--- In
guide-user@yahoogroups.com, paul <astro@p...> wrote:
> Bill et al
> a question has arisen about how Guide calculates
> PA, it appears to measure counter-clockwise, but
> it has been suggested (to me) PA is more frequently
> measured clockwise! but I also see that eg the
> double star catalogues (that I looked at) in Guide
> match the Guide calculation, so
> 1) which is preferred?
> 2) did Guide (Bill :) ever modify a star catalogue
> so PA matched Guide output, or I rather assume that
> that Bill worked it out so Guide PA matches the
> catalogues. do double star catalogues use different
> methods?
Hi Paul,
PA is _always_ measured from north through east; on a star chart in
a conventional, unrotated and unreflected orientation, that
corresponds to an anti-clockwise direction. All catalogs (that I've
ever encountered, anyway) use this convention, eg galaxy and double
star catalogs, and I'm absolutely certain that Guide does likewise.
It's confusing to use words like "clockwise" and "anti-clockwise"
when talking about PA, because the "direction" of measurement is
dependent on the type of optical system in use. Better to stick
to "east" and "west", which don't vary with the optics.
Regards,
Chris