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    Re: Corrections for speed and bearing
    From: Frank Reed CT
    Date: 2005 Jun 6, 22:50 EDT

    Robert E you wrote:
    "Other entry arguments  include the relative bearing of the object being
    sighted. I believe that some  time ago, I posed a question to this list on whether
    or not there is a similar  set of tables for the more sedate speeds as
    experienced by the sea-going  navigator."
    
    It's simple enough in this case that you really don't need a  table. You can
    get the N/S component of your vessel's speed by drawing a simple  triangle on
    a chart or a piece of graph paper or by doing a basic trig problem  (I prefer
    drawing it). And the rest is just proportioning out the correction: a  tenth
    of a minute change in altitude per knot for every six minutes away from  noon
    (or other arbitrary starting point).
    
    And:
    "But this also begs the  question: does this correction really matter for the
    surface navigator who is  not likely to be moving faster than 7 - 10 knots?
    Having myself, taken numerous  sights at sea, including noon sights (just in
    case George questions my  experience!) it seems to me that this factor is not
    terribly significant. Or is  it? "
    
    Let's suppose the Sun's altitude is 60d 10' twenty minutes before  noon and
    60d 30' at noon. If we're motionless then it will be very close to 60d  10'
    twenty minutes after noon (could be as much as 0.7 minutes different close  to
    the equinoxes). Now if I'm sailing towards the Sun's latitude at 10 knots,  then
    these altitudes will shift by (nearly) 6.6 minutes of arc because that's  how
    far we sail in 40 minutes at that speed. That is, we would measure 60d 7.7,
    60d 30', and 60d 13.3. So yes, it matters.
    
    And:
    "you'd been doing me a  big favour if you could provide me (us) with a
    practical example of technique  which you have described, using some real numbers
    and speeds. "
    
    OK. I've  got a bunch of simulations of this with varying degrees of "noise"
    in the sight  data that I've used for experimentation. I'll write it up when I
    get a  chance.
    
    -FER
    42.0N 87.7W, or 41.4N  72.1W.
    www.HistoricalAtlas.com/lunars
    
    
    

       
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