<div dir="ltr">
<div> is proper motion ALWAYS in spherical?</div><div>Yes<br></div><div><br></div><div> if so, is it always EQUATORIAL?</div><div>Usually, but I don't think it needs to be, necessarily; could be converted to Galactic, for instance<br></div><div>Caveat: it can be either specified by separate RA and Dec components, or as total PM and a direction (CCW from North)<br></div><div><br></div><div> which reference frames apply?</div><div>Depends on what the provider of the data used; for modern data one would hope ICRS, but historical data could be any FK</div><div>On the other hand, I suspect that for proper motions with any reasonable precision this won't make a bit of difference<br></div><div><br></div><div> and the reference positions?</div><span class="gmail-im"><div>
For Equatorial it should definitely be BARYCENTER <br></div><div><br></div></span>
<div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr">-------------------------------------------------------------------------------------------------------------<br>Arnold H. Rots Chandra X-ray Science Center<br>Smithsonian Astrophysical Observatory tel: +1 617 496 7701<br>60 Garden Street, MS 67 fax: +1 617 495 7356<br>Cambridge, MA 02138 <a href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a><br>USA <a href="http://hea-www.harvard.edu/~arots/" target="_blank">http://hea-www.harvard.edu/~arots/</a><br>--------------------------------------------------------------------------------------------------------------<br><br></div></div></div><br></div><br><div class="gmail_quote"><div dir="ltr">On Sat, Dec 22, 2018 at 10:25 AM CresitelloDittmar, Mark <<a href="mailto:mdittmar@cfa.harvard.edu">mdittmar@cfa.harvard.edu</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>Hi Arnold! that was fast.</div><br><div class="gmail_quote"><div dir="ltr">On Fri, Dec 21, 2018 at 5:46 PM Arnold Rots <<a href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><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></blockquote><div> </div><div>sure, the most generic Velocity would be unrestricted </div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="auto"><div dir="auto"></div><div dir="auto">Then there are proper motions: 2-D in angular units per time unit, in a spherical coordinate system.</div></div></blockquote><div><br></div><div>this is the main question.. </div><div> is proper motion ALWAYS in spherical?</div><div> if so, is it always EQUATORIAL?</div><div> which reference frames apply?</div><div> and the reference positions?</div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="auto"><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></blockquote><div><br></div><div>then maybe not what was asked for in Victoria.</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="auto"><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></blockquote><div><br></div><div>got it.</div><div><br></div><div>Mark</div></div></div>
</blockquote></div>