[AstroPy] Discrepancy between astropy.constants parsec and au values?

Derek Homeier derek at astro.physik.uni-goettingen.de
Thu Jul 16 08:25:32 EDT 2020


On 16 Jul 2020, at 11:17 am, Hatchell, Jenny <J.Hatchell at exeter.ac.uk> wrote:
> 
> Derek, thanks for looking into this.  I must admit I’ve never calculated a parallax using anything other than the small angle approx., so it’s good to be reminded that there could be a tangent in the calculation.
> 
> The IAU parsec definition is buried in a note at the bottom of a resolution on a completely different topic (zero points for magnitude scales), so it's easy to miss. 
> 
It is actually of course based on the definition of the AU from 2012 B2; I guess they did not
realise they had not clarified the parallax method until the next GA, but since that is even
quoted in the docstring, it is good to become consistent. Fixed for Astropy 4.0.2.
As the difference runs up to ~1.6 AU on a kpc, which would be somewhat below the distance
change between two consecutive parallax measurements for typical Galactic RVs, it is indeed
unlikely to be noticed, but thanks again for the bug report!

Cheers,
					Derek



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