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    Re: Help with Lunar distance.
    From: George Huxtable
    Date: 2010 May 11, 17:59 +0100

    I wrote, earlier today, about Paul Jackson's attempted lunar, which he, and 
    we, had failed to understand.
    
    I copied his posting (and it's copied again below) and mentioned his 
    attachment, which seemed irrelevant.
    
    But Robin Stuart has kindly brought to my attention another posting from 
    Paul on the same day, which did indeed have a relevant attachment. This 
    showed a print-out, with numbers, from a navigation program which appears 
    to be that of Omar Reis, with which some members may be familiar.
    
    Although I haven't investigated it closely, that program appears to be 
    calling for observed altitudes of Moon and planet to be input. These appear 
    to have been-
    
    hs Moon = 21º 06.3', presumably for Moon's lower limb, because it then adds 
    15.0' for semidiameter to give ha = 21º 22.3' for the centre, then 
    {parallax + refraction) correction of - 48.4', to end up with corrected 
    altitude of-
    hc Moon = 20º 33.9'
    
    There are some niggling doubts in my mind about the above procedure, as to 
    whether the right sign has been applied for the (refraction + parallax) 
    correction, and whether some of the arithmetic may be a bit wrong. But 
    that's rather incidental to our present inquiry. And I can't see any 
    accounting for dip, but dip correction plays a very minor part in all this.
    
    Similarly, for Jupiter, the program calls for observed sextant altitude-
    hs Jupiter = 22º 04.0'
    It then corrects for refraction, to end up with,
    hc Jupiter = 22º 02.6'
    
    These corrected hc values are what Paul has been referring to as calculated 
    altitudes. However, those words, "calculated altitude", in this context, 
    has another meaning. It applies to altitudes which have not been taken from 
    an observation at all, but from estimates of the altitudes of the bodies 
    calculated from an ephemeris, together with a rough idea of the observer's 
    position. That's a completely different matter, and quite another way to 
    handle a lunar. That wording fooled me, and possibly has similarly foxed 
    other Navlisters. We all expected calculated altitudes, obtained in this 
    way, to correspond with the predictions of a planetarium program.
    
    But what Paul appears to have given us are not calculated altitudes, but 
    corrected altitudes: corrected, that is, from his own sextant altitude 
    observations, which he has input to the program, but hasn't disclosed to us 
    those observed values. It's those altitude observations that appear to be 
    several degrees too low. That is where he seems most likely to be going 
    wrong, somehow. I ask Paul to confirm whether or not I have it right.
    
    Let me ask a silly question. Is Paul actually measuring his altitudes up 
    from a distant sea horizon, or instead, perhaps, from a "horizon" that's 
    actually a local land skyline?
    
    There's some such factor that's coming in to make his observed altitudes 
    far too low.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ----- Original Message ----- 
    From: "George Huxtable" 
    To: 
    Sent: Tuesday, May 11, 2010 10:11 AM
    Subject: [NavList] Re: Help with Lunar distance.
    
    
    | Paul Jackson wrote, on 9 May-
    |
    | "I took a Lunar distance in position - 36 48.0 S - 174 46.0 E @ 9 Days 18
    | hours 15 min 52 sec on the 9th of may 2010.
    | Calculated Alt moon 20 33.9
    | Calculated Alt Jupiter 22 02.6
    | Distance near limb moon & Jupiter = 7 20.8
    | My Navigator software gives me a lunar time of 2:57:42 GMT how can this 
    be,
    | can you guys at Nav List Help."
    |
    | He added an attachment, which seemed irrelevant.
    |
    | We have agreed that his calculated altitudes for both Moon and Jupiter 
    from
    | that place at that time are seriously in error. Paul seems prepared to
    | dismiss that result and go on to take another lunar. But that won't 
    answer
    | the question of what was going wrong. Where, I ask Paul, did those
    | altitudes that he quoted come from? Did he calculate them himself? If so,
    | how? Or did he take them from some "Navigator software" that he refers 
    to,
    | which he hasn't yet identified to us? In which case, what, exactly, were
    | the numbers that he fed into that program? If he feeds in those same
    | numbers again, does he get the same results?
    |
    | If Paul wishes to ask Navlist for help, which is willingly available, he
    | should be prepared to supply backup information so that we can get our
    | teeth into his problem. Something has gone seriously wrong, and unless,
    | between us, we can identify what it is, the error won't go away.
    |
    | George.
    |
    |
    | contact George Huxtable, at  george@hux.me.uk
    | or at +44 1865 820222 (from UK, 01865 820222)
    | or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    |
    |
    |
    | 
    
    
    
    

       
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