NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Hughes Tables
From: Geoffrey Kolbe
Date: 2007 Jun 07, 19:57 +0100
From: Geoffrey Kolbe
Date: 2007 Jun 07, 19:57 +0100
Robert Eno wrote:
In Ageton's method, the perpendicular is dropped from the geographical position of the celestial body to made a right angle with the observer's meridian.
The Hughes method drops the perpendicular from the observer's DR position to the meridian of the celestial body. In this, it is similar to the method used for the NAO tables used in the Nautical Almanac. Paul Janiczek, co-originator of the NAO tables (with Admiral Tom Davies), wrote a nice piece on how the NAO tables came into being in the last issue of the Navigator's Newsletter.
The disadvantage of the Hughes' method (and others like it where the perpendicular is dropped from the observer's DR position to the meridian of the celestial body) is that the perpendicular splits the azimuth angle - the separate components of which have to be computed and added together, so increasing the number of steps involved. One advantage of the method (says Janiczek) is that the initial entry into the tables is with the Latitude and LHA, so all initial entries for all sightings will be on the same page, reducing the total number of page openings.
It is worth noting that the formulae used in the NAO tables use just sines and cosines, and two sets of tables are required. Whereas, from the scanned page provided by D Walden, the Hughes tables seem to use sines and cosines; tangents and cotangents; secants and cosecants; which requires three sets of tables.
Geoffrey Kolbe
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Many thanks to d walden.
As it turns out, part of the answer to my question was behind me on a bookcase full of navigation texts. Vol 3 of the Admiralty Manual of Navigation (1954) carries a brief description.
On a quick glance and not being a mathematician, it appears that the sight reduction method in Hughes is similar to HO 211 (Ageton)?
Robert
In Ageton's method, the perpendicular is dropped from the geographical position of the celestial body to made a right angle with the observer's meridian.
The Hughes method drops the perpendicular from the observer's DR position to the meridian of the celestial body. In this, it is similar to the method used for the NAO tables used in the Nautical Almanac. Paul Janiczek, co-originator of the NAO tables (with Admiral Tom Davies), wrote a nice piece on how the NAO tables came into being in the last issue of the Navigator's Newsletter.
The disadvantage of the Hughes' method (and others like it where the perpendicular is dropped from the observer's DR position to the meridian of the celestial body) is that the perpendicular splits the azimuth angle - the separate components of which have to be computed and added together, so increasing the number of steps involved. One advantage of the method (says Janiczek) is that the initial entry into the tables is with the Latitude and LHA, so all initial entries for all sightings will be on the same page, reducing the total number of page openings.
It is worth noting that the formulae used in the NAO tables use just sines and cosines, and two sets of tables are required. Whereas, from the scanned page provided by D Walden, the Hughes tables seem to use sines and cosines; tangents and cotangents; secants and cosecants; which requires three sets of tables.
Geoffrey Kolbe
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
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