[QUAR] Re: Expressing position in RDF

[Rob Seaman]

Matthew Graham wrote:

> I'm happy for the philosophical discussion but am also trying to  
> figure out how to actually do something empirical.

Empiricism is a branch of philosophy, of course :-)

> I can see exactly the same arguments that we had about using STC in  
> VOEvent applying to representing positions in RDF. The 90:10 rule  
> should apply and whilst it is wonderful that I can describe any  
> position in any coordinate system using ontology X, why can't we  
> have ontology Y that is small and simple (that word again):
>
> :myStar stc:UTC-TOPO-FK5#RA 134.56

This is an opportunity, not a bump in the road.  First, start on p.  
179 of the ADASS XVII proceedings with Pat Wallace's contribution.   
Use ICRS, not FK5.

Second, what is your use case?  It appears to be the expression of  
coordinates of an unresolved object fixed to the celestial sphere.   
You are seem to be implicitly in a regime in which proper motion can  
be ignored.  Perhaps this should be made explicit in some fashion.   
Note that "unresolved", "fixed", and "no proper motion" are orthogonal  
to the STC issues.

One STC issue you must address (implicitly or explicitly) is  
parallax.  Is the "star" far enough away that this is negligible for  
your purpose(s)?  Another STC issue is whether you are you describing  
spectral observations - i.e., is a doppler correction (for the motion  
of the observer) needed?  Depending on the science to be pursued,  
other coordinate issues will arise.

Third, what is the goal of your application?  Are you describing  
coordinates in support of follow-up observations?  Or are the  
coordinates themselves somehow pertinent to scientific inferences?  In  
the former case, these are targeting coordinates as with VOEvent.  In  
the latter case, perhaps you don't want equatorial coordinates at all,  
but rather some galactic coordinate system.

In short, the celestial coordinates are not inherent to the stars  
themselves.  There are other such quantities, some pertaining to the  
point of view, some to the intervening environment.  My recommendation  
would be the same for representing these:

	X is a star
	X has environment Y
	Y has reddening Z

Not:

	X has reddening Z

Rob