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# Source code for astropy.coordinates.builtin_frames.fk5

    # Licensed under a 3-clause BSD style license - see LICENSE.rst

    import erfa

    from astropy.coordinates.attributes import TimeAttribute
    from astropy.coordinates.baseframe import base_doc, frame_transform_graph
    from astropy.coordinates.transformations import DynamicMatrixTransform
    from astropy.utils.decorators import format_doc

    from .baseradec import BaseRADecFrame, doc_components
    from .utils import EQUINOX_J2000, get_jd12

    __all__ = ["FK5"]

    doc_footer = """
        Other parameters
        ----------------
        equinox : `~astropy.time.Time`
            The equinox of this frame.
    """

    @format_doc(base_doc, components=doc_components, footer=doc_footer)
    class FK5(BaseRADecFrame):
        """
        A coordinate or frame in the FK5 system.

        Note that this is a barycentric version of FK5 - that is, the origin for
        this frame is the Solar System Barycenter, *not* the Earth geocenter.

        The frame attributes are listed under **Other Parameters**.
        """

        equinox = TimeAttribute(default=EQUINOX_J2000, doc="The equinox time")

        @staticmethod
        def _precession_matrix(oldequinox, newequinox):
            """
            Compute and return the precession matrix for FK5 based on Capitaine et
            al. 2003/IAU2006.  Used inside some of the transformation functions.

            Parameters
            ----------
            oldequinox : `~astropy.time.Time`
                The equinox to precess from.
            newequinox : `~astropy.time.Time`
                The equinox to precess to.

            Returns
            -------
            newcoord : array
                The precession matrix to transform to the new equinox

            References
            ----------
            Hilton, J. et al., 2006, Celest.Mech.Dyn.Astron. 94, 351
            """
            # Multiply the two precession matrices (without frame bias) through J2000.0
            fromepoch_to_J2000 = erfa.bp06(*get_jd12(oldequinox, "tt"))[1].swapaxes(-2, -1)
            J2000_to_toepoch = erfa.bp06(*get_jd12(newequinox, "tt"))[1]
            return J2000_to_toepoch @ fromepoch_to_J2000

    # This is the "self-transform".  Defined at module level because the decorator
    #  needs a reference to the FK5 class

    @frame_transform_graph.transform(DynamicMatrixTransform, FK5, FK5)
    def fk5_to_fk5(fk5coord1, fk5frame2):
        return fk5coord1._precession_matrix(fk5coord1.equinox, fk5frame2.equinox)