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.
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
> ================================================================================
>
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/ --------------------------------------------------------------------------Received on 2005-06-20Z15:35:47