Position really needs an "epoch" metadata attribute (coords are relative to the equinox which moves over the years) so the coords need the "epoch" to be specified (e.g. "2000" or "2050"). That this kind of thing can rear its head unexpectedly might be an argument for favoring separate elements to group fields? If it were thrown in with the other attributes of "star" it looks like it qualifies the star, but it only really qualifies the coordinates.