diff --git a/solarmach/__init__.py b/solarmach/__init__.py
index 7c01c0b..51b899c 100644
--- a/solarmach/__init__.py
+++ b/solarmach/__init__.py
@@ -1969,6 +1969,7 @@ def pfss_3d(self, active_area=(None, None, None, None), color_code='object', rss
                 str_number = None
                 symbol = 'square'
 
+            # SkyCoord transformed to cartesian correspond to HEEQ for Stonyhurst
             fig.add_trace(go.Scatter3d(x=[(body_pos.cartesian.x.to(u.m)/R_sun).value],
                                        y=[(body_pos.cartesian.y.to(u.m)/R_sun).value],
                                        z=[(body_pos.cartesian.z.to(u.m)/R_sun).value],
@@ -2334,6 +2335,7 @@ def plot_3d(self, plot_spirals=True, plot_sun_body_line=True, markers=False, num
                 symbol = 'square'
 
             # customdata=[[dist_body], [body_long], [body_lat]]
+            # SkyCoord transformed to cartesian correspond to HEEQ for Stonyhurst
             fig.add_trace(go.Scatter3d(x=[body_pos.cartesian.x.value],
                                        y=[body_pos.cartesian.y.value],
                                        z=[body_pos.cartesian.z.value],
diff --git a/solarmach/pfss_utilities.py b/solarmach/pfss_utilities.py
index 33d284f..9f6541c 100644
--- a/solarmach/pfss_utilities.py
+++ b/solarmach/pfss_utilities.py
@@ -515,7 +515,10 @@ def check_field_line_alignment(coordinates):
 
 def spheric2cartesian(r, theta, phi):
     """
-    Does a coordinate transformation from spherical to cartesian
+    Does a coordinate transformation from spherical to cartesian.
+    For Stonyhurst heliographic coordinates, this means converting to HEEQ
+    (Heliocentric Earth Equatorial), following Eq. (2) of Thompson (2006),
+    DOI: 10.1051/0004-6361:20054262
 
     r : the distance to the origin
     theta : the elevation angle (goes from -pi to pi)