NavList:

   

Reply

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

   

Reply