[guide-user] Symbol troubles, occn's, etc.

Paul Schlyter Mar 29, 2002

> Paul, about the methods for finding an occultation: the one you suggest
> does have some appeal... it would at least work for non-moving objects
> such as stars. There are a few other "speedup" steps that I may apply
> first, though. (Guide's current method for finding occultations is
> somewhat "brute force" in nature, and therefore much slower than it
> really ought to be.)

Even with that "brute force" method you can greatly speed up the
search: use a lower-precision but fast algorithm for the Moon's
position when looking for occultations; when an occultation has been
found, recompute it using a slow but full-precision lunar algorithm.
During the lower-precision search, add an extra "safety margin" to
make sure you don't miss any occultations when searching with that
low-precision algorithm. You will then have to discard some "found"
occultations when recomputing them at full precision, but that's
definitely preferable to missing some actual occultations.

I have some C code for the Moon's position, using the algorithm in
Jean Meeus' older "Astronomical formulae for calculators": it yields
some 10" precision in longitude and some 3" precision in latitude,
which should be sufficient for search purposes if one adds a suitable
safety margin to ensure no events are missed. It was originally
implemented to run reasonable fast a Z80 8-bit micro, i.e. I made it
as fast as I could by e.g. replacing many trig operations with some
multiplies and adds instead. On a modern PC that code is lighting
fast. If you want, you can have a copy of it.

----------------------------------------------------------------
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
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