weirdness in orbital parameters

[Malenfant]

Have I run into a flaw in how orbits are figured out here? I'm trying to test something out here regarding multiple illuminations and I seem to have run into a weird offset.

Here's my Spica STC.

Code: Select all

Barycenter "Spica"
{
RA         201.29835230
Dec       -11.16124491    
Distance  262.05788
}
#-------------------------------
Barycenter "SpicaABCD"
{
   OrbitBarycenter "Spica"

   EllipticalOrbit {             
      Period          186989.398
      SemiMajorAxis   209   
      Eccentricity    0
      ArgOfPericenter 0
   }
}

"SpicaE"
{
OrbitBarycenter "Spica"
SpectralType "K5V"
AbsMag  7.58 # Luminosity = 0.08 Sol
Radius 375983

   EllipticalOrbit {               
      Period          186989.398
      SemiMajorAxis   9791 
      ArgOfPericenter 180
   }
}
#-------------------------------
Barycenter "SpicaABC"
{
   OrbitBarycenter "SpicaABCD"

   EllipticalOrbit {             
      Period          47.80914
      SemiMajorAxis   5.7143   
      ArgOfPericenter 0
   }
}

"SpicaD"
{
OrbitBarycenter "SpicaABCD"
SpectralType "B7V"
AbsMag  -1.14 # Luminosity = 250 Sol
Radius 1740662

   EllipticalOrbit {               
      Period          47.80914
      SemiMajorAxis   34.2857 
      ArgOfPericenter 180
   }
}
#-------------------------------
Barycenter "SpicaAB"
{
   OrbitBarycenter "SpicaABC"

   EllipticalOrbit {             
      Period          1.63299
      SemiMajorAxis   1   
      ArgOfPericenter 0
   }
}

"SpicaC"
{
OrbitBarycenter "SpicaABC"
SpectralType "B5V"
AbsMag  -2.6 # Luminosity = 938 Sol
Radius 2088795
   EllipticalOrbit {               
      Period          1.63299
      SemiMajorAxis   3
      ArgOfPericenter 180
   }
}
#-----------------------

65474 "SpicaA"
{
OrbitBarycenter "SpicaAB"
SpectralType "B1V"
AbsMag  -5.49 # Luminosity = 13400 Sol
Radius 5430867

   EllipticalOrbit {               
      Period          0.0098
      SemiMajorAxis   0.047 
      ArgOfPericenter 0
   }
}

"SpicaB"
{
OrbitBarycenter "SpicaAB"
SpectralType "B4V"
AbsMag  -3.25 # Luminosity = 1710 Sol
Radius 2785060

   EllipticalOrbit {               
      Period          0.0098
      SemiMajorAxis   0.073 
      ArgOfPericenter 180
   }
}

#-------------------------------


And the SSC for the world around Spica E:

Code: Select all

"Spicaworld" "SpicaE"
{
Texture "mars.*"
Radius 6000

   Atmosphere {
      Height 60
      Lower [ 0.43 0.52 0.65 ]
      Upper [ 0.26 0.47 0.84 ]
      Sky [ 0.40 0.6 1.0 ]
      Sunset [ 1.0 0.6 0.2 ]
      # Sunset [ 0.3 1.0 0.5 ]
      CloudHeight 7
      CloudSpeed 65
      CloudMap "earth-clouds.*"
   }


EllipticalOrbit {
Period 10
SemiMajorAxis 10
ArgOfPericenter 180.0
}

Albedo 0.30
RotationPeriod 1000
}


Now, I'm a mite confused here. When Celestia starts up, shouldn't every object described here all be in a line? ABCD all seem to be lined up with eachother. But the planet isn't lined up with ABCD and E (I want Spica E to be directly opposite (ie 180?°) from ABCD relative to the planet, but instead ABCD are off-axis by an angle of about 10-20?°.

I can't see why this is happening, since all the Args of Pericenter are either 0 or 180 degrees - where's this 10-20?° offset coming from? I can't see any other parameter that could be doing this. Is this a bug? Or are planet and star orbits are figured out differently somehow? Or am I just missing something?

[Malenfant]

Here's another wierd thing. I've made the orbital parameters explicit here, and added a textureless 'reference ball'.

Code: Select all

"Spicaworld" "SpicaE"
{
Texture "mars.*"
Radius 6000

   Atmosphere {
      Height 60
      Lower [ 0.43 0.52 0.65 ]
      Upper [ 0.26 0.47 0.84 ]
      Sky [ 0.40 0.6 1.0 ]
      Sunset [ 1.0 0.6 0.2 ]
      # Sunset [ 0.3 1.0 0.5 ]
      CloudHeight 7
      CloudSpeed 65
      CloudMap "earth-clouds.*"
   }


EllipticalOrbit {
                Period              10.000
                SemiMajorAxis       10.000
                Eccentricity         0.000
                Inclination          0.000
                AscendingNode        0.000
                ArgOfPericenter      0.000
                MeanAnomaly          0.000
}

Albedo 0.30
RotationPeriod 1000
}


"Reference" "SpicaE"
{
Radius 1000

EllipticalOrbit {
                Period             100.000
                SemiMajorAxis       10.000
                Eccentricity         0.000
                Inclination          0.000
                AscendingNode        0.000
                ArgOfPericenter      0.000
                MeanAnomaly          0.000
}

Albedo 0.30
RotationPeriod 1000
}


Now, go here:
cel://SyncOrbit/SpicaE/2005-12-02T20:51 ... 6259&lm=71

and have the planet labels and orbits turned on so you can see the two bodies around Spica E.

Why is it that they have exactly the same orbital parameters except for the Period (which surely shouldn't affect the placement of the worlds in the orbit, should it?) - but they're in different locations on their orbit?!

If the Periods are the same then they're in the same place. If they're different, they've moved relative to eachother (eg when Reference's Period is 20, it's about 105 degrees from Spicaworld).

[chris]

Why is it that they have exactly the same orbital parameters except for the Period (which surely shouldn't affect the placement of the worlds in the orbit, should it?) - but they're in different locations on their orbit?!

It will affect the position unless you're viewing the objects at the reference time t0. You can set this value using the Epoch parameter in an .ssc file. The default is J2000.

--Chris

[Malenfant]

EDIT: Oh I see, so they're in the same place only on Jan 1st 2000, right? So I'm looking at them now, five years later, after they've done a few orbits so they'll be in different positions relative to eachother.

I guess the stars are still lined up because most of them have long orbital periods, so they wouldn't have moved much.