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    Re: Graphs of Lunar Distances.
    From: Douglas Denny
    Date: 2010 Oct 25, 17:08 -0700

    Quote: Frank Reed:

    "Douglas assures us that his errors are much larger than this, but that's just because there are appears to be a continuing problem with his calculations.
    Richard Feynman that goes something like: "The first principle of science is that you must not fool yourself and you are the easiest person to fool."


    Indeed. Very true.
    Feynman - a hero of mine.

    And yes, I have discovered there was another bug in what I was doing, but it was not not the programming of the calculator. That is correct.

    It has still not disinclined me from my final conclusion however, that one minute of arc (i.e. half a moa absolute from the 'truth') is about the best one can manage under normal circumstances... equating to two minutes in time accuracy for 30 minutes of arc movement of the Moon per hour.

    I will concede a single measurement is not what one requires for real accuracy in Lunar Distance measurements however; which is adequate in good conditions for an altitude measurement to find a position line in 'normal' navigation.
    For Lunar Distances, an average of at least six measurements is needed and is going to improve accuracy considerably, and this is what was actually done of course, with two other navigators taking sights simultaneously for altitudes wherever possible. Nevertheless all that ignores the serious problem of taking the Lunar distance and altitude sights at sea in a seaway, with the restriction of limited time to take them in twilight only if altitudes are simultaneously taken. No doubt this is why altitudes would have been calculated more often - so lunar distance sights can be take in darkness when it is more convenient for stars.

    The bug was my stupidity in not subtracting the Par-in Alt from the Moon's altitude from the ICE programme values I was using (though adding refraction also was done) - to simulate the altitude as would be observed and to then input this value into the calculator. As might be expected it does not make a huge difference to the overall errors and final result incorporated in the measurement system as clearing the distance is not so sensitive to altitude changes, but it does still make some significant error.

    I have just been outside and taken a Moon/Jupiter Distance.
    Perfect conditions. Bright clear sky; Moon and Jupiter beaming like lighthouse lights.
    Here is my latest effort and the latest results for a typical 'single-shot' sight. It is not bad but still not brilliant in my opinion.


    Lunar / Jupiter Observation

    Observer's position:
    Lat: North 50deg-49.910
    Long: West 000deg-51.300

    Monday 25th.X.2010.
    All times GMT. (UTC)

    MOON/JUPITER. (Jupiter to far limb)

    Time of observation = 23hr-15'-02" GMT
    Moon/Jupiter observed distance by Frieberger Sextant x4 (I think) telescope.
    = 72deg-17'.3
    minus SemiDiam 15'.36
    gives apparent Lunar distance = 72deg-01'.94

    Moon true alt (from ICE) 48deg-03'.7
    plus refn 0'.9
    minus Par-in-Alt 39'.1
    gives 'observed' Moon alt = 47deg-26'.5

    Jupiter true alt (from ICE) = 30deg-58'.0
    plus refn 1'.7
    gives 'observed' alt = 30deg-59'.7

    The Cleared Lunar Distance calculated is = 71.749551deg
    = 71deg-44'.873
    which comparing and interpolating with the Oliv Soft tables gives
    time= 23hr-13'-36".6

    An error of 1min-24".4 in time, equiv to 3/4 of a minute of arc.
    One of my better ones! I must be getting better at it.

    Douglas Denny.
    Chichester. England.
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