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    Re: Longitude by meridian transit/ fishline sextant
    From: George Huxtable
    Date: 2010 Feb 16, 21:30 -0000

    First, a comment about Gary's posting, when he wrote-
    
    "I'm guessing that a "Heath-Robinson arrangement" is the English equivalent
    of the American "Rube Goldberg device,""
    
    Yes, exactly so.
    
    =====================
    
    Next, I have to correct a mistake in my earlier posting on this topic. I had
    said that the numbers, that Gary provided in that manual, added up to give
    GHA Aries, in arc-minutes. They do not. Sorry about that...
    
    I had failed to notice that added in with the other numbers for date, time,
    and year, which combine to provide GHA Aries, is also a contribution for the
    Sidereal Hour Angle (SHA) of the chosen star, so that the end result is the
    Greenwich Hour Angle (GHA) of that star.
    
    And that provides a bit of a problem, because precession causes all those
    star SHAs to drift slowly, generally increasing by varying amounts, which
    depends on the patch of sky that the star occupies. Those changes, between
    then (mid 1970s) and now (2010), for all those stars,  increase those SHAs
    by amounts which vary between 20 and 50 arc-minutes. If there was any
    precision in such an observing method (and, as I've pointed out, there
    isn't) then that precession would need to be taken into account. At the
    moment of meridian passage the GHA of the star, from a current almanac,
    would equal the observer's (Westerly) longitude from Greenwich, the
    Westitude.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ----- Original Message -----
    From: "George Huxtable" 
    To: 
    Sent: Tuesday, February 16, 2010 4:55 PM
    Subject: [NavList] Re: Longitude by meridian transit/ fishline sextant
    
    
    | Wayne, via "Rebecca Lowry" , asked-
    | about "Longitude by meridian transit/ fishline sextant"
    |
    |
    | Cool,
    | A new toy to make....
    |
    | Any idea what the numbers on page 2-109 for the year are?
    | And how do we find the numbers for the current year?
    | Wayne
    |
    | ===========
    |
    | Those numbers sum up to provide GHA Aries, in arc-minutes. But for this
    | application, it's absurd to bother allowing for such a year-to-year
    | variation. If a constant value of around 35 was chosen, unchanging from
    year
    | to year, that would be perfectly compatible with the precision that the
    | method is capable of providing.
    |
    | The claim is, as Mike Boersma repeats, a precision in precision of 10
    miles,
    | which requires latitude and longitude to be within one sixth of a
    | degree.That would require the following quantities components to be known,
    | or set, within the appropriate accuracies, all within 10 arc-minutes.
    |
    | GMT to 40 seconds (quite plausible)
    | A pendulum string, some few metres long, hanging steady and vertical in
    the
    | open air, with no shielding from wind.
    | Another string, a few metres long, set accurately horizontal, with no
    | guidance given as to how to achieve that.
    | That horizontal string to be set precisely in a North-South direction "by
    | using a compass". No mention of variation.
    | Another diagonal string, tensioned to be straight enough to measure an
    angle
    | from, attached part-way up the length of the vertical pendulum, which
    | doesn't kink that vertical string out of straight.
    | A paper protractor, for measuring the angle between two bits of string in
    | space.
    |
    | All these are supposed to combine together to give a precision in the
    result
    | amounting to 10 arc minutes!. With such a Heath-Robinson arrangement,
    anyone
    | who got the answer right within a couple of degrees, or 120 miles, would
    be
    | doing well.
    |
    | There's nothing wrong, in principle, with any component of this scheme.
    But
    | it's a procedure that looks as if it's been assembled by a committee,
    given
    | the job of knocking up some sort of emergency manual. Completely
    | unrealistic.
    |
    | George.
    |
    | contact George Huxtable, at  george@hux.me.uk
    | or at +44 1865 820222 (from UK, 01865 820222)
    | or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    |
    | =====================
    |
    |
    | --- On Mon, 2/15/10, Mike Boersma  wrote:
    |
    |
    | From: Mike Boersma 
    | Subject: [NavList] Longitude by meridian transit/ fishline sextant
    | To: NavList@fer3.com
    | Date: Monday, February 15, 2010, 12:48 PM
    |
    |
    |
    | The US Airforce survival manual from the 1960's included a description of
    | how to make a fishline sextant and tables for determining longitude and
    | latitude from its use.
    | The claimed accuracy is 10 miles. Individual results may vary.
    | Mike Boersma
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