diff --git a/src/astro.cpp b/src/astro.cpp
index baf34b9db..e4fe33bc5 100644
--- a/src/astro.cpp
+++ b/src/astro.cpp
@@ -22,6 +22,8 @@ using namespace std;
 #define KM_PER_AU     149597870.7
 #define AU_PER_LY     (KM_PER_LY / KM_PER_AU)
 
+const double astro::speedOfLight = 299792.458; // km/s
+
 // epoch J2000: 12 UT on 1 Jan 2000
 const double astro::J2000 = 2451545.0;
 
diff --git a/src/astro.h b/src/astro.h
index 065c3ab13..00ab32464 100644
--- a/src/astro.h
+++ b/src/astro.h
@@ -75,6 +75,7 @@ namespace astro
     Point3f equatorialToCelestialCart(float ra, float dec, float distance);
 
     extern const double J2000;
+    extern const double speedOfLight; // km/s
 };
 
 // Convert a date structure to a Julian date