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    Re: Lunar distance accuracy
    From: George Huxtable
    Date: 2007 Oct 28, 16:23 -0000

    I had written-
    
    "But it bears little relation to the difficulties that would have been faced
    | by a real mariner, in a rolling, pitching, vessel, often doused with salt
    | spray, with much of the sky obscured by square sails, and when his
    | corrections depended on measured altitudes, not those deduced from a known
    | position. We need to keep those differences in mind."
    
    |and Frank replied-
    
    | Yes, but we also need to be careful not over-emphasize those difficulties.
    | ... Spars and lines and other "features"
    | in the sky over your head (think branches and power lines on land) make it
    | easier to keep an eye on the Moon in daylight, which is when most lunars
    | were done. Once you've found the Moon, which can remarkably difficult even
    | when the Moon is bright, you note its position relative to the nearest
    spar
    | (or branch) and then you can get back to it easily whenever you want.
    
    That was completely missing the point. I was referring to SAILS, those big
    white things that are dangled from the spars, blown by the wind to propel
    the vessel along, and obscuring much of the sky, especially under square
    rig. Not "spars and lines and other "features"".
    
    From on land, Frank may find that the local lamp-post may be a guide to
    locating the Moon, but from a vessel at sea, especially in a sailing vessel,
    those spars don't stay in the same place for long enough to fulfil that
    purpose, except in a flat calm.
    
    =========================
    
    In the same posting he quibbled about my claim-
    
    "And yet from one observation, to the next on another day, he records
    differences of a large fraction of an arc-minute."
    
    replying
    
    "Large fraction? The standard deviation is 0.25 minutes of arc. Is that a
    large fraction? This is an almost perfect match for my accounts of expected
    lunar distance accuracy. "
    
    I had quoted the scatter in the first four observations, of White's 42, in
    the third column, as-
    
    Aug 27,   -55 sec , -0.46'
    Sept 10,  +11 sec,  +0.09'
    Oct 8,     +30 sec,  +0.25'
    Nov 19,  -35 sec,  -0.29'
    
    Differences of a large fraction of an arc minute. I rest my case.
    
    George.
    
    contact George Huxtable at george---.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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