Orbital precession

[SevenSpheres]

Would be good to be able to specify orbital precession in ssc files. It would probably use a PrecessingOrbit block, similar to PrecessingRotation.

[SevenSpheres]

From @pedro-fixingstuff on Discord:

how I imagine it would work

PrecessingOrbit
{
    Epoch
    SemiMajorAxis
    Period
    Eccentricity
    Inclination
    AscendingNode
    ArgOfPericenter
    MeanAnomaly
    NodePeriod
    PericenterPeriod
}

[SevenSpheres]

From TTarrants on Discord, posting here because it's related

Remember when we talked about the possibility of Celestia displaying precessing orbits?

PrecessingOrbit
{
   Epoch _float_
   Period _float_
   PrecessionAxisInclination _float_
   PrecessionAxisNode _float_
   MeanAnomaly _float_
}

^ Or something to that effect. It might also be interesting to have a block for decaying orbits.

DecayingOrbit
{
   Epoch _float_ # Epoch of semi-major axis.
   DecayRate _float_ # dP/dt in seconds per year at epoch.
   DecayRateChange _float_ # d^2P/dt^2 in seconds per year.
}

(of course negative decay rate values could produce expanding orbits) The WASP-12b paper got me thinking about this because eventually because the orbital period is evolving, there is going to be a mismatch between literature transit times and transit times as they are observed in Celestia. Granted, I haven't done the base amount of effort to determine how big an issue this is going to be over our lifetimes. Probably not much I suspect.

Another concern would be a need to modify the orbital period in some way. Perhaps that could be done automatically. The period at some time t _0 will be a certain ratio of what it was at t _1 regardless of what the user provides as the stated orbital period in EllipticalOrbit.

... or maybe DecayingPeriod might be what we should define since that is usually the observed parameter.

[SevenSpheres]

From pedro again:

From the suggestions thrown here and on other channels about modelling orbital precession and evolution, I came up with the idea to add a one-size-fits-all DynamicOrbit block, which I think could look like this:

DynamicOrbit
{
    Epoch
    Period
    SemiMajorAxis
    Eccentricity
    Inclination
    AscendingNode
    ArgOfPericenter
    MeanAnomaly
    NodePeriod
    ApsisPeriod
    PeriodChange or SemiAxisChange
    LowerClamp
    UpperClamp
}

Most of the names should be self-explanatory, but I imagine the LowerClamp and UpperClamp as ways to limit the changes in semi-major axis/period to a certain range of values or a period of time.