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    Re: Ecliptic coordinates of stars
    From: Peter Monta
    Date: 2015 Mar 29, 21:54 -0700
    Hi Alexander,


    Would anybody know where I can find the ecliptic coordinates of the 57 stars used in celestial navigation to at least 6 significant figures. I am using these coordinates to calculate the SHA and declination of these stars.


    I'm wondering whether you mean equatorial coordinates, which are then but a short step to SHA and declination.  If you really do want ecliptic for some reason, you'll have to transform the (equatorial) catalog data.

    There are many astrometric catalogs, but for bright stars used with celestial navigation you'll probably be happy with a 20th-century catalog like FK5.  One umbrella source for stellar data is SIMBAD:

    http://simbad.u-strasbg.fr/simbad/

    Just type the star name into the "basic search" box and you'll get lots of data, including position, parallax, and proper motion.

    There's always the issue of what precision is needed for your application, but it sounds like you may be interested in the current state of the art just out of curiosity.  If so, there are (I find) many confusing details in fundamental positional astronomy---not only must the reference frame be unambiguously defined (and a good deal of historical baggage "unlearned"), there's also of course the need for reliable observational data on the stars themselves.  The SOFA and NOVAS packages may be useful for their documentation (and also the Explanatory Supplement).  As for actual data, space-based catalogs are best, but Hipparcos is now getting a little old and the proper motions are injecting some uncertainty into current epochs.  Gaia is now taking data on-orbit and should be really great.  I'm not sure how bright it goes, though; it may not be equipped for the very brightest sources.  I think the USNO CCD catalog, UCAC4, covers all stars including bright ones, but it is ground-based, so global systematics are roughly at the 20 mas level, much worse than Hipparcos at its best epoch.

    All this is fun to think about, and it does no harm to use the best star positions available, but for practical purposes when dealing with ground-based CN, the usual error sources will totally swamp the astrometric uncertainties.

    Cheers,
    Peter

       
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