code: def get_apex(self, lat, height=None):
          
          lat = helpers.checklat(lat, name='alat')
          if height is None:
              height = self.refh
  
          cos_lat_squared = np.cos(np.radians(lat))**2
          apex_height = (self.RE + height) / cos_lat_squared - self.RE
  
          return apex_height
docs: Calculate apex height
  
          Parameters
          -----------
          lat : (float)
              Latitude in degrees
          height : (float or NoneType)
              Height above the surface of the earth in km or NoneType to use
              reference height (default=None)
  
          Returns
          ----------
          apex_height : (float)
              Height of the field line apex in km