diff --git a/metar/Datatypes.py b/metar/Datatypes.py
index c1ba23f1..201f6245 100644
--- a/metar/Datatypes.py
+++ b/metar/Datatypes.py
@@ -480,8 +480,8 @@ def getdistance(self, position2):
         long1 = self.longitude
         lat2 = position2.latitude
         long2 = position2.longitude
-        a = sin(0.5(lat2 - lat1)) + cos(lat1) * cos(lat2) * sin(
-            0.5 * (long2 - long1) ** 2
+        a = (sin(0.5(lat2 - lat1))) ** 2 + cos(lat1) * cos(lat2) * (sin(
+            0.5 * (long2 - long1)) ** 2
         )
         c = 2.0 * atan(sqrt(a) * sqrt(1.0 - a))
         d = distance(earth_radius * c, "M")