NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Peter Hakel
Date: 2021 Nov 14, 10:30 -0800
Frank,
I remember having this discussion here on NavList several years ago, when I first found myself adding an ad hoc term to the SHA of this star (I think it was +0.4’ the first time, now it is +0.9’), and other NavListers offered explanations (maybe Paul Hirose and/or Antoine Couette - sorry, I don’t have time to dig through the archives right now). Maybe the proper motion of R.K. indeed has a significant nonlinear component. Interestingly enough, though, its declination does not need to be “fixed” like this at this time.
Anyway, perhaps other tools such as yours or Astron will implement a more accurate model for R.K.’s proper motion. I lean toward leaving it in aries_stars.xls as it is as long as it’s still “good enough” for navigation purposes for the upcoming year. Sirius looks OK so far.
Peter Hakel
================
Re: Navigation Spreadsheets in 2022
From: Frank Reed
Date: 2021 Nov 14, 09:05 -0800
Peter Hakel, you wrote:
"difference up to 0.6' in the SHA of the star Rigil Kent."
Peter, I've been meaning to post about Rigil Kentaurus and proper motion for some time. For the calculationally inclined, let's try this: what are the SHA and Dec of Rigil Kentaurus (alpha Centauri) in the year 2050? And how about 1950? This is tricky because the listed proper motion components in the modern catalogs mostly do not reflect the motion of the center of mass of this double star system. A secondary complication, which I am inclined to ignore, is the issue of "center of light" for the system.
Note: I don't know if that's the source of your 0.6' difference, Peter. It may be related or not. But the proper motion problem applies to all of us who calculate long-term stellar coordinates. I'm still working on this myself. Apart from Rigil K., the only other example where there may be trouble is the star Sirius since Sirius B (the famous white dwarf companion) significantly affects the motion of Sirius proper.
Frank Reed
