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``.

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))