General Question Mean longitude and its effect on Proteus' orbit

[SuprunP]

I've got two Proteus.cfg files.

; === Configuration file for moon Proteus(Neptune) ===
Name = Proteus
EllipticOrbit = TRUE           ; assume unperturbed orbit
HasElements = TRUE             ; orbital elements follow

; === Planetary Mean Orbits ===
;***cfgBatchDataCalculator
Epoch =  2006.49555099247
SemiMajorAxis =  117646138.8      ; by Chode
Eccentricity =  7.21043486568402E-04
Inclination =  .50673265090984
LongAscNode =  .844501641369886
LongPerihelion =  4.32758109853377
[B][COLOR="#ff0000"]MeanLongitude =  4.80178725780075[/COLOR][/B]



; === Physical Parameters ===
Mass = 2.346e19			 ; Approx. Calculated using Phoebe's Density
Size = 200e3
SidRotPeriod = 96968.016	 ; 1.122315 days

SidRotOffset = 4.35902303        ;IAU J2000
Obliquity = 0.50476492           ;IAU J2000
LAN = 3.98195745                 ;IAU J2000

AlbedoRGB = 0.19 0.19 0.19       ; by VF2_Rolf

; === Visualisation Parameters ===
MaxPatchResolution = 4

; === Configuration file for moon Proteus(Neptune) by Rolf Keibel Carl Romanik and Tony Dunn===
Name = Proteus
EllipticOrbit = TRUE           ; assume unperturbed orbit
HasElements = TRUE             ; orbital elements follow

; === Planetary Mean Orbits ===
Epoch =  2006.64792344792
SemiMajorAxis =  117597985.139546
Eccentricity =  2.27683633145637E-04
Inclination =  .506563473268528
LongAscNode =  .844614299027461
LongPerihelion =  5.51442267827102
[B][COLOR="Red"]MeanLongitude =  8.49960185096777[/COLOR][/B]

; === Physical Parameters ===
Mass = 2.346e19			 ; Approx. Calculated using Phoebe's Density
Size = 200e3
SidRotPeriod = 96968.016	 ; 1.122315 days
SidRotOffset = 0
Obliquity = 0.51452              ; inherit from Neptune
LAN = 2.29898                    ; inherit from Neptune
AlbedoRGB = 0.13 0.13 0.13

; === Visualisation Parameters ===
MaxPatchResolution = 4

Apart from discrepancies in Eccentricity and LongPerihelion parameters (although I've found the eccentricity here (Wiki), I can't say the same about LongPerihelion) I've noticed that MeanLongitude parameters are also different. Having read [ame="http://en.wikipedia.org/wiki/Mean_longitude"]this (Wiki)[/ame] article I'm not so sure that I can calculate it myself from those date I have.
But I'm interested in how this parameter can change the 'behaviour' of a celestial body in general and Proteus in particular. What are your thoughts?

P.S. (If you happen to have more reliable data concerning Proteus could you please share it?)

Thanks.

 

[martins]

Your two config files also quote two different epochs. Have you checked that the mean longitudes aren't actually equivalent when you propagate one set to the epoch of the other?

 

[SuprunP]

Thank you, martins.

Have I understood it right that since the time-varying astronomical quantities include the size of the major axis of the orbit of a body the eccentricity varies with time as well?

Thanks.

 

[martins]

This is true only if the elements represent the state vectors at a particular time in a multi-body system ("osculating elements"). Since the orbit is perturbed, the state vectors at a different time will give rise to different osculating elements.

In a true 2-body system, the elements don't change. The only exception is the mean longitude, if expressed "at epoch", i.e. at the current observation time. If the mean longitude is given for a fixed time, that won't change either, obviously.