VOunits draft

[Steve Allen]

On 2009 May 19, at 14:07, louys at alinda.u-strasbg.fr wrote:
> Here is the VO UNITS draft, that I circulate for comments within  
> the DM and VOTable Working group .

I am mystified by the references to SLALIB.  That library does not
include the algorithms which are currently approved by the IAU.
As far as I know the relevant libraries which do implement the
IAU 2000 recommendations are

IAU SOFA
http://www.iau-sofa.rl.ac.uk/

USNO NOVAS
http://aa.usno.navy.mil/software/novas/novas_info.php

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.

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.

--
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