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    Re: Lunar mechanics and Double Alts.
    From: George Huxtable
    Date: 2003 Apr 30, 01:08 +0100

    Herbert Prinz has explained that Doug didn't make an appropriate choice of
    stars for his lunar distances, and I agree with what he said.
    
    There's a way to decide whether a star (or planet) is likely to be an
    appropriate choice for a lunar distance, just by looking at it and the
    Moon. It works well when the Moon is an obvious crescent, as it was on
    Saturday.
    
    Look at the Moon, and in your mind draw a line through it, bisecting it
    equally between the two horns: that is to say, at right angles to the line
    joining the two horns. Then that bisector points, pretty closely, along the
    direction the Moon is travelling through the sky, with respect to the star
    background. Then you need to find a star which is reasonably close to the
    direction of that bisector. If you draw a line joining that star and the
    Moon, the angle at the Moon between the joiner and the bisector shouldn't
    be more than 20deg or so. That's something you can estimate by eye
    perfectly well. It's only stars that are somewhere near the path of the
    Moon across the sky that will show a quickly-changing lunar distance. The
    worst possible choice would be a star at right-angles to the path of the
    Moon, because then the Moon would be moving PAST the star, neither toward
    it or away from it, so the lunar distance wouldn't be changing at all. It
    is only by measuring a changing lunar distance that it's possible to obtain
    the time.
    
    It's hard to use this method near full Moon because then its shape is
    nearly circular and so it's not obvious which way it's moving.
    
    >Reflected Altitudes it is.My wording in all this is incorrect.I forgot the
    >closest or furthest edge can be used,so I just used the edge of the Moon
    >that was closest to each body.
    
    You must always use the most-convex, sharper, outside edge of the Moon, and
    never the inner, fuxxier one, which is just the edge of the shadow from the
    Sun.
    
    >Because of where I live
    >all bodies must have an altitude of about 11* before I can view
    >them.
    
    I wonder if there's a precaution that when taking altitudes above mountain
    tops, one should allow a bit of a gap to ensure that light isn't being
    abnormally bent by air that's been heated (or cooled) by contact with the
    ground. It's not a situation I have ever considered before. Anyone else
    have experience or opinions on the matter?
    
    
    >If L + C didn't take the altitudes of the bodies,thier watches and chrono
    >were suspect by themselves I read,what purpose did the exercise serve?
    
    Well, not a lot, as it turned out. It's a complicated business, and I
    recommend you download the Preston paper I referred to. But Lewis and Clark
    were instructed, before departure, to take lunar observations which could
    be calculated out by experts in the business after their return. So, during
    the journey, Lewis and Clark didn't know their longitudes. However, the
    astronomer who was then given the job didn't understand how it could be
    done without altitude observations, so their lunar observations went
    uncalculated for a couple of hundred years. This is information I have
    taken from the Preston paper.
    
    With hindsight, Lewis and Clark would have done better to measure the
    altitudes, which they knew how to do, and then worked out longitudes as
    they travelled, if they knew how. This information could have been useful
    to them on the return journey.
    
    Finally I have a comment about what Herbert Prinz said, as follows-
    
    >In order to properly perform an LD using observed altitudes, we need
    >four(!) officers on >deck. Short of that, we have to cheat.
    
    I am aware that Herbert was saying that in order to make a point. But I
    would not like readers to get the impression that there's anything
    impossible, or "cheating", or even very difficult, in the job being done by
    one man, if he is good at it. The navy ritual may have called for four
    officers, but there are many examples of a single observer taking valid
    lunars, right up to the days of Joshua Slocum. I don't think Herbert wished
    to imply that one man couldn't do that job, but his phraseing above may
    have given that impression.
    
    George Huxtable.
    
    
    ================================================================
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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