Hello Bernd, Alan, Pete,

I think I must have been having an "off" day when I answered Alan's
question, because there's a simpler way of doing this. For his
situation (adding a fixed zone around the zenith of radius 23 degrees),
one would add these lines to 'horizon.dat':

c 2
0 90 23

For Pete's situation, needing a circle of radius 18 degrees around
the North Celestial Pole, that would change to

c 2
0 44.01 18

For '44.01' (my latitude), substitute your own latitude, of course!

In both cases, one is simply adding a circle of given radius to a
particular alt/az point. The zenith is at alt=90, az=0 (*), and the
north celestial pole is at alt=latitude, az=0.

Pete, you also mentioned: "... Being able to mark the viewing
constraints, such as the neighbor's tall elm tree, would simplify planning."
This can also be done. By default, 'horizon.dat' shows assorted objects
scattered around the horizon, such as trees, houses, etc. There's
some documentation at the bottom of the file describing how you can
move, add, scale, etc. these objects.

If you're feeling truly energetic, you can even add your own objects;
see 'objects.nam' for examples and, near the end, some (admittedly
minimal) documentation. I don't know if anyone has actually done this;
the objects already supplied are probably enough for most needs.

-- Bill

(*) OK, in reality, the azimuth at the zenith is undefined, and you
can use any value you want there; it won't affect how the program
does things.