gas giants, they pull on Earth as well as pulling on the Sun, negating much of their effect.  
 
As an example, imagine if the Sun had an unknown companion, a 10 Jupiter-mass brown dwarf orbiting at 1000 AU.  This would shift the solar system barycenter a great deal.  But being unknown, we wouldn't realize the barycenter was off by that much, because in looking at the Sun / planets barycenter, you can safely ignore anything orbiting external to the planets.  Similarly, when looking at the barycenter distance to Earth, you can ignore everything external to Earth because Jupiter and friends are pulling the Earth with the Sun.
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tallbloke
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Re: unlocking sun from solar system barycentre
Reply #21 - 06/10/08 at 00:22:06
 
Tony, many thanks, that's a much clearer explaination than I've had elsewhere, and I can see where I had misconcieved what is happening.  
 
It's causing lot's more ideas to occur to me, but I'll think them through for a while and use the software to see if I can work them out before I bother you with more dumb questions.  Shocked
 
Please could you explain why the labels for the gas giants changed from V2.0.0 to the latest beta? E.g. from 'jupiter' to 'jupiter barycentre'. Is this an 'internal' reference to the Jupiter system including it's moons?
 
Thanks again for your time.
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Tony
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Re: unlocking sun from solar system barycentre
Reply #22 - 06/10/08 at 13:18:42
 
The switch in names from "Jupiter" to "Jupiter Barycenter" has nothing to do with the version.  That's just the particular simulation you're running.  Either version should be able to open either simulation.  "Jupiter Barycenter" refers to a ficticious object that has the combined mass of Jupiter and its moons, plus their combined momentum.
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frankuitaalst
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Re: unlocking sun from solar system barycentre
Reply #23 - 06/10/08 at 22:48:10
 
I'm not sure about this but was thinking  : if you don't get the coordinates referenced  to the barycenter , but  in absolute coordinates you might first have to calculate het position of the barycenter :  
rbc= sum(mass*robj)/totmass. in three coordinates , and then applying the Pytha rule to the objects position and the barycenter position . I can be wrong . If the coordinates are absolute the outputted values themselves could already  be  the barycentric positions and so no Pytha is needed at all , except to derive the distance . Linear momentum rule should keep the BC at 0,0,0 . Perhaps Tony can answer this.  
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Tony
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Re: unlocking sun from solar system barycentre
Reply #24 - 06/11/08 at 10:33:08
 
Gravity Simulator continuously shifts the entire system so that the first object in the simulation always sits at (0,0,0).  If it drifts to (1,1,1) then all objects are shifted (-1,-1,-1).  This prevents the solar system from drifting light years away from the origin.  Since it uses double precision variables, positions in the solar system can be specified to exact centimeters.  But once it drifts far from the origin, it is only meter-accurate, then km-accurate, then AU-accurate... etc... This causes some strange errors, such as square orbits.  That is why Gravity Simulator continuously re-centers.
 
So when you output data, the Sun should always have a position of (0,0,0).  So you'll have to compute the barycenter yourself in your spreadsheet.
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