Modeling Velocities

[CresitelloDittmar, Mark]

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
ProperMotion and RadialVelocity(?).

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?
   Radial Velocity:
    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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