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    Re: Star - Star Observations
    From: Frank Reed
    Date: 2010 Mar 16, 06:48 -0700

    Brad, I asked previously,
    "Just out of curiosity, could you post some sample RAs and Decs for today from the Skyscout? Five or ten first magnitude stars would be enough to get a sense of the position quality."

    I gather you've moved on from the Skyscout, but humor me. :-) I'm just curious about the accuracy. I haven't touched a Skyscout in years, and I never checked their data. Thanks!

    You wrote:
    "it says I pointed at 'Bayer: 5 UMi' and that the brightness of this object is 4.25, the RA is 14h 27.53m and the Dec is 75.6960 degrees. Page H19 of the Bright Stars table for 2010 states: 'Flamsteed / Bayer Designation: 5 UMi BS=HR no. 5430, RA = 14h 27m 30.9s Dec = +75d 39m 14s and that the brightness is 4.25. That sounds like a precise match to me and took far longer to type into this email than to look up.

    Now lets compare the data. From the Bright Stars table, the RA 14h 27m 30.9s, which I believe is 216d 52m 43.5s. From the Skyscount, the RA is 14h 27.53m, which I believe works out to 216d 52m 57.0s. That works out to a difference of 13.5 seconds. Pretty darned good if you are using the Skyscout for its intended purpose, but out by 2 tenths of a minute for what I want to use it for. But with the Bright Stars table, the inaccuracy is avoided."

    The declinations don't seem to match. Looks like 2.5 minutes of arc difference which sounds unlikely. Is there a typo? No real need to check. You can just post some new stars. As for the RAs, is it true that they are only displayed to the nearest hundredth of a minute of RA in the Skyscout? If so, then of course that means that they would be borderline for this purpose no matter what since 0.01 minutes in RA in hours is equal to 0.15 minutes of RA when expressed as an angle. As you note, the RAs in this case appear to differ by 13.5 seconds of arc. But then again... that doesn't really affect star-star angles so badly. This star is at very high declination. The RA lines converge at the celestial pole so a small difference in RA is a considerably smaller difference in actual position. In this case it's only 3 arcseconds in the actual position (that's 13.5*cos(75)).

    In general, you're going to need star positions that are corrected for aberration and proper motion (in a few cases) so maybe you should just find yourself a decent shareware planetarium program. How about Stellarium? Has anyone checked its position data for stars? It's open source, so you could dig around in the code and find out just what it's doing.

    -FER

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