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> *From: *Antoine Couëtte
> *Date: *2025 Jan 16, 01:11 -0800
>
> Regarding double / multiple Navigation Stars, we observe that the
> maximum geocentric angular distance between center of light and center
> of gravity - or even center of the brightest component and any of the
> just previously mentioned "centers" - never exceeds 6 arc seconds.


The problem is to get star catalog data for the center of gravity. In
the long term, data for either component is not accurate since they
orbit about the common center of gravity. For example, in Simbad the
alpha Centauri primary and secondary are separate entries:

alpha Centauri A

https://simbad.u-strasbg.fr/simbad/sim-id?Ident=*+alf+cen+a&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id

alpha Centauri B

https://simbad.u-strasbg.fr/simbad/sim-id?Ident=*+alf+cen+b&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id

With those data I have their separation 5.4' in the year 3000, an absurd
result. There's a flaw in the calculation: it assumes the proper motion
of each component is along a great circle at constant rate. In reality,
they orbit about their center of gravity. I think the period is about 80
years. A good celestial nav approximation might use the mean proper
motions of both stars, weighted according to their masses.

The Simbad data for alpha Cen are credited to the 2007 re-reduction of
the early 1990s Hipparcos satellite observations. It would be nice to
have data from the ongoing Gaia mission, but I think all navigational
stars are too bright for Gaia. For instance, Simbad credits Hipparcos
for the Polaris data, but Gaia for 61 Cyg A (mag 5.2).

--
Paul Hirose
sofajpl.com

   

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