[Re: VOunits draft] comments

[Rob Seaman]

On May 22, 2009, at 2:56 AM, Anita M. S. Richards wrote:

> 2) MJD
> Sorry, I did not make myself clear enough.  Data providers *do* use  
> MJD as a unit _label_.  This illustrates why the VO needs to be able  
> to understand and translate native usage of units into a standard.   
> Of course Arnold is right, but I think that he has missed the point.


An example of this?  I would think column labels (i.e., what UCDs were  
invented to describe) will often include MJD (whether or not UCDs  
do).  But do they then also express units something like:  "MJD  
(mjd)"?  I would think in most cases the unit labels would be left  
implicit, or very occasionally stated as "MJD (d)".

Or are you saying that there are significant numbers of examples  
resembling "time of mid-eclipse (mjd)"? We need to support  
astronomical usage in the real world, but that doesn't mean that we  
shouldn't attempt to map the underlying issues to VO usage.  Clearly  
the units here remain days, not mjd.  We shouldn't assume that  
anything expressed with unit-like syntax in the astronomical  
literature is actually a unit.

Clocks (and/or calendars) have a rate and a zero point (or phase).   
Units correspond to the rate.  MJD implies both a rate and a zero  
point anchored in the 19th century (and phase anchored to the Sun at  
Greenwich).  The representation (ISO-8601, sexagesimal, floating  
point, etc.) is a presentation issue on top of both rate and phase.   
It is not conventional, but MJD could be expressed in sexagesimal.   
The time scale (eg, STC) issues are above (or below) all of this.

Arnold and Steve are encouraged to improve/correct my statements :-)

Rob