Use J2000 ecliptic (rather than equatorial) coordinates as starting point for precession.

We will assume that these are the coordinates returned by VSOP87.

2 files changed, 5 insertions, 2 deletions

diff --git a/org/madore/ephem/Nutation.java b/org/madore/ephem/Nutation.java
index bc6e721..8c37fd2 100644
--- a/org/madore/ephem/Nutation.java
+++ b/org/madore/ephem/Nutation.java
@@ -64,6 +64,7 @@ public final class Nutation {
     }
 
     public static Frames.Rotation matrix(double t) {
+	// Transform mean equatorial coordinates of date to true equatorial coordinates of date.
 	Frames.Rotation mat1 = Frames.Rotation.rotx(Precession.epsilon.v(t)*Comput.arcsecond);
 	Frames.Rotation mat2 = Frames.Rotation.rotz(-getFunc(Variable.PSI).v(t)*Comput.arcsecond).apply(mat1);
 	Frames.Rotation mat3 = Frames.Rotation.rotx(-(Precession.epsilon.v(t)+getFunc(Variable.EPSILON).v(t))*Comput.arcsecond).apply(mat2);
diff --git a/org/madore/ephem/Precession.java b/org/madore/ephem/Precession.java
index af8a1dc..33453c5 100644
--- a/org/madore/ephem/Precession.java
+++ b/org/madore/ephem/Precession.java
@@ -3,6 +3,7 @@ package org.madore.ephem;
 public final class Precession {
 
     // The following angles are in arcseconds, argument is in Julian centuries since J2000.
+    // Source: IERS Conventions 2010, §5.6.4
     public static final double epsilon0 = 84381.406;
     public static Comput.Polynomial psi
 	= new Comput.Polynomial(0, 5038.481507, -1.0790069, -0.00114045, 0.000132851, -0.0000000951); 
@@ -16,8 +17,9 @@ public final class Precession {
 	= new Comput.Polynomial(0.014506, 4612.156534, 1.3915817, -0.00000044, -0.000029956, -0.0000000368);
 
     public static Frames.Rotation matrix(double t) {
-	Frames.Rotation mat1 = Frames.Rotation.rotx(epsilon0*Comput.arcsecond);
-	Frames.Rotation mat2 = Frames.Rotation.rotz(-psi.v(t)*Comput.arcsecond).apply(mat1);
+	// Transform J2000 ecliptic coordinates to mean equatorial coordinates of date.
+	// Frames.Rotation mat1 = Frames.Rotation.rotx(epsilon0*Comput.arcsecond);
+	Frames.Rotation mat2 = Frames.Rotation.rotz(-psi.v(t)*Comput.arcsecond);
 	Frames.Rotation mat3 = Frames.Rotation.rotx(-omega.v(t)*Comput.arcsecond).apply(mat2);
 	Frames.Rotation mat4 = Frames.Rotation.rotz(chi.v(t)*Comput.arcsecond).apply(mat3);
 	return mat4;