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> Date: Fri, 22 Jun 2001 14:46:56 -0400
> From- Smith_Peter@EMC.COM
> Subject: Re: Sextant accuracy with short distance to horizon
> To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
> X-Virus-Scanned: by AMaViS snapshot-20010407
>
> Steven Wepster [mailto:wepster@MATH.UU.NL] said:
> >                                   ... I don't know for shure what
> > Peter and  Russell meant by 'dip short' tables: my 1981 Bowditch
> > Vol.II has a different table 14, but it has a table 22 'Dip of the
Sea
> > Short of the Horizon'. This table gives the dip of objects _in
front
> > of_ the horizon, so it should not be used for a normal altitude
above
> > the horizon.
> >...
>
> The table for "Dip of the Sea Short of the Horizon" is for just the
> situation Dan Allen was in: the horizon was blocked by an
intervening
> island, so he had to use the point where the island met the water as
> his horizontal reference instead of the horizon. Normal dip tables,
as
> in the Nautical Almanac, give the correction between the horizon and
> the true horizontal. "Dip of the Sea Short of the Horizon" is for
the
> special case, as here, where one is using a point on the sea's
surface
> closer than horizon, but at a known distance from the observer.
> To quote from Bowditch's Explaination of Tables for table 14 (1995
> edition):
>
>       If land, another vessel, or other obstruction is between
>       the observer and the sea horizon, use the waterline of the
>       obstruction as the horizontal reference for altitude
>       measurements, and substitute dip from this table for the
>       dip of the horizon (height of eye correction) given in the
>       Nautical Almanac.
>
>  -- Peter

I got that. But I missed the point that Dan really _did_ use such a
short horizon :(.  --Thanks for pointing that out to me.
_Steven

   

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