Added equatorialToCelestiaCart to convert RA,dec style coordinates to the celestial Cartesian coordinates used everywhere in Celestia.

src/astro.cpp

@@ -10,6 +10,7 @@
 #include <cmath>
 #include <iomanip>
 #include "celestia.h"
+#include "mathlib.h"
 #include "astro.h"
 
 using namespace std;
@@ -27,6 +28,8 @@ const double astro::J2000 = 2451545.0;
 // epoch B1950: 22:09 UT on 21 Dec 1949
 #define B1950         2433282.423
 
+static Mat3f equatorialToCelestial = Mat3f::xrotation(degToRad(-23.4392911));
+
 
 float astro::lumToAbsMag(float lum)
 {
@@ -162,6 +165,21 @@ UniversalCoord astro::universalPosition(Point3d heliocentric,
 }
 
 
+// Convert equatorial coordinates to Cartesian celestia (or ecliptical)
+// coordinates.
+Point3f astro::equatorialToCelestialCart(float ra, float dec, float distance)
+{
+    double theta = ra / 24.0 * PI * 2;
+    double phi = (dec / 90.0 - 1.0) * PI / 2;
+    double x = -cos(theta) * sin(phi) * distance;
+    double y = -cos(phi) * distance;
+    double z = -sin(theta) * sin(phi) * distance;
+
+    return (Point3f((float) x, (float) y, (float) z) * equatorialToCelestial);
+}
+
+
+
 astro::Date::Date(int Y, int M, int D)
 {
     year = Y;

src/astro.h

@@ -63,6 +63,8 @@ namespace astro
     UniversalCoord universalPosition(Point3d heliocentric,
                                      Point3f starPosition);
 
+    Point3f equatorialToCelestialCart(float ra, float dec, float distance);
+
     extern const double J2000;
 };