diff --git a/src/rotate_p.scad b/src/rotate_p.scad
index 0a7f77df..ab8aab65 100644
--- a/src/rotate_p.scad
+++ b/src/rotate_p.scad
@@ -17,12 +17,13 @@ include <__private__/__to3d.scad>;
 
 function _q_rotate_p_3d(p, a, v) = 
     let(
+        half_a = a / 2,
         axis = v / norm(v),
-        s = sin(a / 2),
+        s = sin(half_a),
         x = s * axis[0],
         y = s * axis[1],
         z = s * axis[2],
-        w = cos(a / 2),
+        w = cos(half_a),
         
         x2 = x + x,
         y2 = y + y,