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    Re: Star-sight discrepancy
    From: George Huxtable
    Date: 2005 Aug 25, 11:01 +0100

    Tom Shanklin wrote-
    >I don't think it was due to Index Error or Dip.  The I.E. (determined before
    >the sight, but not after) on my sextant was .8' on the arc, so I subtracted
    >that from Hs.  My height of eye was 40ft, taken from the outer deck of Bolsa
    >Chica State Beach Lifeguard Headquarters (N33 41 W118 03): 35ft to top of
    >balcony+ 5'10" for my own height= 40ft app.= DIP corrn -6.2' also subtracted
    >from Hs.  I've double checked all my calculations and have calculated it
    >using a Javascript program provided on a website
    >(http://www.geocities.com/CapeCanaveral/2265/sight.htm). The only thing
    >left is the horizon.  Which definetly wasn't clear.  There was a fogbank
    >hanging offshore.  The eastern edge would have had to be somewhere between 3
    >and 26 n.m.s offshore (I could see Oil Island Eva along with cargos lined up
    >offshore outside Long Beach, but strangely only Blackjack peak of Catalina
    >Is: low hanging haze/fog).  I'm thinking maybe having the horizon 'cut
    >short' might have caused my error.  Plus it was hard to find my stars until
    >it got pretty dark (light pollution).  So a darkening, fog obscured horizon
    >shot through a whole horizon mirror (saving for a trad.).  Approximately how
    >far away does fog need to be to get an accurate fix?
    Above, he has provided some, but not enough, of the information needed for
    us to check over his observation. What would be interesting to learn is-
    Presumed lat and long of observing position.
    Name of star observed.
    Sextant reading.
    Corrections made to that reading
    Greenwich date, and time by watch, of observation.
    Known error of watch.
    He computed his error to be 5 nautical miles, but doesn't give its sign. If
    his horizon has been cut short by a fog bank at an unknown distance, that
    would cause the observed altitude of the star, above that false horizon, to
    be greater than the calculated altitude above the (invisible) real horizon.
    Was the 5-mile error that way round?
    If it was possible to descend to the shoreline at sea-level, rather than
    observe from a 35 ft. platform, that would have been better in those
    circumstances. Then the horizon would become much closer, and therefore
    less likely to be obscured by any fog.
    >I've double checked all my calculations and have calculated it
    >using a Javascript program provided on a website
    Beware; among all the gold that's available on the internet there is a
    significant amount of dross. Just because somethig can be found on a
    website does not mean that it's good, or correct. Indeed, the converse
    applies, too often for comfort. Try working it longhand, with a calculator,
    and see if you get the same answer. It's a good way to learn.
    Tom asked-
    >Approximately how
    >far away does fog need to be to get an accurate fix?
    Fog, at sea-level, needs to be at least as far away as the horizon distance
    to allow an accurate fix. The difficulty is to recognise such distant fog
    when you see it. Sometimes, a distant fog bank can produce an appearance of
    a horizon when it just ain't so. And it's particularly difficult to
    distinguish the presence of such fog under twilight conditions; just the
    time when sea-surface fog is likely to materialise.
    In a reply, Bill has written-
    >What is important is your actual height-of-eye above water
    >level. First you need to determine that as precisely as possible.
    >Next wave height comes into play.  Whether a 6 foot wave is measured from
    >peak to trough, or mid point to peak, has been discussed on the list.
    >Nonetheless, you might want to subtract some form of wave height from your
    >height of eye (on a small boat you want to be on top of a crest and discount
    >wave height--on a large ship split the difference).  From the shore? Betting
    >on 1/2 total broadcast wave height.
    Well, yes, but such corrections, for tidal height and wave height, will
    amount only to a small fraction of the total dip, and will never reach a
    value of 5 miles. Nevertheless, for accurate work, they shouldn't be neglected.
    >previously noted by another list member, 6 seconds time would account for an
    >approx 1.5 nm error in latitude, possibly less in longitude.
    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.
    >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.
    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.
    >I have tried to get a simple answer/rule of thumb from the list at least
    >twice on how to adjust for a hazy horizon (sharp horizon obscured by a band
    >of white) or anomalous dip.
    The answer to that question is "can't be done",  which is perhaps why Bill
    failed to get his answer. If the horizon can't be seen there's no way to
    account for a band of white of unknown extent which may be covering it. As
    far as anomalous dip is concerned, it's not apparent, which is why it can
    be so insidious. What Bill needs is a dipmeter, which we have discussed
    before, and it may be useful to do so again. He would have to make his own,
    or find an antique. You can measure the altitude of an object, higher than
    60 degrees, with a sextant, once in the normal way and once behind your
    back, but that isn't easy.
    At night things are best. If you take a number of star sights, spread over
    different azimuths around the horizon, anomalous dip will enlarge you
    resulting "cocked hat", but its centre should give an unaffected true
    position. Simply observing one or more pairs of stars, at roughly opposite
    azimuths, will do the trick.
    >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.
    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.
    >  Others seeking an answer on "ideal" atmospheric
    >conditions also seem to come up short.
    That's because it's such a difficult question to answer. As long as you can
    avoid low-altitude observations, keeping to 5 degrees and upwards, the
    problems occur, not in the direction of light from the body, so much as in
    the direction of the horizon. Without a dipmeter, you can't determine what
    that is. In the past, mariners sometimes tried comparing the air and sea
    temperatures, but that was never a reliable predictor of dip.
    Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.

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