diff -Nur gpxviewer.old/gpxviewer/gpx.py gpxviewer/gpxviewer/gpx.py
--- gpxviewer.old/gpxviewer/gpx.py	2011-08-23 11:46:58.062708863 -0300
+++ gpxviewer/gpxviewer/gpx.py	2011-08-23 11:48:01.812602991 -0300
@@ -33,7 +33,14 @@
   lon2 = radians(lon2)
   
   # Great Circle Distance Formula
-  arc = acos((sin(lat1) * sin(lat2)) + (cos(lat1) * cos(lat2) * cos(lon2 - lon1)))
+  arc = ((sin(lat1) * sin(lat2)) + (cos(lat1) * cos(lat2) * cos(lon2 - lon1)))
+  try:
+      arc = acos(arc)
+  except ValueError:
+      if arc > 1.:
+          arc=0.
+      else:
+          arc=pi
   
   d = R * arc
   return d