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    Re: Longitude by Lunar Occultation
    From: Paul Hirose
    Date: 2019 Jan 13, 15:16 -0800

    On 2019-01-13 10:01, Robin Stuart wrote:
    >          If you could confirm the TT of geocentric conjunction of the Moon 
    and star (T*_0) in R.A. then I think we could draw some concrete conclusions.
    
    RA (true equator & equinox of date) at 2019-01-18 19:33:52.697 TT
    moon     84.6970085° (DE431)
    zet Tau  84.6970084
    moon     84.6970064  (DE406)
    
    Out of curiosity I included the old DE406 ephemeris from ca. 1997. It's
    pretty good — the conjunction is only 11 milliseconds later.
    
    
    >   For reference here’s the relevant data that comes to Skyfield. It is as it 
    appears in the original Hipparcos catalog.
    > magnitude              2.970000
    > ra_degrees            84.41118447
    > dec_degrees           21.14259299
    > parallax_mas           7.820000
    > ra_mas_per_year        2.390000
    > dec_mas_per_year     -18.040000
    > epoch_year          1991.250000
    
    The zeta Tauri star catalog data I used above came from this SIMBAD page
    
    
    http://simbad.u-strasbg.fr/simbad/sim-id?Ident=zet+tau&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id
    
    where they have been adjusted to epoch J2000.0. Those data are based on
    the 2007 Hipparcos re-reduction of van Leeuwen. With the CDS VIZIER site
    I compared the original Hipparcos catalog (HIP1) to the re-reduction
    (HIP2). Both are at epoch J1991.25. The upper line matches Skyfield.
    
    RA           dec          px   pmRA  pmDec
    084.41118447 +21.14259299 7.82 2.39 -18.04 HIP1
    084.41118462 +21.14259289 7.33 1.78 -20.07 HIP2
    
    Now test what effect the star catalog differences have on zeta Tauri RA
    at 2019-01-18 19:33:52.697 TT.
    
    84.6970133 HIP1
    84.6970084 HIP2
    84.6970084 SIMBAD
    84.6970084 SIMBAD sans radial vel.
    
    That shows the SIMBAD adjustment from J1991.25 to J2000.0 is accurate
    (declination was identical too) and radial velocity can be ignored.
    (There are only about 20 stars in the Hipparcos catalog where that's not
    true. All have high parallax and proper motion.)
    
    The two different catalog data yield apparent positions .059″ apart at
    present. (That includes the discrepancies in declination.)
    
    In order for the moon and zeta Tauri RA to match, using HIP1,
    
    TT = 2019-01-18 19:33:52.724
    84.6970133 zeta Tauri
    84.6970133 moon
    
    All computations with IAU 2006 precession, 2000B nutation, JPL DE431
    ephemeris.
    

       
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