//Originally by Zool, revised by The_Duck
//Converts a true anomaly into an eccentric anomaly.
//For elliptical orbits this returns a value between 0 and 2pi
//For hyperbolic orbits the returned value can be any number.
//NOTE: For a hyperbolic orbit, if a true anomaly is requested that does not exist (a true anomaly
//past the true anomaly of the asymptote) then an ArgumentException is thrown
public static double GetEccentricAnomalyAtTrueAnomaly(this Orbit o, double trueAnomaly)
{
double e = o.eccentricity;
trueAnomaly = JUtil.ClampDegrees360(trueAnomaly);
trueAnomaly = trueAnomaly * (Math.PI / 180);
if (e < 1) //elliptical orbits
{
double cosE = (e + Math.Cos(trueAnomaly)) / (1 + e * Math.Cos(trueAnomaly));
double sinE = Math.Sqrt(1 - (cosE * cosE));
if (trueAnomaly > Math.PI)
{
sinE *= -1;
}
return(JUtil.ClampRadiansTwoPi(Math.Atan2(sinE, cosE)));
}
else //hyperbolic orbits
{
double coshE = (e + Math.Cos(trueAnomaly)) / (1 + e * Math.Cos(trueAnomaly));
if (coshE < 1)
{
throw new ArgumentException("OrbitExtensions.GetEccentricAnomalyAtTrueAnomaly: True anomaly of " + trueAnomaly + " radians is not attained by orbit with eccentricity " + o.eccentricity);
}
double E = JUtil.Acosh(coshE);
if (trueAnomaly > Math.PI)
{
E *= -1;
}
return(E);
}
}