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Re: Sextant accuracy with short distance to horizon

Subject: Re: Sextant accuracy with short distance to horizon
Smith_Peter@XXX.XXX
Date: Fri Jun 22 2001 - 14:46:56 EDT

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



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