[fitswcs] RADESYS = 'GAPPT'

[patrick.wallace at stfc.ac.uk]

Steve Allen writes:

> I cannot find anything in the published standard which points out
> that the interpretation of GAPPT depends upon DATE-OBS or MJD-OBS.

I'm not sure what at what level the point is being made.  Because
apparent place (or for that matter the newfangled CIRS place) is a
"current coordinates" concept, and is affected by aberration and
precession-nutation, its interpretation requires you to know when
the observation was made.  It isn't terribly critical (small
fraction of 1 arcsec per day), so the precise way you use the date
information won't matter much.  But you have to know when the
picture was taken, one way or another.  If the documentation fails
to say so, perhaps it is assumed to be self-evident (not necessarily
so).


Patrick Wallace
____________________________________________________________________
Space Science & Technology Department                +44-1235-531198
STFC Rutherford Appleton Laboratory
Harwell Oxford
Didcot, Oxfordshire, OX11 0QX, UK         Patrick.Wallace at stfc.ac.uk
____________________________________________________________________


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