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    Re: Error in taking lunar distance
    From: George Huxtable
    Date: 2008 Feb 12, 16:10 -0000

    I think an important point is being lost, in this discussion of determining
    lunar distance from a thin crescent Moon, by both Frank and Jim. It boils
    down to Frank's sweeping-statement - "Celestial navigation back then was a
    daytime activity."
    
    If a navigator chooses to restrict himself to daytime lunars (with the Sun)
    then yes, the dimness of the Moon crescent against an illuminated sky
    presents a real challenge in perceiving exactly where its edge should be,
    because of low contrast. However, this is just the situation for which
    night-time star-lunars were intended.
    
    If the crescent being observed is shortly after New Moon, all the observer
    has to do is to wait until the Sun has set, and the sky background has
    darkened, when the Moon outline will then show up, clear and sharp as ever
    (in the case of a waning crescent, he must get up before dawn). And that's
    the time when the horizon is visible for measuring altitudes. And it's also
    the time for measuring a star-lunar, ideally choosing a star, near the
    ecliptic, that's makes a good angle with the Moon, somewhere around 90
    degrees.
    
    For the rest of the month, Sun-lunars are available, and indeed preferred by
    many observers. But for just those days when the Moon is a thin crescent,
    star-lunars provide a way of shortening the dead-period, around New Moon,
    when lunars are impossible.
    
    Frank writes, in another sweeping-statement- "That's really all that was
    necessary since dead reckoning for longitude was "good enough" for ten days
    or more ...", regarding ten days without an observed longitude with
    equanimity. I doubt whether any real navigator, approaching an unseen coast
    after a week or more against contrary winds, under square rig, would share
    Frank's confidence in his DR. He would, by then, be desperate to get a
    longitude, and that's what a star-lunar could often provide.
    
    George Huxtable.
    
    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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