diff --git a/simgear/ephemeris/moonpos.cxx b/simgear/ephemeris/moonpos.cxx
index 8952be5c..55070a56 100644
--- a/simgear/ephemeris/moonpos.cxx
+++ b/simgear/ephemeris/moonpos.cxx
@@ -85,6 +85,8 @@ void MoonPos::updatePosition(double mjd, double lst, double lat, Star *ourSun)
     cosN, sinN, cosvw, sinvw, sinvw_cosi, cosecl, sinecl, rcoslatEcl,
     FlesstwoD, MlesstwoD, twoD, twoM, twolat, alpha;
   
+  double max_loglux = -0.504030345621;
+  double min_loglux = -4.39964634562;
   double conv = 1.0319696543787917;    // The log foot-candle to log lux conversion factor.
   updateOrbElements(mjd);
   actTime = sgCalcActTime(mjd);
@@ -227,4 +229,8 @@ void MoonPos::updatePosition(double mjd, double lst, double lat, Star *ourSun)
 
   // Convert from foot-candles to lux.
   log_I += conv;
+
+  // The moon's illuminance factor, bracketed between 0 and 1.
+  I_factor = (log_I - max_loglux) / (max_loglux - min_loglux) + 1.0;
+  I_factor = SGMiscd::clip(I_factor, 0, 1);
 }
diff --git a/simgear/ephemeris/moonpos.hxx b/simgear/ephemeris/moonpos.hxx
index 0f87b9d3..b601d425 100644
--- a/simgear/ephemeris/moonpos.hxx
+++ b/simgear/ephemeris/moonpos.hxx
@@ -42,6 +42,7 @@ private:
     double age;        // the moon's age from 0 to 2pi
     double phase;      // the moon's phase
     double log_I;      // the moon's illuminance outside the atmosphere (logged)
+    double I_factor;   // the illuminance factor for the moon, between 0 and 1.
     // void TexInit();  // This should move to the constructor eventually.
 
     // GLUquadricObj *moonObject;
@@ -70,6 +71,7 @@ public:
     double getAge() const;
     double getPhase() const;
     double getLogIlluminance() const;
+    double getIlluminanceFactor() const;
 };
 
 inline double MoonPos::getM() const
@@ -122,4 +124,9 @@ inline double MoonPos::getLogIlluminance() const
   return log_I;
 }
 
+inline double MoonPos::getIlluminanceFactor() const
+{
+  return I_factor;
+}
+
 #endif // _MOONPOS_HXX_