def transformProperMotions(self, phi, theta, muphistar, mutheta):
        """
        Converts proper motions from one reference system to another, using the prescriptions in section
        1.5.3 of the Hipparcos Explanatory Volume 1 (equations 1.5.18, 1.5.19).

        Parameters
        ----------

        phi       - The longitude-like angle of the position of the source (radians).
        theta     - The latitude-like angle of the position of the source (radians).
        muphistar - Value of the proper motion in the longitude-like angle, multiplied by cos(latitude).
        mutheta   - Value of the proper motion in the latitude-like angle.

        Returns
        -------

        muphistarrot - Value of the transformed proper motion in the longitude-like angle (including the
        cos(latitude) factor).
        muthetarot   - Value of the transformed proper motion in the latitude-like angle.
        """
        c, s = self._getJacobian(phi,theta)
        return c*muphistar+s*mutheta, c*mutheta-s*muphistar
Converts proper motions from one reference system to another, using the prescriptions in section
        1.5.3 of the Hipparcos Explanatory Volume 1 (equations 1.5.18, 1.5.19).

        Parameters
        ----------

        phi       - The longitude-like angle of the position of the source (radians).
        theta     - The latitude-like angle of the position of the source (radians).
        muphistar - Value of the proper motion in the longitude-like angle, multiplied by cos(latitude).
        mutheta   - Value of the proper motion in the latitude-like angle.

        Returns
        -------

        muphistarrot - Value of the transformed proper motion in the longitude-like angle (including the
        cos(latitude) factor).
        muthetarot   - Value of the transformed proper motion in the latitude-like angle.