Get spherical CIRS coordinates at a given epoch (ra, dec, distance).

        Calculate coordinates in the Celestial Intermediate Reference System
        (CIRS), a dynamical coordinate system referenced to the Celestial
        Intermediate Origin (CIO). As this is a dynamical system it must be
        calculated at a specific epoch.

def cirs_radec(self, epoch):
        """Get spherical CIRS coordinates at a given epoch (ra, dec, distance).

        Calculate coordinates in the Celestial Intermediate Reference System
        (CIRS), a dynamical coordinate system referenced to the Celestial
        Intermediate Origin (CIO). As this is a dynamical system it must be
        calculated at a specific epoch.
        """
        r_au, dec, ra = to_polar(self.cirs_xyz(epoch).au)

        return (Angle(radians=ra, preference='hours'),
                Angle(radians=dec, signed=True),
                Distance(r_au))