Modeling Velocities

[CresitelloDittmar, Mark]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.ivoa.net/pipermail/dm/attachments/20181222/19e92a43/attachment.html>