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    Variation of dip.
    From: George Huxtable
    Date: 2001 Apr 22, 1:23 AM

    Dan Hogan wrote, in a thread headed- Re: Any happy/unhappy Astra IIIB
    owners here?
    >I started out navigating with a Davis plastic sextant in 1963(?) or there
    >abouts. I used it on trips to/from Hawaii, Baja, and up/down the coast. For a
    >small boat it kept an accuracy of 4 Nmi+/-. Used it up to 1984, when I left
    >it laying on a plastic cushion on the way to Santa Catalina Island one summer
    >day, it tried to turn into a ball. Then I bought a used Huson, I used it
    >until I got talked into exchanging it for a Freiberger.
    >IMO a top of the line sextant is required only if you are going try Lunars.
    >Otherwise, for *PRACTICAL* small boat navigation, most modern sextants will
    >work admirably.
    >Consider these errors:
    >        Your eyesight
    >        Boat motion
    >        Your plotting error
    >        Your method of sight reduction
    >        Any time error
    >        And, any errors in the charts
    >Dan Hogan WA6PBY
    >Catalina 27 "GACHA"
    >Navigation-L List Owner
    I agree fully with what Dan says: however, he has omitted an important
    contribution to sextant error: that of variation in atmospheric dip, due to
    unusual layering in the temperature of the atmosphere near sea level. These
    are conditions that can give rise to mirages, if there's any object in
    sight to be seen as a mirage. But if the horizon is clear, there's no clue
    to the navigator that dip is abnormal.
    On one occasion, a US Navy vessel in the Gulf of Santa Catalina (off San
    Diego, Cal.) reported a measured dip  of 7.2 minutes. This should be
    compared with the expected value from dip tables of 4.7 minutes, so
    producing an error of 2.5 minutes in latitude.
    Four days later, in the same area, the same vessel measured a dip that day
    of minus 2.5 minutes, this time 7.2 minutes out from the expected value, in
    the opposite direction.
    The vessel carried an instrument designed to measure the actual dip that
    was occurring at the time, the Blish Prism. Otherwise, its navigator would
    have been quite unaware that anything was amiss, and would have made
    corresponding errors in his position on the chart.
    I've been informed by Andrew Young, an authority on optics of the
    atmosphere and a resident of San Diego, that the area is one where such
    disturbances of dip may occur frequently. This can be especially the case
    in October (when the above measurements were made), when what are known
    locally as "Santa Ana" conditions blow hot air from the desert out over the
    sea. The region is just where Dan does his sailing. There may well be many
    other areas of the world, frequented by craft, where similar conditions can
    Other oceanographic cruises, such as that of the Carnegie, measuring dip
    mainly out in deep-ocean conditions, have not recorded such large
    The fact remains that in certain areas and at certain times it is possible
    for navigators to find themselves in conditions that can put their sextant
    positions out by several minutes, without any evidence to show what is
    happening. It can happen to the best navigator, with the most expensive
    sextant. And it can dwarf all those other errors listed by Dan.
    More about the Blish Prism shortly.
    George Huxtable.
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.

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