public static double GetVolumetricMeanRadius(this ICalculatableRadius model) =>
 (model.EquatorialRadius + model.PolarRadius) / 2;
 public static double GetEllipticity(this ICalculatableRadius model) =>
 1 - (DoubleHelper.GetMin(model.EquatorialRadius, model.PolarRadius) / DoubleHelper.GetMax(model.EquatorialRadius, model.PolarRadius));