cos Z = sin(lat) sin(dec) + cos(lat) cos(dec) cos(hr)
Z is the zenith distance and hr is the hour angle.
cos A = (sin(lat) cos (Z) - sin (dec)) / cos(lat) sin (Z)
A is the Azimuth.
http://astro.umsystem.edu/atm/ARCHIV.../msg00355.html
http://astro.umsystem.edu/atm/ARCHIV.../msg00357.html
