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    The Moon is a "nuisance"
    From: Frank Reed CT
    Date: 2006 Oct 11, 15:24 -0700

       In mid-August, Robert Eno started a discussion about the "*&^%$#@
    Moon" and how Moon sights seemed to be more prone to error than Sun
    sights or star sights. Except for a few specific details that imply a
    little more calculational work with a Moon sight, I tend to agree with
    everybody who described this as basically an "old navigator's tale"
    (not you, Robert.. the 'old navigators' who started it ). It is
    essentially a prejudice with little relevance to anyone attempting
    celestial navigation today. But it sure was a popular prejudice! I
    found an example in a little navigation manual from 1903:
    "Latitude by Meridian Altitude of the Moon:
       The Moon is more or less of a nuisance, and is not used by expert
    navigators when it can be avoided. The declination changes so rapidly
    that even minutes of time have to be taken into account, and one is
    likely to be deceived as to its semi-diameter because of irradiation,
    which makes the Moon at times look larger than it is. Furthermore, in
    using the Moon we have to allow for parallax."
    (--from 'Elements of Navigation' by Henderson, New York, 1903. In his
    preface, he describes is goal in assembling the book: "Fundamental
    principles have been explained, but no attempt has been made to
    elucidate the higher mathematics of the subject. Students who have
    tried to learn navigation from books like Captain Lecky's 'Wrinkles in
    Practical Navigation', which is addressed to navigators only, or from
    Bowditch's 'American Navigator', which is only for mathematicians,
    will, it is hoped, appreciate this little book.")
    
       The idea that "minutes of time" presented a difficulty for the
    navigator seems strange today, but when you compare with the way a Noon
    Sun sight was worked back then, it starts to make some sense.
    Considering the Sun first... Naturally, the Sun's declination changes
    from day to day and you have to take that into account at Noon to get
    the latitude right. But the interpolation is easy because the rate of
    change in the Sun's declination is small. In fact, it was common to use
    the DR longitude to interpolate the declination, almost as if the
    chronometer was still a luxury (today we wouldn't even think of doing
    it that way... GMT is always available, it's ubiquitous, and "the
    chronometer" is more of an idea than an instrument). By contrast, since
    the Moon changes its declination rapidly, a navigator would have to be
    much more careful with the interpolation. The exact minute and second
    at the time of the observation have to be recorded to get the correct
    value for the declination. Using exact GMT for a latitude sight must
    have seemed down-right counter-intuitive for a navigator back then.
    >From today's navigation perspective (or rather from the perspective of
    late 20th century celestial navigation), this interpolation is a
    piece-o-cake, and the tables in the Nautical Almanac are crafted to
    make the work trivial. If you can't interpolate the Moon's declination,
    you don't really know how to use the Nautical Almanac. But a century
    ago, the task must have seemed quite a bit more intimidating.
    
       By the way, this business about the Moon being a "nuisance" has
    nothing to do with lunar distances --just ordinary lunar altitudes. But
    I suspect that there was some pleasure taken in scoffing at anything
    "lunar" in this period when lunar distances had finally been declared
    obsolete and were no longer required for navigators' examinations (a
    solid fifty years after they had ceased to be generally practical). In
    other words, I am speculating that disdain for lunar distances, in
    1903, may have contributed to a disdain for lunar altitude sights, too.
    
    -FER
    A quick path to the August Moon thread(s):
       http://www.fer3.com/arc/sort2.aspx?y=200608sd
    
    
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