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    Re: Lunars using Bennett
    From: George Huxtable
    Date: 2008 Jul 4, 08:15 +0100

    Perhaps, I'm starting to understand better what Ken Muldrew has been telling
    us about these Canadian surveyors' astronomical observations. Although Ken
    has clearly stated that Sun altitude was never measured, for the lunar, he
    is less specific about the Moon, but I will assume that the Moon was being
    treated in the same way.
    
    Let me summarise the matter in my own words, and then if I still have at
    wrong, Ken can correct me.
    
    To clear a lunar distance observation to get Greenwich time, the navigator
    needs the altitudes of the two bodies, for example Moon and Sun. But they
    don't need to be actually measured for that purpose; they can be calculated
    instead, from a rough assessment of the lat and long, well enough to arrive
    at a good estimate of Greenwich time, and hence a much better value for
    long. Because these navigators were travelling slowly over land, they always
    knew an approximate position, by reference to their previous position
    assessment. So they NEVER needed to measure Sun or Moon altitude (for the
    lunar) and never in fact did so. Their Sun and Moon altitudes, for the
    lunar, were always calculated instead..
    
    Having found a value for Greenwich time, so that their watch-error was then
    known, two altitudes were then needed, one near or at the meridian to define
    latitude, and one (a time-sight) as near as possible to East or West, to
    define longitude. But these altitude measurements played no part in the
    lunar distance calculation, and didn't need to be taken anywhere near the
    same moment as the lunar.
    
    =====================
    
    If I have that right, then it differs from Ken's final paragraph, which
    states-
    
    " My point was just to say that to get Greenwich time from a lunar you need
    to measure two altitudes and the lunar distance, they just don't need to be
    the altitudes of the moon and the other body used for the lunar distance."
    
    But those altitudes were NOT measured "to get Greenwich time from a lunar",
    were they? As Ken explained, the altitudes, for the lunar, were calculated,
    not measured. Those altitudes that Ken refers to were measured, not to get
    Greenwich time, but to get position, having determined Greenwich time. They
    could have been altitudes of any convenient body, not necessarily the Moon
    and Sun, just as he says.
    
    I'm trying to clear the matter up, not to make it more complicated, but I
    wonder if I've got it right? Perhaps Ken will say.
    
    George.
    
    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.
    
    
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