> Hi Paul,
>
> In such a case, Guide will sit and spin until you hit the Escape
> key to stop it.
>
> I should be able to fix this (at least in most cases) by including
> some code that says, "if a star is more than about 7 degrees
> from the ecliptic, it absolutely cannot be occulted by the moon."
> The strict limit is 6.6 degrees, as discussed by Jean Meeus in
> _Astronomical Algorithms_.

You can refine that rule to "if a star is more than 1.3 degrees from
the lunar orbit, it cannot be occulted by the moon". Of course the
regression of the lunar nodes complicates matter a little.. :-) ..but
over a time interval of one year, you can compute the orientation of
the lunar orbit at the start of the year and at the end of the year:
if the star is at least 1.3 degrees away from both these orbits, it
cannot be occulted during that year, and thus you won't have to
investigade 12 or 13 conjunctions to figure out whether there's an
occultation occurring too.

Many years ago I wrote some lunar occultation prediction software,
which produced a list of visible occultations during one calendar
year as seen from a location of your choice. It originally ran on a
Z80 based CP/M-80 machine (i.e. an 8-bit CPU with no floating-point
hardware), thus I wanted to minimize the amount of number crunching
as much as possible. So this software worked in three steps:

1. Read the ZC and figure out which stars can be occulted this
year. I then used that "must be within 1.3 degrees of the lunar
orbit" algorithm mentioned above. This step output a "reduced"
ZC, valid for that one year.

2. Read that "reduced" ZC, investigate each and every lunar
conjunction and compute Besselian elements for all occultations
occurring during that year.

3. Read the table of Besselian elements, and figure out which of
these occultaitons are visible from a particular location. Outputs
a table of local predictions.

Step 2. was by far the most time consuming step here -- on that old
Z80 computer I often left it running overnight. But thanks to Step
1, the execution time of Step 2 could be reduced by a factor of
perhaps 4 or 5, i.e. it was enough to let it run ONE night. Without
Step 1, half a week might have been needed instead... :-)

==================================================================

Another thing about those occultation maps output by Guide, i.e. the
maps showing on which parts of the Earth the occultation will be
visible: did you ever consider adding the option of a polar
projection of thes maps? A lot of occultations are visible at high
latitudes only, and for such occultations it's preferable to have a
map in polar projection showing the area of visibility.

I'm not asking for a lot of fancy projection options here, but merely
two options:

1. The current equatorial centered map.

2. A polar centered map, of either hemisphere down to, say, 30 deg
latitude of that hemisphere (beyond that, map distortion gets too
large).

Perhaps this could be included in Guide 9 ?

----------------------------------------------------------------
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pausch at saaf dot se
WWW: http://hem.passagen.se/pausch/index.html
http://home.tiscali.se/~pausch/