NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Lunar distances - shot clearance methods
From: Henry Halboth
Date: 2004 Sep 11, 13:50 -0400
From: Henry Halboth
Date: 2004 Sep 11, 13:50 -0400
It is high time that a few of the comments/questions raised concerning my posting regarding Arnold's Method of clearing Lunar Distances be addressed. Sorry for the delay. Preliminarily, thanks to all those who did respond. Your comments were certainly insightful and conducive to the advancement of our discussions. Many of the older methods of navigation are fading into obscurity or are sometimes simply misrepresented - this forum provides an informative arena for there correct preservation. If you get it wrong, you are going to be stomped on, which often is not the case with published material. As regards Arnold's tables being somewhat restrictive by reason of constraints in altitude correction, please note the use criterion to be the bodies apparent altitude or the correction thereto; how either is calculated is not mandated and may be at the user's option - whatever refinement in the way of temperature and latitude corrections for refraction and parallax may certainly be applied at discretion. One cavalier treatment, not previously mentioned, is that Arnold advocates a standardized observed altitude correction to obtain the apparent altitude; under a Rule III, he advocates, across the board ... "To the moon's observed altitude, add 12', if the lower limb be taken, but if the upper limb be taken, subtract 20'. to the observed altitude of the sun's lower limb add 12', and from the star's observed altitude subtract 4', and you will have their apparent altitudes." Of course, we know this to be technically incorrect - perhaps it is simply a reflection of the often expressed opinion that an error of a few minutes of arch in altitude does not materially affect the result in clearing the distance. As to the longevity of Arnold's work, addressed by Frank, it is probably worth noting that his work is not primarily a navigational text. It is a wide ranging compendium of nautical knowledge, trivia if you will, specific to the era it which it was written; it possesses no ongoing values that might attract it perpetuation by a hydrographic office or on which subsequent authors/publishers might build on a continuing basis, as occurred both with Bowditch and Norie, although it seems Norie has pretty much run it's course, as the last edition I was able to obtain was that published in 1917. Subjects covered by Arnold in some 844 pages, exclusive of tables, include sailing directions, ship maneuvering instructions, sailing instructions, practical sea gunnery, familiar subjects in astronomy, the pepper trade on the West Coast of Sumatra, UIS Customs Duties, coins of the US, instructions to Masters of Ships, quarantine laws, laws of pilot and pilotage, regulation of seamen in the Merchant Service, marine assurance, etc. It is really a snapshot in time and contains priceless information, some of which has long since been forgotten, of a particular era in seagoing history and the United States. It bears a frontispiece of testimonials, albeit unsigned, by prominent marine figures of the day. There are about 103 pages devoted primarily to Lunar Distances and the associated astronomy and, as far as my personal interests were served, allowed a greater understanding of the lunar theory than did either Bowditch or Norie which, in my view, were more prone to simply providing rules of calculation rather than in appealing to the reader's understanding of the problem.