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    Re: Historical Lunars : take in account 'delta-T' or ignore it ?
    From: Antoine Couëtte
    Date: 2009 Dec 14, 07:49 -0800

    Dear Paul,


    Thank you very much for your second detailed reply in [NavList 11143].

    You are quite right : better not shooting a Lunar in the environment of the given example since the Sun is so low. However, I did it on purpose because low altitude refractions increase some effects which have become quite visible in this example, such as : Low body "apparent height deceleration" due to the increasing refraction, and also body shape distortion due to differential refraction.

    Since it is also very well known that below @ 10° / 12° , refractions can NOT be reliably predicted for low apparent altitudes. So for our SUN Lower Limb example which is below 6° of apparent elevation, any difference of 0.1' or 0.2' in predicted value between various software is definitely NOT significant.

    True, I should (almost) never have submitted such a "way off resonable limits" example !

    Still - would reply an old sea wolf - there might be cases when better such a Lunar than no lunar at all.


    To recap, lower and lower altitude refraction random errors do render Lunar distances more and more inaccurate .... So, why should we bother with the (subtle ?) distinction between the Great Circle joining the refracted centers and the shortest Great Circle joining refracted limbs ? Well, this still remains an interesting subject to study and play with ...

    ---

    A possible explanation for the "discrepancy" you mentioned between the NAL and AAL refraction values might come from the fact that AAL values are given for a single given CONSTANT wavelenght, while NAL gives values for the observed average wavelenghts - i.e. INCREASING WAVELENGHTS when heights decrease - since all bodies apparent colors start moving towards "red" when height decrease. This MIGHT be a resonable explanation. Any other explanations from NavList Members ?

    *******

    For comparison, I have checked my results with your own published results :

    Position angles : Moon to Sun 265.18d ( you specified 265 ), and
    Sun to Moon 39.87d ( you specified 40 ).

    Also, I find the following Center to Center separation velocities :

    GEOCENTRIC : .007664 d/minute (you specified .00767) and

    TOPOCENTRIC : .007767 d/min (you specified .00492). Something is unclear to me here because apart from this specific topocentric result, all our other values match quite closely.

    *******

    I will certainly read with a lot of interest your upcoming comments on pending topics (including "refracted" semi-diameters centers and values as I would guess it).


    *******

    And finally, as regards the "exotic signs" you could read in [NavList 11087] these are essentially due to the fact that since I cannot get the degree symbol (°) with my laptop computer when directly typing a post on NavList (possible exception for the just preceeding degree symbol which I just copied and pasted from your own post), I have taken the habit to type everything on Word and then copy/paste onto Navlist. This is when the exotic signs ( • , o , � ) come to play.

    Sorry about that ... But maybe should I (we ???) harshly coplain to our Forum Moderator ... ( Silence on board !, don't let him know for now !!! )

    :-)))

    Well, thank you again Paul for all the time you have spent here !

    Best Regards, Merry Christmas and Happy New Year 2010


    Antoine


    Antoine M. "Kermit" Couette

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