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    Re: Polaris' PM in RA?
    From: Sean C
    Date: 2020 Sep 24, 07:07 -0700

    Roger,

    You asked:

    "Are you also correcting for [nutation]?"

    I double-checked and yes, I am correcting for proper motion, then precession, then nutation, and finally aberration. I am following the procedures outlined in Meeus' book fairly closely (at least I think I am).

    And Frank,

    You wondered if I had perhaps forgotten to convert the proper motion from seconds of time to seconds of arc.

    I am starting with the J2000 right ascensions in decimal degrees and the PM in mas/yr. I am then multiplying the PM by the time interval in years between epochs. All of the corrections I mentioned above are being done in decimal degrees. I convert the final apparent RA to h:m:s later to get GHA by subtracting from GAST. So no, a conversion error does not seem to be the problem.

    I did take your advice and had a look at Rigil Kentaurus and Arcturus - and something was definitely wrong there, too. Which is odd, because I was sure I checked Rigil K. when I was troubleshooting specifically because it has such a large PM in RA. Anyway, that got me looking at the RA, Dec. and PM values I have more closely. It is now clear to me that I did not get my data from SIMBAD. At least, not all of it. But now I have no clue where I did get it from. It's been so long since I started this project that I can't remember.

    I compared more of the data in my spreadsheet to SIMBAD and the SAO and FK5 catalogues and wherever I got these values from was apparently not a great source. Some of them make sense and match the other sources, but many are off. I really don't think I would have used multiple sources in different formats ... but I suppose it's possible. However, I just tried using the data for a couple stars directly from the FK5 catalogue and the results were much better. So, I think we may have found the culprit. I am now going to try changing all of my stellar data to the FK5 values and I will report back after more testing.

    I don't know why I didn't use the FK5 catalogue in the first place, especially considering the fact that Meeus mentions it specifically in his book. And I'd really like to figure out where I got the data I was using, but I think that just might remain a mystery.

    Thank you, Roger and Frank, for the help. I can't tell you how much I appreciate it!

    Regards,

    Sean C.

    PS

    Anyone care to take a stab at why the sources I've looked at disagree so much when it comes to proper motion? The FK5 catalogue agrees with Meeus that Polaris' PM in RA is around 3000 mas/yr. So where did SIMBAD get 44 ... and where did Wiki get 198?

       
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