diff --git a/includes/defines.h b/includes/defines.h
index 2aed70221..88997b0a9 100644
--- a/includes/defines.h
+++ b/includes/defines.h
@@ -21,6 +21,9 @@
 #ifndef M_PI
 #define M_PI 3.14159265f
 #endif
+#ifndef M_GRAV
+#define M_GRAV 6.67300e-1
+#endif
 
 #define IMGCONNS 3
 #define TIMEOUT 100