[Difx-users] J2000 to date conversions {External}

[Leonid Petrov]

Chris,

   Why do you need it? Words like "equinox of date" are related to
efforts to simplify computations in the pre-computer epochs.
What do you want to achieve at the end?

Leonid

> On 2022-01-11 20:42, Phillips, Chris (S&A, Marsfield) via Difx-users 
> wrote:
> Hi al
> 
> Apologies for using the DIFX mailing list for a decidedly non-DIFX
> question. I’m guessing there is a number of people on this list who
> can help me with this query.
> 
> I am trying to convert some J2000 coordinates to “date
> coordinates” (ie precess to a specific date).
> 
> The initial idea was to use python astropy (rather than perl/slalib)
> to get with the times. However I am getting inconsistent results, and
> cannot find any documentation on how to do this in astropy.
> 
> As an example I am trying to convert 1934-638 from J2000 to epoch
> 2020-1-1. Various different approaches are here:
> 
> 1934-638 19h39m25.026s -63d42m45.63s J2000
> astropy 19h41m15.463s  -63d39m55.69s
> perl 19h41m13.563s -63d39m56.94s
> SLA 19h41m10.545s -63d40m02.77s
> Perl SLA 19h41m10.559s -63d40m02.76s
> Online 19h41m15.5s     -63d39m56s
> http://www.robertmartinayers.org/tools/coordinates.html
> 
> “Perl” is code I wrote years ago for Astro:Coord (on CPAN) based
> on Fortran code at Hobart observatory
> SLA is a hacked version from Caltech VLBI package “PRECESS” which
> uses SLALIB
> PerlSLA is a perl version which makes the same calls (mostly for
> coding consistency check)
> Online is a random webpage found via google.
> 
> Clearly astropy and the online version use the same algorithm (or
> online is using astropy under the hood).
> 
> My perl version is similar to astropy, but slalib is different by a
> lot. I remember years ago looking into this sort of stuff
>  (maybe B1950-J2000 rather than epoch precession) and SLALIB used a
> very different implementation to astronomical
> Almanac, following a paper which basically said the almanac
> implementation was wrong.
> 
> FYI, the relevant code is:
> 
> Astropy:
> 
> from astropy.coordinates import SkyCoord, FK5
> from astropy.time import Time
> 
> c = SkyCoord('19h39m25.026s', '-63d42m45.63s', frame='icrs')
> t = Time('2020-01-01 00:00:00', scale='utc')
> 
> my_date = FK5(equinox=Time(t))
> c_date = c.transform_to(my_date)
> 
> I’m a little dubious this is correct, given the use of
> “Equinox”, while I really want to change Epoch.
> 
> The SLALIB implementation is:
> 
>       CALL SLA_MAPPA(2000D0, DATE, AMPRMS)
>       CALL SLA_MAPQKZ(R2000, D2000, AMPRMS, RA, DA)
> 
> Where DATE is the MJD (checked to be consistent).
> 
> Does anyone have code which they trust to test these conversions (or
> comments)?
> 
> Thanks
> Chris
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