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    Re: Back sights
    From: George Huxtable
    Date: 2010 Mar 19, 15:28 -0000

    Brad has the right idea about backsights. I imagine that's just the way the 
    index adjustment was made, at sea.
    
    There are a few things to bear in mind.
    
    It's a rather indirect procedure, in that it relies on first measuring the 
    altitude of a near-zenith star. That depends on both the index-index error 
    adjustment in the forward direction, then the 0º to 90º scale calibration 
    being error-free, then a precise observation of the star in the forward 
    direction, then another precise observation in the back direction, from the 
    same part of the horizon.  But on the other hand, as there was never a 
    telescope involved in  back observations, they would never be very precise, 
    at the best of times.
    
    The disc confined within 5º of the zenith only offers a tiny fraction of 
    the total sky, which is unlikely to offer a bright star at a particular 
    time, so it may be necessary to wait until one turns up.
    
    Because one is shooting at a moving target, a star, careful timing, or 
    better, careful interleaving, of forward and back observations would be 
    called for. If a star at culmination is chosen, then its angle, from the 
    North point of the horizon, would be unchanging, and it would be best to 
    alternate fore and back observations from a due-North spot on the horizon, 
    chosen with reference to another star near that spot, the observer facing 
    alternately North and South. Then, to add a bit more data, another pair of 
    altitudes could be measured with respect to the South point of the horizon.
    
    A normal quadrant / octant, for forward observation,  uses some sort of 
    telescope or at least a peep-tube to put the eye in the right place. In the 
    back mode, there's no such guidance. So I wonder how sensitive the 
    observation is to error, from the observer's eye being out of the plane?
    
    But I have never ever handled such an instrument, or attempted a back 
    sight, so I'm guessing, a bit.
    
    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: "Brad Morris" 
    To: 
    Sent: Friday, March 19, 2010 1:24 PM
    Subject: [NavList] Re: Back sights
    
    
    | Hi George
    |
    | Thank you for the information about backsights.  That was precisely what 
    I was after!
    |
    | You wrote:
    | The big problem with the backsight was this: With the familiar geometry 
    of a foresight, the index error of the instrument was quickly obtained by 
    aligning an object with itself. That was not possible with a backsight. It 
    might be possible to check one end of its scale, by aligning the fore 
    horizon with the aft horizon, if they could both be seen together, but that 
    observation would include twice the dip, an unpredictable quantity. Some 
    index mirrors had a special facet ground exactly 90º from the main surface, 
    to aid such alignment, which could be as good as was the precision of that 
    set angle. But otherwise, I imagine that mariners accepted any backsight 
    index-error as it came, without checking. On land, given appropriate 
    distant landmarks, I can imagine ways of doing the job with some difficuly. 
    However, there's no sign that Lewis and Clark ever verified the error in 
    their backsights. If any reader can suggest ways of dealing with such 
    offset error, on land or sea, I am ready to learn.
    |
    |
    | I think I may have the answer to the index error for backsights. 
    Consider NAV1268 at the National Maritime Museum in Greenwich.  It is a 
    backsight Octant.  As is typical of Octants, it is a vernier type, with the 
    arc from -2 degrees to +101 degrees.  Due to the length of the vernier 
    itself, however, the octant can only measure to 95 degrees.  The 
    measurement beyond 90 degrees is the key.  Using a FORESIGHT, measure the 
    altitude of a star whose apparent altitude is greater than 85 degrees.  Why 
    greater than 85 degrees?  There is a doubled region between 85 degrees and 
    95 degrees, in which we can measure the altitude of a star with EITHER a 
    foresight or a backsight observation.  Since it is possible with either 
    method, we must perform the observation with BOTH methods.  In knowing what 
    the altitude is with a foresight observation, we therefore know what the 
    vernier must read for a backsight observation, given the same star.  Set 
    the octant's vernier to the arc for a backsight observation and adjust the 
    backsight horizon mirror until the altitude is correct.  Viola!
    |
    | Best Regards
    | Brad
    |
    |
    |
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