def orientation(self):
        """
        The angle in radians between the ``x`` axis and the major axis
        of the 2D Gaussian function that has the same second-order
        moments as the source.  The angle increases in the
        counter-clockwise direction.
        """

        a, b, b, c = self.covariance.flat
        if a < 0 or c < 0:    # negative variance
            return np.nan * u.rad  # pragma: no cover
        return 0.5 * np.arctan2(2. * b, (a - c))