# Modeling Velocities

Arnold Rots arots at cfa.harvard.edu
Fri Dec 21 23:46:05 CET 2018

```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.

Then there are proper motions: 2-D in angular units per time unit, in a
spherical coordinate system.

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.

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.

Cheers,

- Arnold

On Fri, Dec 21, 2018, 17:22 CresitelloDittmar, Mark <
mdittmar at cfa.harvard.edu wrote:

> DMers,
>
> I'm sure you are all preparing for the holidays and not wanting to think
> about data models, but I'll be out until just after the new year, and
> wanted to get this out.
>
> I'm starting to move to the Measurement model documentation.  At the
> Victoria interop hack-a-thon, it was called out that we NEED to include
>
> STC-1.33 and earlier drops of the Measurement model had a simple Velocity
> type to serve all velocities.  I can restore that, as a minimum, but it
> sounds like there is interest in these specific flavors as important
> "source properties" to model.  To do that, I'd need some more details:
>    Proper Motion:
>     o should this be tied to a Spherical coordinate space?  or is it
> sometimes given in other spaces?
>        in other words, is it always the velocity in the directions of
> (ra,dec), (l,b), etc?
>     o wikipedia tells me this is 1 dimensional, in the 'line-of-site' to
> an object (observer, sun)
>         Is this just the velocity in the inverse direction of the Radial
> axis of a standard Spherical space (Equatorial, Galactic, etc )?
>
> These are outside my wheel-house, so please let me know of any specifics
> that I need to fold in.
>
> Mark
>
>
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