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    Re: Celestial data for the planet Mercury
    From: Paul Hirose
    Date: 2016 Feb 23, 21:27 -0800

    On 2016-02-22 7:03, Dave Walden wrote:
    > ************************
    > 
    http://aa.usno.navy.mil/cgi-bin/aa_geocentric.pl?ID=AA&task=6&body=1&year=2016&month=2&day=1&hr=11&min=0&sec=0.0&intv_mag=1.0&intv_unit=1&reps=5
    > 2016 Feb 01 11:00:00.0     19 14 15.421     - 20 38 03.89
    > ************************
    > http://ssd.jpl.nasa.gov/horizons.cgi#results
    > 2016-Feb-01 11:00     19 14 15.4172 -20 38 03.896
    > ***************
    > http://vo.imcce.fr/webservices/miriade/?formsMercury2016-02-01T11:00:00.0019 14 15.42394
    > -20 38 3.8918
    
    
    Nobody is curious about the discrepancies between those positions? For
    example, the format of the USNO coordinates implies about 10 mas (milli
    arc second) accuracy. But compared to JPL HORIZONS, the difference is 54
    mas. OK, that's not actually unreasonable. Maybe the USNO values carry
    an extra decimal place to ensure negligible loss of accuracy due to
    roundoff. I can go along with that.
    
    But what about HORIZONS vs. IMCCE? The coordinates differ by 95 mas.
    That's far more than a few counts in the last place!
    
    Maybe the time scale is not the same for all three sources? Mercury
    moves about 1.75 arc seconds per minute of time, so 95 mas is equivalent
    a time error of 3.2 seconds. UT1 is about .026 s ahead of UTC and 68
    behind TT. Neither number is close to 3.2, so a time scale mistake
    doesn't look like a good explanation for the position discrepancy.
    
    No do I think we can blame differences between ephemerides. Here is the
    geocentric geometric place of Mercury at 2016 Feb 1 11:00:00 UTC from
    three major JPL ephemerides going as far back as 1997:
    
    19h13m19.3193s -20°39'56.415  DE430
    19h13m19.3193s -20°39'56.416  DE422
    19h13m19.3193s -20°39'56.412  DE406
    
    I think IMCCE has their own ephemeris, but could it differ from JPL by
    95 mas? Of course that's insignificant for navigation. But when precise
    computations differ by that much, frequently there's a mistake
    somewhere. I know that from painful experience!
    
    

       
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