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    Re: lunars with and without altitudes
    From: George Huxtable
    Date: 2006 Nov 24, 10:31 -0000

    Frank Reed has offered more suggestions about his proposed method of
    position finding without a horizon, based on Moon parallax. It's a
    clever, interesting, and amusing concept, but we would be wise not to
    take it too far, or too seriously, as a practical tool.
    
    He has claimed that a skilled observer can measure lunar distances
    reliably to 0.1 arc-minutes; claims which have been treated with
    scepticism by some contributors, me included.
    
    Now he has put forward some rather unlikely scenarios, in which his
    proposed method might come into play.
    
    | Suppose you're sailing in the Bermuda race in June, 2007 and you
    decide to  enter
    | under the celestial navigation rules (apparently you get a 2% time
    advantage
    | if you do so). It is not particularly uncommon in the Atlantic
    between  New
    | England and Bermuda to find that the stars and Sun and Moon are
    visible but
    | the horizon is lost in haze (and in addition, this area is prone to
    extreme,
    | unpredictable refraction). So shoot a set of lunar distances,  and
    work up your
    | position from them. Don't worry about the calculational work;  the
    rules
    | permit computing devices for working any and all sights. You  get a
    true position
    | fix, and you get bragging rights to spare. As long as  the Moon is
    in the sky,
    | you can get a line of position and combine it with  whatever other
    positional
    | information you may have.
    
    Well, what precision does Frank expect to achieve, in measuring his
    lunar distances, not this time from on land, but from a yacht
    bucketing about in the Gulf Stream? Has he taken lunars under similar
    conditions, and if so, with what resulting accuracy?
    
    | Or perhaps you're playing modern-day Arctic explorer sailing up
    above the
    | Arctic Circle next summer, near Spitzbergen perhaps, among ice flows
    and  sea
    | fog and weird refraction --no place to measure altitudes unless you
    have no
    | other option. You were smart enough to bring a spare GPS to replace
    the primary,
    | which has failed, but not quite smart enough to bring batteries for
    the
    | spare.  Luckily, one member of your group is a Navigation List
    lunarian and has
    | brought along a sextant. The horizon is a mess, so you shoot a lunar
    distance
    | between the Sun and the Moon (at known GMT), wait four hours, and
    shoot
    | another. Cross the lines of position and you've got a position fix
    (accurate to
    | about +/- 6 nautical miles *if* you're skilled enough with  your
    sextant to
    | measure angles to +/- 0.1 minutes of arc). [note that you can  try
    this in a
    | hypothetical case to verify that it works]. Of course, if you have
    a bubble
    | sextant, you would almost certainly prefer to use that. Like
    anything  in celestial
    | navigation, there is a time and a place for every trick.
    
    Frank has acknowledged that a high Moon is needed to extract
    worthwhile accuracy from such a procedure. How high can the Moon be,
    from Spitzbergen? Once every 18 years (as it happens, this year) the
    Moon can reach an altitude of 40 degrees from Spitzbergen at some
    short period in each month. Most of the time, it's going to be much
    lower than that. So how does measuring a lunar distance angle to 0.1
    arc-minutes, if that can be done, result in a position to +/- 6
    nautical miles?
    
    We keep on asking Frank to provide a proper procedure for reducing
    such an observation, with a numerical example. He keeps insisting that
    he has done so, but the limited information that has been supplied is
    as yet insufficient for his steps to be followed. For example, I have
    pointed out how celestial positions, calculated from the Nautical
    Almanac, are insufficiently precise to provide the claimed precision,
    yet he has still not explained what the observer needs to have aboard
    to provide what's needed, and how it is to be used.
    
    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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