Modeling Velocities

[Arnold Rots]

   is proper motion ALWAYS in spherical?
Yes

  if so, is it always EQUATORIAL?
Usually, but I don't think it needs to be, necessarily; could be converted
to Galactic, for instance
Caveat: it can be either specified by separate RA and Dec components, or as
total PM and a direction (CCW from North)

  which reference frames apply?
Depends on what the provider of the data used; for modern data one would
hope ICRS, but historical data could be any FK
On the other hand, I suspect that for proper motions with any reasonable
precision this won't make a bit of difference

  and the reference positions?
For Equatorial it should definitely be BARYCENTER

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Arnold H. Rots                                          Chandra X-ray
Science Center
Smithsonian Astrophysical Observatory                   tel:  +1 617 496
7701
60 Garden Street, MS 67                                      fax:  +1 617
495 7356
Cambridge, MA 02138
arots at cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
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On Sat, Dec 22, 2018 at 10:25 AM CresitelloDittmar, Mark <
mdittmar at cfa.harvard.edu> wrote:

> Hi Arnold!  that was fast.
>
> On Fri, Dec 21, 2018 at 5:46 PM Arnold Rots <arots at cfa.harvard.edu> wrote:
>
>> There are simple Cartesian coordinates, in 1, 2, or 3 dimensions. Don't
>> dismiss this as unimportant: it's used in all solar system and orbit
>> ephemerides. In units of length per unit of time.
>>
>
> sure, the most generic Velocity would be unrestricted
>
> Then there are proper motions: 2-D in angular units per time unit, in a
>> spherical coordinate system.
>>
>
> this is the main question..
>   is proper motion ALWAYS in spherical?
>   if so, is it always EQUATORIAL?
>   which reference frames apply?
>   and the reference positions?
>
>
>
>>  Then there are radial velocities: 1-D unit of length per unit of time
>> along the line of sight. However, this is rarely used in astronomy  - only
>> really in models or when ephemerides are transformed to a spherical
>> coordinate system.
>>
>
> then maybe not what was asked for in Victoria.
>
>
>>
>> Far more common is a (radial) Doppler velocity. But that is not properly
>> a velocity in a spatial coordinate system. It is a pseudo velocity along
>> the redshift coordinate axis, expressed in unit of length per unit of time.
>> That redshift axis is absolutely crucial. If you set this up as a
>> velocity in a spatial coordinate system, you are setting yourself up for
>> trouble later on.
>>
>
> got it.
>
> Mark
>
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