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    Unexpected USNO height correction precepts
    From: Antoine Couëtte
    Date: 2018 Sep 17, 08:22 -0700

    Hello to all,

    I just lately happened to scrutinize Greg Rudzinski's twilight fix published in " Its-fourplanet-fix-time-Rudzinski-aug-2018-g42485 " and during the course of our private exchanges, I happened to stumble on some unexpected height correction precepts from the USNO ( http://aa.usno.navy.mil/data/docs/celnavtable.php ).

    I have been surprised that - for height corrections purposes - the USNO indicates that they consider Planets as having an apparent semi-diameter and that "for the solar system bodies" (hence including all the planets as can be checked from the published Height Corrections "Sums") it considers that the Planet lower limb is assumed to have been observed. Hence it performs a systematic semi-diameter correction for the planets themselves. In addition to this quite strange procedure - as I habe been trained to use Planet SD = 0 - the USNO also specifies that the Venus phase effect correction is ignored. This is a bit regrettable since nowadays with the huge computation power at our hands such effect can be very easily taken in account, even if it is only significant for Venus for which it can reach 0.4' under extreme cases.

    I dug through my files and retrieved an ancient Venus daylight observation which is the scope of the enclosed document.

    What can be immediately seen is that for this specific observation, the Venus Intercept as determined from the USNO explanations and precepts shows a systematic error of 0.7 NM which could otherwise have been avoided, had more adequate and correct procedures been used.

    Everything is explained in detail in the enclosed document.

    I trust that our Venus configuration on that day was some kind of an exceptional case, with VENUS being extremely difficult to observe in full daylight since it was also almost perfectly lined with the Sun and not even 30° away from it as can been seen from the USNO computed data.

    However the 2 juniors officers onboard could then spot and shoot Venus on that morning. As a prize they enjoyed an invitation for dinner offered by the Captain in the very small local Haurei restaurant in the magnificent and gorgeous Island of Rapa Iti on that evening.

    It was quite easy to check the for correctness of our observations since the Skipper himself shot the SUN by the same time. So he exactly knew what kind of intercept to expect from our VENUS observations. As earlier indicated with VENUS being exactly in the same azimuth as the SUN it was really an easy check for the Captain.

    I wish to get the advice of reputable and succesful User's Software Programmers of our NavLsit community on that specific case : Andrés Ruiz, Peter Hakel, Paul Hirose, Stan K. and maybe others.

    Would you agree that the USNO precepts could be better fine-tuned ? or do you think that this topic is only about "splitting hair" ? Nonetheless - and if I am not mistaken in my enclosed computations - we are encountering a case here where the intercept derived from the USNO precepts and recommendations seems to be in error by 0.7 NM which could otherwise have been avoided.

    Thanks for your kind understanding and for your replies.


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