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    Re: Celestial Navigation Data from USNO--alternative
    From: Bill Ritchie
    Date: 2019 Nov 28, 11:01 +0000

    re / Ritchie-nov-2019-g4624


    Yes, Antoine, I suspect the difference is probably due to Delta T, or possibly oblate Earth handling.


    For Delta T, Astron uses the NASA formulae in http://eclipse.gsfc.nasa.gov/SEhelp/deltatpoly2004.html.

    Between 2005 and 2050, this is

     ΔT = 62.92 + 0.32217 * (y – 2000) + 0.005589 * (y – 2000) * (y – 2000)

    Using this for 2037/11/24 14:00:00 Astron gives Delta T = 83.139.


    For the years 2019-2025, Astron overrides the above with the values from the United States Naval Observatory 2019 REVIEW. (No longer accessible on USNO website). I created the polynomial

    ΔT = 69.34 + 0.53 * (y - 2019) - 0.007 * (y - 2020) * (y - 2020)

    which fits the review tabular values for those years within 0.01”.

    For 2025/12/31 00:00:00 the NASA formula gives 75.049 and USNO review data 72.779, a difference of over 2 secs. I do not have the knowledge to extrapolate this review result to years after 2025, so Astron uses the NASA formula for the subject year, 2037.

    Whilst testing this change, I made the following note.

    Separate checks on a 10 second reduction in Delta T gave an increase in GHA of around 5 seconds of arc in Moon GHA. This could become a rule of thumb! For the Moon, corresponding changes in declination will vary depending on her declination. They are around zero when declination is near maximum/minimum, increasing sinusoidally to a maximum of about 40% (occasionally 47%) of the GHA change when declination is zero.”


    For oblate earth correction to parallax correction, Astron code is

    const FLATTENING = 1 / 298.257223563;   // WGS84 value

    const LatCorr = FLATTENING * HP / MINSinDEGREE * SineD(A_LAT) * SineD(A_LAT);

    (SineD is bespoke function using degrees i/o radians)



    Bill Ritchie

    50N 003W

    On Wed, Nov 27, 2019 at 7:34 PM Antoine Couëtte <NoReply_Couette@fer3.com> wrote:

    re / Ritchie-nov-2019-g4624

    Nice comparison indeed !

    Which TT-UT (DeltaT) are each of these authors using ?

    For the Moon, it seems worth knowing DeltaT in order to perform fully meaningful comparisons.


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