STC Questions

[Arnold Rots]

David Berry had also weighed in on this subject, about a month ago,
arguing that epoch should not be associated with a spatial coordinate
frame.  I have been convinced by the combined arguments of David and
Francois.  So, an epoch will be an optional attribute of
astroCoordsType as well as regionType; it will be a double.
And the order of precedence will be as follows:

The spatial coordinate values represent those at a time equal to:
- the epoch, if present; otherwise:
- the time of observation, if present; otherwise:
- the equinox of the coordinate system, if applicable; otherwise:
- 2000.0 for ICRS; otherwise:
- UNKNOWN

Note that this is only relevant (at least I think I have this list
complete) for objects for which RA and Dec are time-variable due to:
- proper motion
- parallax
- orbital motion: binary stars, solar system objects,
  extra-solar planets, galactic rotation

Epoch should not be confused with equinox which is a property of the
coordinate system (as correctly pointed out in the previous messages),
if applicable.

  - Arnold

Francois Ochsenbein wrote:
> 
> Quick answer...
> 
> >
> >Quick note....
> >
> >Francois states that
> >>    The epoch of a coordinate system represents the date to which the
> >>    spherical angles are reduced.
> >
> >I believe that you are defining Equinox here, not Epoch.
> >The epoch is the time of observation.
> >
> 
> OK, my original point
> 
> > 2. The Question of Epoch (Question (4) of the RFC page)
> >
> >    The epoch of a coordinate system represents the date to which the
> >    spherical angles are reduced. After reflexion, it looks more logical
> >    to attach  the Epoch to the AstroCoords structure (section 4.6)
> >
> 
> would be better rephrased as
> 
> 2. The Question of Epoch (Question (4) of the RFC page)
> 
>    The epoch of the coordinates represents the date to which the
>    spherical angles are reduced. After reflexion, it looks more logical
>    to attach  the Epoch to the AstroCoords structure (section 4.6)
> 
> (suppression of "system"). To a coordinate system it could be possible
> to attach a default epoch.
> 
> --Francois
> 
> ================================================================================
> Francois Ochsenbein       ------       Observatoire Astronomique de Strasbourg
>    11, rue de l'Universite F-67000 STRASBOURG       Phone: +33-(0)390 24 24 29
> Email: francois at astro.u-strasbg.fr   (France)         Fax: +33-(0)390 24 24 32
> ================================================================================
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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