[obs-tap]observable quantities in observations and their UCDs
igor.chilingarian at obspm.fr
Wed Jun 23 11:19:11 PDT 2010
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,
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