[obs-tap]observable quantities in observations and their UCDs

[Igor Chilingarian]

On Wed, 23 Jun 2010, Anita M. S. Richards wrote:

>> As I already mentioned in our discussion here at CDS, I do not think that 
>> "velocity" is observable in any sense. I would agree that it can be a 
>> spectral coordinate, but we NEVER directly observe velocities. Velocities 
>> are results of the [model-dependent] data analysis.
>
> Hm, but what about proper motions?

Still they are not velocities.
1) Normally, a proper motion is a data analysis results for a set of 
observations (time series)

2) Even in rare cases of Solar system objects (or meteors in the Earth 
atmosphere or satellites) we can measure a trace, i.e. a change of *angular 
coordinates* in time. To derive velocity, one needs to know the distance (and 
take into account projection effects. The velocity is not observed directly.

With best regards,
 						Igor