That's pretty neat. Any chance you could share the code for the calculations, or at least a source where I could get that info? I've been wanting to write something like this for my PocketPC Phone that could use the phone GPS to determine location and then do the calculation. (I have Ephemeris currently, but manual entry of coordinates is a PITA).
We got most of our info from the USNO.gov site but it's mostly basic trigonometry.