VOunits draft

[Arnold Rots]

Aside from my more impassioned please below, there are two issues that
I would like to raise:

1. Why not just refer to the Units section in FITS WCS Paper I?

2. By outlawing cgs units you are going to alienate the HEA community.
   As much as I dislike cgs units (after all, I grew up as a radio
   astronomer), I do need to put a word in defending the interests of
   HEA - they will be completely lost without their ergs and cm.

  - Arnold

Anita M. S. Richards wrote:
> 
> Dear Steve,
> 
> Thanks very much for your careful reading of the document.
> 
> 
> >
> > I may be stealing the words of Arnold Rots when I write
> > MJD is not a unit.  It is a measure of time which has
> > an origin distinction akin to that of K or degC for temperature.
> 
> I apologise if the document is worded clumsily, as I remember Sebastien 
> making a related point although the sutlety was lost on me.  However, MJD 
> (like some aspects covered by ISO-8601) is a label for a unit of elapsed 
> time.  We need to make sure that we can handle common use cases and that 
> involves knowing the origin of coordinate systems.  Perhaps someone can 
> come up with a more semantically correct term for unit labels like MJD.

NO NO NO!!!
MJD (or JD, for that matter) is NOT a unit.
It is a measure of time that happens to have the unit 'd'.

> 
> > Does the VO effort intend to address the difference between
> > meters and seconds in "TT units" and "TCB units"
> > and "TCG units"?
> > I.e., as seen on page 12 of IERS conventions 2003 (tech note 32)
> > a quantity of length or time has a different numerical value "x"
> > depending on the reference frame in which its value is expressed
> > x_{TDB} = x_{TCB} * (1 - L_B)
> > x_{TT}  = x_{TCG} * (1 - L_G)
> > where L_G is defined as 6.969290134e-10
> > and L_B is approximately 1.55051976772e-8
> > and whereas the IAU recommends measurements be in TCG or TCB units,
> > a common practice is to express them in TDB/TT units.
> 
> I think that if there is a use case and a suggested solution, that could 
> be included, although the conversion rules are not for us to define (if 
> possible!), but for us to point to libraries or quote where they are 
> defined.

I think this is a strong argument for specifying a complete (and
consistent) AstroCoordSystem in STC: the Time Scale in that element -
whether TT, TCG, TDB, or TCB - should resolve this ambiguity.
And, if I may add, this argues that a full STC element should be
considered the right thing to do for space-time (and related)
coordinate, rather than just picking a smattring of individual
elements from the STC schema.

> 
> Best wishes
> 
> Anita
> 
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> 
> Dr. A.M.S. Richards, UK ARC Node/AstroGrid,
> Jodrell Bank Centre for Astrophysics, Alan Turing Building, 
> University of Manchester, M13 9PL
> +44 (0)161 275 4124
> and
> MERLIN/VLBI National Facility, Jodrell Bank Observatory, 
> Cheshire SK11 9DL, U.K. +44 (0)1477 571321 (tel) 571618 (fax)
> 
> "Socialism or barbarism?" Rosa Luxemburg (1915)
> 
> On Tue, 19 May 2009, Steve Allen wrote:
> 
> 
--------------------------------------------------------------------------
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/
--------------------------------------------------------------------------