Correct sidereal rotation period for precession

This matches the behavior of precessing orbits

Author: pedro-fixingstuff

src/celengine/parseobject.cpp

@@ -206,7 +206,7 @@ CreateKeplerianOrbit(const AssociativeArray* orbitData,
     {
         double periodCorrection = 0.0;
         if (elements.nodalPeriod != 0.0)
-            periodCorrection += 1.0 / elements.nodalPeriod;
+            periodCorrection -= 1.0 / elements.nodalPeriod;
         if (elements.apsidalPeriod != 0.0)
             periodCorrection += 1.0 / elements.apsidalPeriod;
 

src/celephem/rotation.cpp

@@ -276,7 +276,9 @@ PrecessingRotationModel::getPeriod() const
 Eigen::Quaterniond
 PrecessingRotationModel::spin(double tjd) const
 {
-    double rotations = (tjd - epoch) / period;
+    // Correct the sidereal rotation period for precession of the node
+    double periodCorrection = precessionPeriod != 0.0 ? 1.0 / precessionPeriod : 0.0;
+    double rotations = (tjd - epoch) / (1.0 / (1.0 / period - periodCorrection));
     double wholeRotations = std::floor(rotations);
     double remainder = rotations - wholeRotations;