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    Re: Lunar trouble, need help
    From: Frank Reed
    Date: 2008 Jun 26, 07:31 -0400

    George H, you wrote:
    "Where on Earth does "Tan app. Altitude x 60,53� " come from? For one thing, 
    it's quite the wrong way up, showing refraction increasing with altitude 
    rather than the other way round, and I suspect that must be the result of an 
    accidental slip in his email text, rather than in his calculation. It would 
    be more correct (but for high altitudes only) if the tan were changed to 
    cot, or (which amounts to the same thing) altitude were changed to 
    zenith-angle, and if, at the same time, the constant was changed from 60.53" 
    to 58.293" (see Meeus, also Smart)." 
    
    Gee, George, give the guy a break. OF COURSE, it was simply a slip in his 
    email text. He meant "tan(zenith distance)". That's obvious. As for the 
    correct refraction constant, his is probably lifted from a historical text. 
    A single arcsecond of difference is not significant to this topic. 
    
    And you wrote:
    "Then that amended formula would provide reasonable predictions, for 
    altitudes greater than 20�. But it breaks, down, and badly, at lower 
    altitudes, and then a more complicated expression is required." 
    
    This is well-known and presumably OBVIOUS for anyone who has looked at the 
    matter for more than two minutes. 
    
    And you wrote, regarding Kent's account of finding local time:
    "Well, I have struggled a bit with that, without fully understanding what
    Kent is doing, which may well be my problem rather than his. It seems a very 
    roundabout procedure." 
    
    What he is describing is an ordinary time sight (to give local time in order 
    to compare it with the Greenwich Time derived from the lunar observation).
    The reason his version is so "roundabout" is because he's describing the 
    procedure for getting local time from observations of stars (or other bodies 
    besides the Sun). This is a "textbook" procedure. It's the sort of long and 
    involved calculation that was used to drive students nuts on their exams. It 
    is almost entirely irrelevant to real navigational practice. Real navigators 
    used the Sun for local time and carried it on a watch if they needed it 
    later. And of course the steps required to get local time from a Sun sight 
    are much shorter, and anyone except a complete novice could work them. And I 
    would add that this again highlights the problem of using historical 
    textbooks and navigation manuals to understand the history of navigational 
    practice. Just because the textbook provides a long complicated procedure 
    for dealing with some sight has no bearing on whether that method was used 
    at sea. 
    
     -FER 
    
     
    
     
    
     
    
    
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