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    More on lunars. was: Re: Exercise #6, Lunars at sea
    From: George Huxtable
    Date: 2008 Jun 2, 08:42 +0100

    I have switched to a more relevant threadname.
    
    Frank Reed wrote-
    
    "By the way, if you want to think about lunar distance observations
    differently, each lunar generates a line of position, which you could plot
    on a chart just like any other LOP. But unlike standard altitude LOPs, an
    error of 0.1 minutes of arc in a lunar distance observation implies a 6 n.m.
    error in the lunar LOP. One advantage is that you can shoot lunars when the
    horizon is hidden. Cross two lunar LOPs and you get a rough position fix."
    
    Earlier, I questioned that second sentence, suggesting that the 6 miles
    given should have been 3. But questioning the rest of that paragraph, also,
    may help our understanding.
    
    It calls for some clear thinking about what we are discussing, a lunar
    distance observation. Does that, in itself, generate a line of position? Not
    on its own, no. A lunar distance itself is nothing more than a measurement
    of the position of the Moon in its orbit. From that, you get Greenwich time,
    nothing else. You can set your watch from it, that's all. It tells you
    NOTHING about your position, just the same as reading Greenwich time from
    your chronometer tells you nothing about where you are.
    
    It is generally true that in order to correct a lunar it is normal to
    measure the altitudes of the two bodies involved as well as the lunar
    distance itself. But that's not necessary, and it's quite possible (but a
    bit complicated) to correct a lunar by computing where the two bodies must
    be in the sky, without making any altitude observations at all. All it calls
    for is a reasonable estimate of latitude, which a mariner could usually
    provide. Indeed, to "shoot lunars when the horizon is hidden", as Frank
    suggests, that is the only way the job can be done. It's what Frank's
    on-line computer will do for you.
    
    If, however, the lunar corrections are based on a measurement of the two
    altitudes, then high precision in those altitudes isn't necessary. That
    makes it possible to measure star-Moon lunars at night, when there's only an
    indistinct moonlit horizon, though still, care needs to be taken to avoid a
    "false horizon".
    
    As long as it's been possible to precisely measure both altitudes, under
    favourable circumstances (sharp horizon), then each such altitude can be
    used to generate a LOP, once GMT has been deduced. That's the standard
    process of Sumner-line navigation, and they can then be crossed to give a
    position. Alternatively, those precise altitudes, to get position, may be
    measured at a more propitious moment, away from the lunar itself, the
    chronometer having been set from the lunar so that Greenwich time is then
    known.
    
    So what I am saying here is that a lunar observation itself may provide no
    position line at all, or the observations taken with it may provide two
    position lines, and so a fix. Which is why I question Frank's statement that
    "each lunar generates a line of position, which you could plot on a chart
    just like any other LOP". It doesn't. If it did, which direction would that
    LOP run?
    
    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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