Expressing position in RDF

[Rob Seaman]

I'm no expert, but the intent seems to be to remove the Earth from the  
equation entirely.  Of course, I doubt they account for the rotation  
of the Milky Way or motions of the Local Group :-)


On Oct 15, 2008, at 11:36 AM, Ed Shaya wrote:

> Rob,
>
> You may be right on the equinox staying fixed for the foreseeable  
> future.
> Here is the definition of the zero of right ascension for ICRS:
>
> "Its origin of right ascensions was implicitly defined by fixing the  
> right ascension of 3C 273B to the Hazard et al. (1971) FK5 value  
> transferred at J2000.0."
>
> The ICRS is an inertial reference frame.  So, was B1950 and J2000.
>
> What will they do in 2015?  They may decide to maintain the above  
> quoted definition by pegging the right ascension of 3C 273B to its  
> current ICRS value.  Or they may update everything so that the  
> origin of right ascension moves to the vernal equinox of 2015 or  
> so.  Standards are permanent only until the next standard arrives.
>
>
> Ed
>
> Rob Seaman wrote:
>> ICRS = International Coral Reef Symposium
>> ICRS = International Christian Retail Show
>> ICRS = International Cartilage Repair Society
>> ICRS = International Cannabinoid Research Society
>> ICRS = International Celestial Reference System
>> "International" is a quaint adjective to attach to celestial  
>> coordinates.  (I much prefer the chutzpah of "Universal" Time :-)   
>> Presumably it was a placeholder for the International in IAU, this  
>> new standard's momma organization.  ICRS is fixed with respect to  
>> the ICRF, a grid of 608 VLBI (way distant) sources stationary in  
>> space and orientation by construction.  See http://aa.usno.navy.mil/faq/docs/ICRS_doc.php 
>>  or Pat Wallace's paper from ADASS XVII.
>> Precession and nutation don't go away - the Earth still wobbles  
>> like a top - but these effects don't appear in ICRS coordinates.   
>> The Vernal Equinox has been defined out of existence because ICRS  
>> is no longer an equatorial coordinate system, unlike FK5 and all  
>> those J's, B's, etc.
>> The implications for the Art of Astronomy are still unclear.  Very  
>> smart people, but too few engineers perhaps, were involved in  
>> defining ICRS.  The net effect for the purposes of this virtual  
>> observatory discussion appears to be to avoid the need to specify  
>> just one scalar - the equinox.  All the other STC-like and non-STC  
>> issues persist.  Real observatories face issues like all their  
>> equatorial mount telescopes turning into tilted alta-az  
>> telescopes :-)
>> Rob
>> ---
>> On Oct 15, 2008, at 6:06 AM, Ed Shaya wrote:
>>> Matthew,
>>>
>>> Precession must have been explained before somewhere in VOEVENT  
>>> discussions, but here is another try at emphasizing it.
>>>
>>> Precession rotates the equatorial system at the rate of about 50.3  
>>> arcsec per year (approximately 360 degrees divided by 25,700 yr),  
>>> or 1 degree every 71.6 years.  Thus, one year of precession is  
>>> almost as large as the distance from the center of the HST/WFPC2  
>>> field to the edge.  We avoid vast confusion by stating the  
>>> positions in a given frame fixed at a particular epoch (hence  
>>> J2000, B1950 etc).
>>> In two years GAIA will fly and there will be a vastly superior  
>>> reference frame, if all goes well (knock wood).  Logically, we  
>>> will all flock to this new reference frame which will probably be  
>>> set at a current epoch like 2015.  So, in not too many years your  
>>> VOEVENT database will be a mixed bag of J2000 and G2015.  With  
>>> about 12.5 arcminute disagreements for the positions of anything  
>>> with low declination,  10 times larger than the JWST field of view  
>>> from center to edge.
>>> Just the facts.  You can do what you want with them.
>>> You could, in 2015, replace all positions with G2015 positions,  
>>> for instance.  But the harder issue is how do you inform all  
>>> applications (including someone's one line IDL procedure) that  
>>> query your database that this change has been made?
>>>
>>> Ed
>>>
>>> PS - And hopefully there will be an S2020 from SIM.
>>>
>>>
>>> Matthew Graham wrote:
>>>> Hi,
>>>> RDF is just like XML so questions of production, presentation and  
>>>> storage are really quite secondary. It's the representation of  
>>>> information that is the primary concern and the issue here is  
>>>> does the IVOA have a succinct way of representing celestial  
>>>> positions in RDF. As Ed, I think, said, RDF and ontologies are  
>>>> supposed to remove the need for context to provide meaning to the  
>>>> information and so how the RDF statement is used is also quite  
>>>> secondary. The statement means what it says whatever.
>>>> SPARQL is the query language for RDF and is very SQL-like so an  
>>>> inequality is just a constraint on the search predicate. The  
>>>> exact syntax is dependent on how verbose the RA expression is.
>>>> I would not use a SPARQL query to do intersections, though - we  
>>>> have STC and Footprint Services for that. There might an RDF  
>>>> statement that a data object has a footprint associated with it  
>>>> and but then the object will the URI for the footprint itself.
>>>>   Cheers,
>>>>   Matthew
>>>> On Oct 14, 2008, at 7:35 PM, Roy Williams wrote:
>>>>> Matthew
>>>>>
>>>>> I like Doug's question, asking how the RDF will be used:  
>>>>> produced, presented, stored etc. Another question in the same  
>>>>> genre asks what *queries* will run against this knowledge base.  
>>>>> I suspect that "RA" will be used in an arithmetical inequality  
>>>>> -- "Give me all sources with RA>240.0", also as part of a cone  
>>>>> search or polygon. Is it Sparql, the query language for RDF? How  
>>>>> would an inequality be framed as part of a Sparql query?
>>>>>
>>>>> More abstract, we could utilize RDF at a higher level -- not  
>>>>> details of coordinate systems, as in this naked quantity "RA".  
>>>>> How about the RDF handles "Regions" (of spacetime). With  
>>>>> Regions, the questions are no longer arithmetical, but rather  
>>>>> boolean choices, about intersection: "Does the Region in which  
>>>>> this event lies intersect with the spacetime coverage Region of  
>>>>> the Catalina Survey?"
>>>>>
>>>>> Roy
>>>>>
>>>>>
>>>>> -- 
>>>>>
>>>>> California Institute of Technology
>>>>> 626 395 3670
>>>>>