Standardising units and formats (and ref frames?) in transmission

[Arnold Rots]

One might argue that negative values are OK for JD, and possibly for
MJD, but one has to watch precision.

There is little appetite (also in FITS WCS) to extend ISO-8601; it is
not clear that there are any advantages, since for those ranges (M)JD
is more than adequate.

'd' will have to be defined as equal to 86400 s (and it is in the FITS
WCS proposal). Clearly, UTC becomes shaky when one would attempt to
extrapolate it beyond its current limits - we have some idea about the
history of the earth's rotation, but no agreement on ancient leap
seconds. So, if either TT or UTC would have to go, we should retain TT.

There is indeed also 'a', and conceivably 'cy', but I would be happy
with just 's' and 'd'. However, if we are talking about historical
records, there is the issue of the tropical year ('try'?). I am
beginning to think that I will have to relent and allow that one.

  - Arnold

Rob Seaman wrote:
> On May 18, 2009, at 12:08 PM, Arnold Rots wrote:
> 
> > I'm afraid we have to accommodate time in ISO-8601 as well as MJD  
> > (or JD); both have specific application. Besides, there are times  
> > that cannot be expressed in ISO-8601 (since it has limited range).
> 
> A couple of points here.  As Arnold says, even if a single time  
> representation were possible, it could not be ISO-8601.  Similarly,  
> MJD has an initial horizon of 1858 and even JD runs negative before  
> the 48th century BC.  MJD will be acceptable for many purposes, but  
> the VO also hopes to support theoretical data or extrapolations far  
> into the past or future.
> 
> >  I believe that, as far as units are concerned, both 's' and 'd'  
> > have a role to play.
> 
> And likely years as well.
> 
> Another issue - without digging too deeply into the leap second morass  
> - is that conversion between overtly similar units may be difficult or  
> fundamentally impossible.  A day is not equivalent to 86,400 seconds.   
> One is defined by reference to SI, the other by reference to Mother  
> Earth.
> 
> Rob
> 
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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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