docs: def parang (hourangle, declination, latitude):
      """Calculate the parallactic angle of a sky position.
  
      This computes the parallactic angle of a sky position expressed in terms
      of an hour angle and declination. Arguments:
  
      hourangle
        The hour angle of the location on the sky.
      declination
        The declination of the location on the sky.
      latitude
        The latitude of the observatory.
  
      Inputs and outputs are all in radians. Implementation adapted from GBTIDL
      ``parangle.pro``.
  
      """
      return -np.arctan2 (-np.sin (hourangle),
                          np.cos (declination) * np.tan (latitude)
                          - np.sin (declination) * np.cos (hourangle))