NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Polaris' PM in RA?
From: Paul Hirose
Date: 2020 Sep 24, 14:02 -0700
From: Paul Hirose
Date: 2020 Sep 24, 14:02 -0700
A diagnosis is difficult since Sean has not given his results. However, one frequent point of confusion is the exact meaning of the right ascension proper motion: seconds of arc vs. seconds of time, seconds per Julian century vs. milliseconds per Julian year, etc. Another issue is coordinate rate vs. great circle rate. In other words, does the unit of RA proper motion get smaller as you approach the poles, or is it always the same size as the declination unit? The latter is the modern tendency. Typically, both proper motions are given in mas (milli arc seconds) per Julian year (exactly 365.25 days). In other words, they are both in great circle units. For navigational stars the most reliable catalog is probably the re-reduction of the Hipparcos satellite observations (van Leeuwen, 2007). That seems to be the default catalog used by SIMBAD. SIMBAD right ascension proper motions are in great circle units: mas per Julian year. A note to that effect appears if you hover the mouse pointer over the place. http://simbad.u-strasbg.fr/simbad/sim-id?Ident=polaris&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id If you demand a high degree of accuracy it's not easy to validate your own software. You can't be sure how far to trust other applications until they have been tested. One good test is the stellar reduction example in every edition of the Astronomical Almanac. It's challenging since the fictitious star is close and has high proper motion. If your software succeeds here (within its intended accuracy, not necessarily matching the Almanac to the last digit), it's almost certainly OK. https://archive.org/details/binder1_202003/page/n189/mode/2up The Almanac example stops at the geocentric coordinates. For a reduction all the way to azimuth and altitude, see the Patrick Wallace example. It's rather intimidating, but if there's anything weak in your code this will reveal it. https://syrte.obspm.fr/iauWGnfa/ExPW04.html For a detailed discussion on these matters use the "contact me" link at my site. -- Paul Hirose http://sofajpl.com/
