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    Re: Star - Star Observations
    From: Brad Morris
    Date: 2010 Mar 16, 22:42 -0400
    Hi Frank
     
    I haven't tossed out the Skyscout, I just use it to identify which stars I want.  I use the Bright Stars list to get RA and Dec.  You can see the quality of the Skyscout data below.
     
    I have managed to get the spreadsheet kicking out the refraction correction AND the aberration correction in RA and Dec for the referenced objects. 
     
    The Bright Stars data is from my 2009 Astronomical Almanac, not 2010 as would be expected.  Someone was hogging the computer.
     
    The aberration is calculated using Bowditch 1849 Table XLII Aberration in Right Ascension and Declination.  Someone please check my aberration numbers :-)
    The aberration in my spreadsheet agreed with the example Bowditch supplies.  I don't know if it works for every case.   Mostly it seems like very small numbers.
     
     
    OBSERVATION 1  GMT Date 17Mar Time 01-15
    14 OmiLeo and Mirzaam (2BetaCMa)
     
          14 Omi Leo   Skyscout  RA 9h 41.15m  Dec 9.8920                Bright Stars (2009)RA 9h 41m 39.4s   Dec 9d 50m 55s
          2BetaCMa     Skyscout  RA 6h 22.77m  Dec  -17.9558            Bright Stars (2009)RA 6h 23m  7.1s    Dec -17d 57m 40s
     
          14 Omi Leo    Aberration in RA 13.06", Aberration in Dec -5.47"
          2BetaCMa      Aberration in RA   1.78", Aberration in Dec 13.11"
     
          Apparent Distance 56d 22m 24.47s  (sextant set to 56d 22m 20")
     
          Objects pass thru each other :-))
     
    OBSERVATION 2 GMT Date 17Mar Time 01-45
    Almaaz (7E Aur) and Menkalinam (34Beta Aur)
     
           7E Aur       Skyscout RA 5h 1.97m  Dec 43.8230                  Bright Stars (2009)RA 5h 2m 39.1s Dec 43d 50m 11s
           34Beta Aur Skyscout RA 5h 59.67m Dec 44.9475                 Bright Stars (2009)RA 6h 0m 13.6s Dec 44d 56m 51s
     
           7E Aur       Aberration in RA 10.95", Aberration in Dec 5.86"
           34Beta Aur Aberration in RA 4.97",   Aberration in Dec 7.21"
     
           Apparent Distance 10d 20m 7.89s  (sextant set to 10d 20m 10s)
     
           Objects pass thru each other :-))
     
    OBSERVATION 3 GMT Date 17Mar Time 02-10
    Sirius (9Alpha CMa) and Alphard (30 Alpha Hya)
     
             9Alpha CMa   Skyscout RA 6h 45.22m Dec -16.7175          Bright Stars (2009)RA 6h 45m 33.8s Dec -16d 43m 47s
             30Alpha Hya   Skyscout RA 9h 27.68m Dec -8.6678           Bright Stars (2009)RA 9h 28m 3.3s   Dec -8d 42m 01s
     
             9Alpha CMa   Aberration in RA 4.98", Aberration in Dec 12.14"
             30Alpha Hya  Aberration in RA 8.21", Aberration in Dec 9.71"
     
             Apparent Distance 40d 25m 23.33s (sextant set to 40d 25m 20s)
     
             Objects pass thru each other :-))
     
    Best Regards
    Brad
     
     
     
     

    From: navlist-bounce@fer3.com [navlist-bounce@fer3.com] On Behalf Of Frank Reed [FrankReed@HistoricalAtlas.com]
    Sent: Tuesday, March 16, 2010 9:48 AM
    To: NavList@fer3.com
    Subject: [NavList] Re: Star - Star Observations

    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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