diff --git a/src/apexpy/apex.py b/src/apexpy/apex.py
index <HASH>..<HASH> 100644
--- a/src/apexpy/apex.py
+++ b/src/apexpy/apex.py
@@ -57,6 +57,8 @@ class Apex(object):
     Notes
     -----
     The calculations use IGRF-12 with coefficients from 1900 to 2020 [1]_.
+    
+    The geodetic reference ellipsoid is WGS84.
 
     References
     ----------
Update apex.py

adding a note to the `Apex` class to address #<I>