This comes out of the discussion in #4254, as well as some offline discussion about astroplan development.

If you execute the gist at https://gist.github.com/eteq/08dba433082e57b7f2af (with astropy master, pyephem, and skyfield installed), you see this:

PyEphem (Alt, Az, Dist): (55.4920153458 deg, 239.031895666 deg, 356424.848107 km)
Skyfield (Alt, Az, Dist): (55.32651159 deg, 239.298837015 deg, 356413.96585 km)
Skyfield(de430) (Alt, Az, Dist): (55.3265117716 deg, 239.298837058 deg, 356413.965628 km)
jplephem/Astropy (Alt, Az, Dist): (56.0578669005 deg, 239.043478848 deg, 356423.493806 km)
jplephem/Astropy(ICRS) (Alt, Az, Dist): (56.0596259457 deg, 239.03677597 deg, 356421.587215 km)

That demonstrates that there are disagreements about the as-observed location of the moon at the ~0.5-1 degree level (and ~10 km distance), but between astropy and the other two, as well as between skyfield and pyephem.

It's quite possible this level of difference is driven by subtle differences in how the various frames are defined rather than any sort of implementation bug... But we should try to get to the bottom of the differences, if possible.

And once we've figured it out, even if there are no bugs to be fixed, hopefully this investigation can provide a thorough battery of tests to be added to the testing suite.

cc @mhvk @bmorris3 @brandon-rhodes