i favor locally-temporally directions for this sort of thing over culturally-determined arbitrary coordinate systems. my favorite is the triad
where the sun rises today (the spirit of dawn and ascending)
where the sun sets today (the spirit of dusk and descending)
the projection of the apex (splitting the large angle between the two, so i guess if you were doing this on the equator at an equinox, you'd just pick one of the two ambiguities) (the spirit of transition, turning points, and balance)

but your SSE is excellent