NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant accuracy with short distance to horizon
From: UNK
Date: 2001 Jun 22, 12:47 PM
From: UNK
Date: 2001 Jun 22, 12:47 PM
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