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    Re: Can I Navigate Without an Assumed Position?
    From: Antoine Couëtte
    Date: 2017 Sep 24, 12:43 -0700

    After my flights, replying here-after to both RobinJouan-sep-2017-g40138 and to RonJones-sep-2017-g40134.

    Dear Ron and dear Yves, dear All,

    1 - re : RonJones-sep-2017-g40134

    Much better this time with your data Ron ! Through automatic Stars position computation I am now getting at UT=04:28:24 for all Bodies :

    19MAY2003 3 Body Problem : N48°08.47'-W123°26.02' with Standard dispersion equal to 0.0' (i.e. inferior to 0.05'), which are essentially the exact same results as RonYves and Peter, and certainly the same as Andrés Ruiz, should you happen, Andrés to contribute further down the line.

    19MAY2003 2 Body Problem : N48°08.48'-W123°26.01' with Standard dispersion equal to strictly 0.0' (as for any 2 body fix), with same comments as here-above.

    Comment # 1 : I would assume that you derived your published coordinates, Ron, through Nautical Almanac interpolation, did you not ? Over decades now I have consistently observed that my own coordinates computation match the Nautical Almanach [carefully interpolated] ones to most generally better than 0.15', except in some few cases where the NA is known to be a bit less less accurate (e.g. Moon Computation, or Sun "approximate" GHA adjustment under some specific circumstances as regularly discussed here). So nothing to be surprised here. We are all on the same ship.

    Comment # 2 : I took advantage of my earlier blunder (re : Couëtte-sep-2017-g40125) to recompute both problems for the same time (04:28:24) but on 19MAY2013, i.e. some 10 Years later. Results are as follows:

    19MAY2013 3 Body Problem : N48°05.0'-W123°52.8' with Standard dispersion equal to 0.1' (instead of 0.0' on 19MAY2003).

    19MAY2013 2 Body Problem : N48°04.9'-W123°26.02' with Standard dispersion equal to 0.0'.

    Obviously the computed Longitude is not the same essentially because Aries GHA has changed appreciably. Questions : (1) Can any one explain why the dispersion has slightly increased from 0.0' to 0.1', and (2) most importanly, can somebody simply explain why the overall latitude has decreased by some 3.4' over a 10 year period ? What effect has become significant over this time period ?

    2 - Re RobinJouan-sep-2017-g40138

    Dear Yves,

    It is a bit surprising that your own 100% self derived results through ASTROLAB total automation - vs. your earlier results using Ron's published coordinates - seem to be so slightly different from your earlier published results.

    Except for the case when you might have inadvertently hit a wrong ASTROLAB key - erroneous key hitting keeps happening to me quite frequently (see : Couëtte-sep-2017-g40125)  - I can see no reasonable explanation for such difference, and would somebody else find one ?

    In order to help clarifying this, I am hereafter giving you a more detailed solution of my computed Celestial Coordinates. I am publishing accurate results believed to be better than 0.01 '.

    I agree 100 % with you Frank, that this is an accuracy overkill - but sometimes it turns out to be very helpful to check the computations strict mathematical side - vs. their sensible physical one.

    Yves, since you are probably using the "old computation" system, I am giving you relevant check results in that computation frame:

    19 May 2003, Reference UT1 = 04:28:24 TT-UT = 64.5 s (the difference between using the TT-UT value displayed here vs. the actual TT-UT value is absolutely insignificant in our case here)

    Precession RE : IAU 2006 (P03) with displayed results accurate to better than +/- 0.0001" as follows :

     𝜛ₐ =  1.58742 "  ,   𝜫ₐ = 174.9131483°  ,  pₐ+ 𝜫ₐ =  174.8659659 °  𝜺ᵐ (Mean Ecliptic) = 23.43884°

    Nutation : RE : IAU 2000A with displayed results accurate to better than +/- 0.0015" as follows :

    𝜟𝜓 =  -15.60395 "  𝜟𝜀 = + 4.32502 "

    Aries GHA = 303.5363842 °

    All stars - DenebArcturus, Pollux and Spica - computed from the Hipparcos New Catalog (2007) as published in 2007, i.e. from the catalog reprocessed from the [first] Hipparcos Catalog published by 2000. Radial velocities in ICRS (2000.0)

    HIPNEWCAT (2007) is believed to be slightly more accurate than the [first] Hipparcos Catalog

    For the stars, the following effects are taken in account : Proper Motion, Parallax, Precession, Nutation and Sun Gravitational light deflection. Only the diurnal aberration effects are not included here-under, which are Observer's dependent and inferior to about 0.5".

    DENEB           GHA 353°09'029 DEC N47°17'156, comparison with Ron's published results : GHA 353°09'0 DEC N45°17'2

    POLLUX         GHA 187°09'826 DEC N28°01'245, comparison with Ron's published results :  GHA  187°09'8  DEC N28°01'2

    SPICA             GHA 102°11'608 DEC N11°10'772, comparison with Ron's published results :  GHA  102°11'6  DEC N11°10'8

    ARCTURUS   GHA 089°34'836 DEC N19°09'920, comparison with Ron's published results :  GHA  089°34'8  DEC N19°09'9

    Again, we can observe the excellent if not outstanding agreement to better than +/- 0.05' between results computed and given here-above and the results earlier published by Ron, including for Arcturus which has such a big proper motion. Last note : BTW, Arcturus is believed to be a very special extragalactic star since its huge proper motion vector is almost perpendicular to our Galaxy plan ...

    Dear Yves, hope it helps,

    Kermit

       
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