As noted in #2244 and #3217, eventually we should support using the JPL ephemerides. A good place to start would be @brandon-rhodes https://github.com/brandon-rhodes/python-jplephem

It needs some mechanism by which a user can opt to download the (large) ephemeris files, as we would not want to include those in astropy by default.