Binary Planet/Moon orbits acting weird.

[eternal brook]

Why is it that when Dardes (my planet) and Enera (my moon) are played from January 1, 2000 onwards, Dardes gradually catches up to Enera? How do I fix this?

[grand falcon]

Hard to say without looking at the script, but if one is faster than the other it probably means that orbital periods are not identical, where in a binary system that should be the case.

[eternal brook]

Orbit
{
RefPlane "Equator"
PeriodDays 7.26184472
SemiMajorAxisKm 9504.03
Eccentricity 0.0549
Inclination 107.03
AscendingNode 16.5043
ArgOfPericenter 138.15
MeanAnomaly 20.405
}
}

Here are the orbital characteristics for Dardes

Orbit
{
RefPlane "Equator"
PeriodDays 7.26184472
SemiMajorAxisKm 19039.495
Eccentricity 0.0549
Inclination 120.034
AscendingNode 16.5043
ArgOfPericenter 138.15
MeanAnomaly 20.405
}

And the orbit for Enera.

[grand falcon]

A binary system follows specific laws of physics, from the manual:

To make a bi­nary star sys­tem or a bi­nary planet sys­tem, make sure that the fol­low­ing pa­ram­e­ters are the same for
both bod­ies in the sys­tem: Ec­cen­tric­ity, In­cli­na­tion, As­cend­ingN­ode, MeanAnomaly. The Ar­gOf­Peri­cen­ter pa­ram­e­‐
ter of the first body must dif­fer by 180 de­grees from the same pa­ram­e­ter of the sec­ond body. It’s also nec­es­sary to
cal­cu­late the Semi­Ma­jo­rAxis pa­ram­e­ter for each body based on their masses us­ing these sim­ple for­mu­lae:
Body 1: a1 = R * M2 / (M1 + M2)
Body 2: a2 = R * M1 / (M1 + M2)
where R is the dis­tance be­tween the bod­ies, i.e. sum of their semi­ma­jor axes a1 + a2;
M1 and M2 are masses of bod­ies.

Set the same Inclination for both bodies, and one ArgOfPericenter at 318.15. If you remove PeriodDays for both, SpaceEngine should calculate the correct period given the masses of the two bodies.

If we put the values as they are into the calculations, we find that Dardes should be 2.0033075442546027 times the mass of Enera. If it's not like this, then the period is incorrect for the masses, and letting SpaceEngine decide what it is could be the best solution.

[eternal brook]

How do I know what R is supposed to be?

And now Dardes is take 11.247 days to orbit the barycenter

[grand falcon]

where R is the dis­tance be­tween the bod­ies, i.e. sum of their semi­ma­jor axes a1 + a2;

The period is directly related to the masses of the bodies.
this is the formula:
Period = sqrt(R^3 / (M1 + M2))
where R is again the sum of semimajor axes. Forgot to mention that in those formulae Semimajor axis is in AU.

May I see the entire script?

[eternal brook]

Hey, do I divide both or just one of the values?

[grand falcon]

Sorry, what value are you referring to?
I hope this is less confusing:

[eternal brook]

DwarfPlanet "Lukteus"
{
ParentBody "Lukteus-Kophine"
Class "Aquaria"
Mass 0.00207
Radius 1195.4053

TidalLocked        true
Obliquity          0
EqAscendingNode    0

AlbedoBond         0.721
AlbedoGeom         0.523
Brightness         2

NoLife             true

NoClouds           true

NoOcean            true

Atmosphere
 {
 
    Model          "Pluto"
    Height          156.403
    Density         0.02314
    Pressure        9.504e-6
    Greenhouse      0.0505
    Bright          10
    Opacity         1
    SkyLight        3.3333
    Hue             0.025
    Saturation      1
    Composition
    {
    
       N2      76.506
       CH4     22.943
       CO      0.551
     }
     
}

NoAurora           true

NoRings         true

NoAccretionDisk true

NoCometTail     true
Orbit
{
   RefPlane        "Equator"
   SemiMajorAxis   1.9540702e-5
   Eccentricity    0.0054
   Inclination     127.605
   AscendingNode   154.095
   ArgOfPericenter 209.0093
   MeanAnomaly     17.50432
   
 }

}

Moon "Kophine"
{
ParentBody "Lukteus-Kophine"
Class "Aquaria"
Mass 2.518e-4
Radius 592.30443

TidalLocked        true
Obliquity          0
EqAscendingNode    0

AlbedoBond         0.234
AlbedoGeom         0.512
Brightness         2

NoLife             true

NoClouds           true

NoOcean            true

NoAtmosphere       true

NoAurora           true

NoRings         true

NoAccretionDisk true

NoCometTail     true
Orbit
{
   RefPlane        "Equator"
   SemiMajorAxis   1.60640401e-4
   Eccentricity    0.0054
   Inclination     127.605
   AscendingNode   154.095
   ArgOfPericenter 29.0093
   MeanAnomaly     17.50432
   
 }

}

[night bobcat]

Delete the SemiMajorAxis tag for both and replace it with PeriodDays, then set it to your desired length

Orbit
{
RefPlane "Equator"
PeriodDays Value
Eccentricity 0.0054
Inclination 127.605
AscendingNode 154.095
ArgOfPericenter 29.0093
MeanAnomaly 17.50432

 }

}

Orbit
{
RefPlane "Equator"
PeriodDays Value
Eccentricity 0.0054
Inclination 127.605
AscendingNode 154.095
ArgOfPericenter 209.0093
MeanAnomaly 17.50432

 }

}