One is the fixed stuff and one varies run by run. According to the standard, this simply allows you to have an extension if you want to. As far as I can see nobody ever has and this is simply a line using up space to no purpose. More on this below. I couldn't find one in the FITS standard.
Size is expressed in decimal degrees.
Change DATE-OBS to include time as well, not only date. I once came across a subroutine that claimed to compute Airmass. IIRC, it was actually computing zenith distance. It certainly didn't ask for barometric pressure. 2043.0 to 2044.0 and the centre is 1022.5.