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    Re: Refraction, sunset, and HO229 beginner question
    From: Henry Halboth
    Date: 2004 Jul 25, 15:30 -0400

    Determination of Longitude or an LOP by sunrise/sunset observations was
    and is an accepted navigational  method and is covered in many older
    texts, as well as in those dealing with lifeboat navigation. In solution,
    a negative altitude is employed which may not be compatible with short
    tabular methods, but works perfectly well with a time sight solution. I
    suppose some argument can be advanced with respect to uncertainties of
    refraction, but the method is otherwise entirely useable.
    On Thu, 22 Jul 2004 09:46:43 -0500 Jim Theriot  writes:
    > Bill said:
    > > The idea struck me that at sunrise and sunset I could get a LOP
    > without a
    > > sextant.  One barrel of my binoculars, a heavy-duty
    > neutral-density
    > filter,
    > > a watch, Nautical Almanac and HO229 tables would suffice.
    > I was struck by a possibly similar idea -- I am not sure if it's the
    > same
    > idea,
    > because Bill's comments are mostly about use of almanac and tables,
    > not
    > about
    > the basic principle.  My idea was simply that for any observation
    > you make
    > with watch and sextant where the object appears on the horizon, the
    > same
    > observation could be made with watch only (or watch and binoculars).
    >  Then
    > you run the computations, using using zero for the sextant reading,
    > making
    > all
    > the appropriate corrections (but using zero for the sextant index
    > error
    > correction!), and you should get identical results.  Possibly that's
    > what
    > Bill was getting at.
    > Furthermore, it seems that on a moonless night you might be able to
    > get
    > a LOP from the setting of a star or planet more easily using this
    > method,
    > since you don't have to see the horizon, you only need to note the
    > instant
    > at which the star or planet blinks out.
    > I wanted to test this theory on a sailing trip a couple of months
    > ago (I
    > live inland, not in sight of a 'real' horizon), but weather and
    > circumstances
    > prevented it.
    > Regards,
    > Jim

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