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    Re: Lunar trouble, need help
    From: George Huxtable
    Date: 2008 Jun 16, 10:46 +0100

    I'm sure Frank has put his finger on the answer to Jeremy's odd lunar 
    distances, and the way to readjust them, especially as Jeremy isn't sure, 
    now, whether Moon and Sun were side-by-side, or superimposed. That settles 
    the matter, as I see it; they must have been superimposed..
    But Jeremy's observations raise a few more matters of interest.
    First, his prowess with a sextant is most impressive, a standard I could 
    never have matched even in the days when my eyesight was much sharper than 
    it is now. With time-intervals of 30, 40, 50 seconds or so he has taken 
    precise observations of different bodies, with differing sextant 
    orientations, from at sea, and got precise answers (when readjusted). I 
    wonder what magnification of scope he was using, if his on-board equipment 
    equipment offers a choice.
    He wrote-
    "I shot an altitude of the sun, then the moon, then 5 lunar distances, 
    followed by another sun and finally another moon altitude.  (in retrospect, 
    I should have shot the 2nd moon first then finally the second sun)."
    Yes, his "retrospect" is correct; that would have put the mean timing of 
    Moon altitude and the mean timing of  Sun altitude both very close to the 
    same moment as the mean time of the lunar distances.
    and added-
    "My first trouble was with the moon altitudes.  The Hs of the sun was nearly 
    2x and on opposite bearings as the moon so I was getting massive flashes of 
    the sun where it hit my horizon mirror and bounced back through the scope. 
    I have a feeling that my altitudes of the moon are none too accurate."
    Well, they don't need to be VERY accurate, because they are needed only to 
    calculate a correction. But I am interested in his problem of sunlight 
    getting into the scope, and the way he has explained it.
    Yes, sunlight can be a real problem when observing Moon altitudes, 
    especially when the two bodies are nearly opposite in azimuth. But Jeremy 
    also refers to "The Hs of the sun was nearly 2x and on opposite bearings as 
    the moon". But it wasn't; the Moon was roughly twice the altitude of the 
    Sun! And is the height of the body being observed relevant anyway? I think 
    not. The problem is at its worst when the Sun can just peep around the edges 
    of the index mirror, and illuminate the horizon mirror with scattered light 
    (but direct light is usually masked out by the frames), and that always 
    occurs at a Sun altitude of around 60�, depending a bit on the details of 
    the sextant design. In this case the Sun was only at 33 deg altitude, behind 
    the observer, so the light was coming in around the side of his head, or 
    over it, but just how was it getting into the scope so badly? Just what was 
    the sunlight bouncing off? Is Jeremy clear about that?
    By stating chronometer times of GMT, Jeremy has pre-empted the answer, of 
    course. If you know your GMT, there's no need for a lunar;  you can simply 
    cross position lines from the measured altitudes of Sun and Moon, and get a 
    much more precise answer, in both lat and long, that a lunar could ever 
    provide. The only point of taking a lunar is to determine GMT (and rather 
    imprecisely, at that) and correct your on-board clock from it. Then, armed 
    with that time, you can work out the true position of one or more bodies, 
    measure altitudes, and cross the resulting position lines. Of course, in 
    this case, he wasn't setting us an exercise, he was asking for our help ( 
    and got it, I'm pleased to see).
    I'm still a bit puzzled by Kent Nordstrom's calculations. He wrote, in 5462,
    "...In my calculation I have
    assumed that the LD was measured on the nearest edges on both the moon and
    the sun. This means that in the calculation the semi-diameters for both
    bodies have been added to the measured distance, for the moon +15m 25s and
    for the sun +15m 45s."
    Well, leave aside the fact that we now know the lunar distance was measured 
    somewhat differently, and accept Kent's original numbers, which were-
     "mean LD 86 d 10 m 18 s (same as Jeremy)
    After reduction of altitudes and distance (my old fashioned way) is
    -         distance 86d 14m 45,64s"
    Kent would then have added two those semidiameters, totalling 31'10", to the 
    stated LD of 86� 10' 45", to end up with a LD between centres of 86� 41' 
    55", and then put that through the clearing process for refraction and 
    parallax, ending up with 86� 14' 46"  (neglecting here that 2nd difference). 
    I am just very surprised by that result. As Frank has pointed out, when LD 
    is near 90�, it is little-affected by such corrections, yet here the 
    clearing process has resulted in a change of over 27'.
    Of course, now we have to recalculate the whole thing from a different 
    starting-point anyway, so there may not be much virtue in chasing-down those 
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Navigation List archive: www.fer3.com/arc
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