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Subject: Re: Sextant accuracy with short distance to horizon
From: Steven Wepster (wepster@XXX.XXX)
Date: Mon Jun 25 2001 - 10:57:19 EDT
> Date: Fri, 22 Jun 2001 14:46:56 -0400
> From: Smith_Peter@XXX.XXX
> Subject: Re: Sextant accuracy with short distance to horizon
> To: NAVIGATION-L@XXX.XXX
> X-Virus-Scanned: by AMaViS snapshot-20010407
>
> Steven Wepster [mailto:wepster@XXX.XXX] 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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