A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunars by Moon declination. was: [NAV-L] Thomas Jefferson and Lunar Obs.
From: George Huxtable
Date: 2005 Mar 26, 17:33 +0000
From: George Huxtable
Date: 2005 Mar 26, 17:33 +0000
Well, here's a rum state of affairs. Most senders of postings, challenged with an apparent identified error, would either defend it or explain what's gone wrong. Not so Frank Reed, it seems. Asked (twice) to explain what seems to be an erroneous quantity in his recent posting of text from Dunbar, in which 5" was printed, where 15" would be expected, he has (twice) declined to say whether it's an error in Dunbar's text or an error in Frank's (or another's) transcription. And in the following contemptous terms- >No one else seems interested in this, so I don't think it's worth the >effort, "onus" or not
. How does he presume to know how many readers may be interested, I wonder? If it's too much "effort" for him as author to put the matter right, does he then expect his readers to put in equivalent effort to do the same? There's an unwritten understanding, on this list as well as in most other means of communication, that authors take responsibility for the accuracy of what they write. And yet, in regard to his original posting, Frank Reed wrote- >There are certainly transcription errors, especially on numerical matters, >but the original scans are available on the web site so you can check for >yourself. I hope that other contributors to Nav-L will avoid this laid-back couldn't-care-less attitude to the accuracy of what they post. Perhaps Dunbar got it wrong: or perhaps Frank got it wrong when it was transcribed, and then failed to notice the unlikely resulting value. Is he just avoiding admitting that? ================= If any other reader happens to take an interest in resolving this minor matter, Frank's instructions (which didn't work on my old Mac) were- "So let's try this web link: _http://memory.loc.gov/ammem/_ (http://memory.loc.gov/ammem/) At that address, enter "lunar Dunbar" (without quotation marks) in the "Search all collections" box. That will call up the letters back and forth between Dunbar and Jefferson on this topic." Frank doesn't identify the particular letter his posting referred to, except as "From William Dunbar to Thomas Jefferson in 1804 (Library of Congress web site)". Nor can I say how many items there are in this collection of letters, though Frank may know. and the text of Dunbar's letter in question (or perhaps a paragraph from it) states- "I mentioned in my last that one very simple method had occured to me of ascertaining in certain Circumstances the Longitude of places, which is much better calculated for travellers by land than Voyagers by Sea; the method is such that a Single observer with a good altitude instrument, altho' deprived of the use of a time keeper, may still make useful observations for the advancement of geographical Knowledge. I shall now just mention the principles & shall hereafter Send you some examples of the Calculation. The excellence of the usual lunar method of determining the Longitude depends (supposing her theory to be perfect) upon her quick change of place from west to east; but it cannot be denied that it requires great dexterity to make good observations, which is evident from the disproportion of the times to the distances in the hands of the best Observers, and this arises from the slow progress of the moon which Causes the Contact to appear to be continued for many seconds of time; were this observation similar to a meridian altitude, it might certainly be taken to any desireable accuracy, that is, were the motion of the moon from North to South in place of from West to east, the moon's altitude when brought upon the meridian by the rotation of the earth would furnish an easy & very Correct mode of ascertaining the Longitude: Now altho' the proper motion of the moon is from West to East, yet her orbit makes so considerable an angle with the equinoctical circle, that there are two portions of each lunation when the moon's change of declination is very rapid, exceeding 6 in 24 hours, that is 5" of a degree in one minute of time; if therefore under favorable Circumstances we take the moon's greatest altitude near the meridian, we shall thence be enabled to ascertain the moon's declination at the moment of her passing our meridian; we must then find the time at Greenwich when the moon had that declination and also the time when the moon passed the meridian of Greenwich, from which data the Longitude is easily found: this method will require the use of some interpolations and an equation for the Correction of the Moon's altitude on the Meridian, because her greatest altitude will not be on the meridian, but to the East or West according as She is increasing or diminishing her North polar distance. I have communicated this method to my Worthy friend Mr. Briggs who is pleased with the idea & intends giving it consideration." The matter in question is simply this; whether Dunbar referred to the Moon's change of declination as 5" of a degree in one minute of time, or 15". Or whatever. George. ================================================================ contact George Huxtable by email at firstname.lastname@example.org, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================