diff --git a/src/unitvector.hh b/src/unitvector.hh
index e2fbdf17e5aa38c0466cdf6735aabc05e4b3687e..200dfd48950a05d72b21b2b85d1881d294d9db3a 100644
--- a/src/unitvector.hh
+++ b/src/unitvector.hh
@@ -18,6 +18,11 @@ public:
         return *this;
     }
 
+    /** \brief Length of the great arc connecting the two points */
+     static double distance(const UnitVector& a, const UnitVector& b) {
+        return std::acos(a.data_ * b.data_);
+    }
+
     /** \brief Write LocalKey object to output stream */
     friend std::ostream& operator<< (std::ostream& s, const UnitVector& unitVector)
     {