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    Re: Star-sight discrepancy
    From: Bill B
    Date: 2005 Aug 25, 16:09 -0500

    George
    
    Bill wrote:
    >> As
    >> previously noted by another list member, 6 seconds time would account for an
    >> approx 1.5 nm error in latitude, possibly less in longitude.
    >
    George relied:
    > Not necessarily so. For example, near to noon, a 6-second time error would
    > have no effect at all on calculated latitude. What I said was that in the
    > worst case (which applies near the equator, when stars can rise and fall
    > vertically near the East-West horizons), the maximum error in overall
    > position would be 1.5 nm for a 6-second time error. That effect would be
    > predominantly an error in longitude.
    
    Agreed, I was attempting to keep it simple and establish 6 seconds time
    would not account for 5 nm.  If there were "should haves," I should have
    said "may account for...approx," or "no greater than." On the other hand, if
    I recall T was shooting a star, not the Sun.   You do raise an
    interesting point regarding stars.  Wouldn't a star also have a brief
    period--relative to its declination/observed altitude--where change in
    observed altitude appeared to stall while it makes its meridian passage?
    
    Bill wrote:
    >> A fog bank 26 nm away is probably not
    >> the main problem, but you may have anomalous dip. One of the list gurus,
    >> Frank Reed, had written of spring and fall anomalous dip shifts off the CT
    >> coast similar to the magnitude of error you observed.
    
    George replied:
    > I had pointed out some time ago that the coast of Southern California,
    > together with the Red Sea, was a notorious haunt of anomalous dip, when the
    > wind was blowing air, Sun-heated over desert sand, over the adjoining sea.
    > A 5-minute discrepancy in dip would be unusually high even there, however,
    > and I would guess that anyway such effects would have largely died down by
    > sunset. So Tom would be unfortunate to be experiencing such extreme
    > unexpected values for dip, but I agree that it's possible.
    
    Now that you mention it, I do recall your post.  I apologize if not
    remembering it, or including it in my response to T, was perceived as a
    slight. When I mention the gurus, I do include you on my list.
    
    Bill wrote:
    
    >> It seems there are
    >> differing views on how best to deal with fog--get as high as you can vs get
    >> as low as you can.
    
    George replied:
    >
    > I doubt that. For fog, I doubt that anyone will argue with the rule "get as
    > low as you can". Where it's good advice to "get as high as you can", is in
    > wave or swell conditions, to put the waves or swell onto a horizon that's
    > as far away as possible.
    
    I was thinking back to a post by one of our professional mariners, who spoke
    about being on the bridge--which was above the fog.  If I recall he could
    see the tops of buildings poking out of the fog along the shore/banks.  My
    assumption, perhaps erroneous, was that if a ship were in a *patch* of fog,
    it may be possible to get above it, and if very lucky see a clear horizon.
    Perhaps wishful thinking, and probably a low percentage shot.
    
    Bill
    
    
    

       
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