VOunits draft

[Anita M. S. Richards]

>
> I'm mentioned yotta and yocto because they are part of SI and it keeps being 
> asserted that we must support ALL of SI.  It seems more important to support 
> table columns in units of parsecs (something we definitely will want to do) 
> rather than columns in units of yoctohertz (something I am skeptical will 
> prove of practical value).

I very much hope that we will do all of these - and Mpc and even Ypc....

>
> So will folks have to convert 1 megaparsec to 30.9 Zettameters (Zm)?  Bear in 
> mind that the distance estimate may be accurate only to +/- 50% and may scale 
> with the hubble constant...

One folk will write a piece of code (or has already done so) which 
everyone else can use seamlessly without even realising.  If it does not 
give the game away by converting 30.9 to 30.8888888756789...

Th Units WG does not take any responsibility for cosmological constants, 
though - the value of H0 in the published data is what the user will get - 
caveat emptor!  If there are tools which perform conversions using H0 or 
anything else, that and the value used should be visible.

The question of precision is rather separate but in my astronomer (and 
referee...) hat I am delighted to see it discussed, even though in my 
editor of Units doc hat, it is yet another headache...

> I'm not adverse to using standards.  The question is whether we are choosing 
> standards solely because some authority has blessed them.

er, don't we all want everyone to use standards which the IVOA has 
adopted? But hopefully not written in tablets of stone...

>
>> Maybe I'm wrong, but what I've understood from the discussion is that the 
>> defintion of a common unit system is needed for 'internal' purpose (eg 
>> service-to-service communication). At the level of the human client, any 
>> 'traditional' unit is allowed. Am I right?
>
> The choice of internal units must still make sense astronomically and 
> computationally.  But I think that VO facilities must also be considered to 
> include their user interfaces.  If our internal systems are required to use 
> zettameters and our external interfaces parsecs, some agent must perform the 
> conversion.

Indeed, but better to have as few agents as widely used as possible for 
consistency.  And NB the Units workpackage is only talking about providing 
tools for conversions which are the same for any given data - e.g. not 
most magnitude systems...


all the best

Anita