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    Re: Any "on line" Star Position Computation available ?
    From: Paul Hirose
    Date: 2011 Jun 14, 13:54 -0700

    Antoine Couette wrote:
    > -The 2000.0 coordinates for Beta Cet DIPHDA / DENEB KAITOS) - FK6 N� 22 
    > , HIP 3419 - differ by over 2 arc seconds if you pick them up in FK6, or 
    > if you pick them up in the HIPNEWCAT / ICRS reference. There is a very 
    > significant 0.14 s difference in RA (equivalent to a 2 arc second 
    > difference ) and also an already significant 0.3 arc second difference 
    > for its Declination,
    Kermit, there was a clue in my previous message: "The Hipparcos catalog 
    values are ICRS at epoch J1991.25. This is also true for the new 
    reduction." The FK6 coordinates are also in the ICRS, but at epoch 
    J2000.0. So to compare the catalogs we must apply proper motion to 
    update the Hipparcos coordinates to J2000.0:
    0h43m35.3709s -17°59'11.783"  Hipparcos at J2000.0
    0h43m35.3714s -17°59'11.778"  FK6 at J2000.0
    VizieR displays the same values - it converts Hipparcos to J2000.0 - but 
    with less precision. My Hipparcos coordinates were calculated with the 
    free Tinyac program for Windows.
    Incidentally, users of that program will get an error message if the 
    "auto ∆T" feature is used and the computation is for a date in 2011 or 
    later. That's because we are now beyond the limits of the internal ∆T 
    table. You have to calculate a value and enter it by hand. The Web site 
    explains (see "Bugs"):
    In my defense, the program was only supposed to be a prototype, not for 
    release. I decided to put it online because it performed extremely well 
    in testing.
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