diff --git a/lib/geo.js b/lib/geo.js
index 71334af7..998ed933 100644
--- a/lib/geo.js
+++ b/lib/geo.js
@@ -254,8 +254,8 @@ function geoDistance(x1, y1, x2, y2, options) {
     return Math.pow(Math.sin(a / 2.0), 2)
   }
 
-  var c = Math.sqrt(haversine(x2 - x1) + Math.cos(x2) * Math.cos(x1) * haversine(y2 - y1));
+  var f = Math.sqrt(haversine(x2 - x1) + Math.cos(x2) * Math.cos(x1) * haversine(y2 - y1));
 
-  return 2 * Math.asin(c) * EARTH_RADIUS[type];
+  return 2 * Math.asin(f) * EARTH_RADIUS[type];
 }