Index: celestiacore.cpp
===================================================================
--- celestiacore.cpp	(revision 4094)
+++ celestiacore.cpp	(working copy)
@@ -3140,7 +3140,7 @@
                               Body& body,
                               double t,
                               double distance,
-                              Vec3d /*viewVec*/)
+                              Vec3d viewVec)
 {
     double kmDistance = astro::lightYearsToKilometers(distance);
 
@@ -3161,6 +3161,47 @@
 
     displayApparentDiameter(overlay, body.getRadius(), kmDistance);
 
+	// Display the phase angle
+
+	// Find the parent star of the body. This can be slightly complicated if
+	// the body orbits a barycenter instead of a star.
+	Selection parent = Selection(&body).parent();
+	while (parent.body() != NULL)
+		parent = parent.parent();
+
+	if (parent.star() != NULL)
+	{
+		bool showPhaseAngle = false;
+
+		Star* sun = parent.star();
+		if (sun->getVisibility())
+		{
+			showPhaseAngle = true;
+		}
+		else
+		{
+			// The planet's orbit is defined with respect to a barycenter. If there's
+			// a single star orbiting the barycenter, we'll compute the phase angle
+			// for the planet with respect to that star. If there are no stars, the
+			// planet is an orphan, drifting through space with no star. We also skip
+			// displaying the phase angle when there are multiple stars (for now.)
+			if (sun->getOrbitingStars()->size() == 1)
+			{
+				sun = sun->getOrbitingStars()->at(0);
+				showPhaseAngle = sun->getVisibility();
+			}
+		}
+
+		if (showPhaseAngle)
+		{
+			Vec3d sunVec = Selection(&body).getPosition(t) - Selection(sun).getPosition(t);
+			sunVec.normalize();
+			double cosPhaseAngle = sunVec * ((1.0 / viewVec.length()) * viewVec);
+			double phaseAngle = acos(cosPhaseAngle);
+			overlay.oprintf("Phase angle: %.1f%s\n", radToDeg(phaseAngle), UTF8_DEGREE_SIGN);
+		}
+	}
+
     if (detail > 1)
     {
         if (body.getRotationModel()->isPeriodic())