<div dir="auto">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. <div dir="auto"><br></div><div dir="auto">Then there are proper motions: 2-D in angular units per time unit, in a spherical coordinate system. </div><div dir="auto"><br></div><div dir="auto">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.</div><div dir="auto"><br></div><div dir="auto">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.</div><div dir="auto">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. </div><div dir="auto"><br></div><div dir="auto">Cheers,</div><div dir="auto"><br></div><div dir="auto"> - Arnold </div></div><br><div class="gmail_quote"><div dir="ltr">On Fri, Dec 21, 2018, 17:22 CresitelloDittmar, Mark <<a href="mailto:mdittmar@cfa.harvard.edu">mdittmar@cfa.harvard.edu</a> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div><div><div><div><div><div><div><div><div><div><div>DMers,<br><br></div>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.<br><br></div>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(?). <br><br></div>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:<br></div> Proper Motion: <br></div> o should this be tied to a Spherical coordinate space? or is it sometimes given in other spaces?<br></div> in other words, is it always the velocity in the directions of (ra,dec), (l,b), etc?<br></div> Radial Velocity:<br></div> o wikipedia tells me this is 1 dimensional, in the 'line-of-site' to an object (observer, sun)<br></div><div> Is this just the velocity in the inverse direction of the Radial axis of a standard Spherical space (Equatorial, Galactic, etc )?<br></div> <br></div>These are outside my wheel-house, so please let me know of any specifics that I need to fold in.<br><br></div>Mark<br><br></div>
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