When I first posted I thought there might be a fairly prosaic answer hidden somewhere. That's why I was less than accurate as to dates (not mid-December, but Dec 4), no moon (there really was a crescent to the south west but very thin).

The next time I'm in the area, possibly volunteering again at Big Bend NP, I'll make a much more detailed observation...bringing my D70, long lens, etc. Might be a fascinating trip. Oh yes, I'd bring warmer clothes.

GeorgieB